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ScienceWeek
SCIENCEWEEK E-BOOK
Life and Death: Frontiers in the 21st Century
Part 2: Cell Biology of Reproduction
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Section 1 - Contents
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1. Introduction
2. Oogenesis
3. Spermatogenesis
4. Fertilization
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Section 2
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1. INTRODUCTION
In this context, the term "diploid" refers in general to a
chromosome state in which each type of chromosome is represented
twice. In contrast, the term "haploid" refers to a chromosome
state in which each chromosome is represented singly. In humans,
somatic cells are diploid and gametes are haploid.
In this context, the term "meiosis" (reduction division) refers
to the process whereby a nucleus divides by two divisions
(meiosis I and meiosis II) into four nuclei, each containing half
the original number of chromosomes, in most cases forming a
genetically nonuniform haploid. This is a necessary aspect of
eukaryotic sexual reproduction, for without it fertilization
would usually double the chromosome number every generation.
HISTORICAL NOTE: ON THE RECOGNITION OF THE ROLE OF SPERM CELLS.
"It is only within the past century that the sperm's role in
fertilization has been known. Anton van Leeuwenhoek [1632-1723],
the Dutch microscopist who co-discovered sperm in 1678, first
believed them to be parasitic animals living within the semen
(hence the term spermatozoa, meaning 'sperm animals'). He
originally assumed that they had nothing at all to do with
reproducing the organism in which they were found, but he later
came to believe that each sperm contained a preformed embryo.
Leeuwenhoek (1685) wrote that sperm were seeds (both sperma and
semen mean 'seed') and that the female merely provided the
nutrient soil in which the seeds were planted. In this, he was
returning to a notion of procreation promulgated by Aristotle
[384-322 BC] 2000 years earlier. Try as he might, Leeuwenhoek was
continually disappointed in his attempts to find the preformed
embryo within the spermatozoa. Nicolas Hartsoeker, the other co-
discoverer of sperm, drew a picture of what he hoped to find: a
preformed human ('homunculus') within the human sperm . This
belief that the sperm contained the entire embryonic organism
never gained much acceptance, as it implied an enormous waste of
potential life. Most investigators regarded the sperm as
unimportant.
"The first evidence suggesting the importance of sperm in
reproduction came from a series of experiments performed by
Lazzaro Spallanzani [1729-1799] in the late 1700s. Spallanzani
demonstrated that filtered toad semen devoid of sperm would not
fertilize eggs. He concluded, however, that the viscous fluid
retained by the filter paper, and not the sperm, was the agent of
fertilization. He, like many others, felt that the spermatic
'animals' were parasites.
"The combination of better microscopic lenses and the cell theory
led to a new appreciation of spermatic function. In 1824, J. L.
Prevost and J. B. Dumas claimed that sperm were not parasites,
but rather the active agents of fertilization. They noted the
universal existence of sperm in sexually mature males and their
absence in immature and aged individuals. These observations,
coupled with the known absence of spermatozoa in the sterile
mule, convinced them that 'there exists an intimate relation
between their presence in the organs and the fecundating capacity
of the animal.' They proposed that the sperm entered the egg and
contributed materially to the next generation... [But] it was
only in 1876 that Oscar Hertwig [1849-1922] and Herman Fol [1845-
1892] independently demonstrated sperm entry into the egg and the
union of the two cells' nuclei."
Scott F. Gilbert: Developmental Biology. 6th Edtion. Sinauer
Assoc. 2000, p.185.
ON OOGENESIS
"Egg development varies from species to species in details, but
usually follows [a general scheme]. Oogenesis begins after the
primordial germ cells migrate to the gonad and become oogonia. A
finite number of mitotic divisions occur; in the human female,
these divisions are complete before birth. Many more potential
eggs are generated than will ever be shed; thus most oogonia
regress and die before they complete oogenesis. When the final
number of oogonia is obtained, the chromosomes replicate and the
cells differentiate into primary oocytes, which enter prophase of
the first meiotic division. At this stage, meiosis is usually
arrested, and each oocyte enters a resting state until the female
becomes sexually mature. This can take anywhere from a few days
to many years, depending on the species.
"At sexual maturity, hormones direct primary oocytes to mature
one (or a few) at a time. The nuclear membrane of the oocyte is
known as the germinal vesicle. The steroid hormone progesterone
triggers oocyte maturation, causing germinal vesicle breakdown
and the completion of meiosis I. Nuclear division is equal, but
the cytoplasm divides very unequally, producing one large
secondary oocyte, from which the egg will eventually arise, and a
small polar body. Division of the large secondary oocyte is again
very unequal, as meiosis II generates another small polar body
and the large ootid, which develops into the ovum.
"Because both meiotic divisions are very unequal, the ovum
retains more than 95% of the cytoplasm of the primary oocyte.
Oocytes are one of the largest cell types. This makes sense
because the ovum must supply almost all of the cytoplasm and
initial food supply for the embryo that is formed upon
fertilization. In contrast, the only known contribution of the
sperm cell is its genetic material and the minimal contents of
the male pro-nucleus."
WM Becker and DM Deamer: The World of the Cell. 2nd Edition.
Benjamin/Cummings 1991, p.675.
ON SPERMATOGENESIS
"Male gametogenesis differs in several ways from the process by
which eggs are formed during oogenesis. The male gametes of
animals, called sperm cells, are produced in the testes by the
process of spermatogenesis. In mammals, spermatogenesis occurs in
close association with Sertoli cells that provide protection and
nutritional support for the developing sperm; in fact, the
various stages of sperm development usually occur while the newly
forming sperm lie embedded within recesses of the Sertoli cell
surface. Spermatogenesis begins with mitotic division of the male
primordial germ cells or spermatogonia, which continue to
proliferate throughout most of the organism's adult lifetime,
thereby allowing sexually mature males to continually produce
large numbers of new sperm cells.
"Some of the cells generated during the mitotic proliferation of
spermatogonia eventually differentiate into primary
spermatocytes, which then embark upon meiosis. The first meiotic
division transforms primary spermatocytes into secondary
spermatocytes, which are subsequently converted by the second
meiotic division into spermatids. In contrast to the comparable
phase of oogenesis, no dramatic growth or morphological changes
accompany meiosis in male gametogenesis. However, the mitotic and
meiotic divisions that lead to the formation of spermatids are
accompanied by an unusual type of cytokinesis that fails to
completely divide the cytoplasm; as a result, the spermatids
remain connected to each other by cytoplasmic bridges. Unlike
female gametogenesis, where meiosis produces egg cells that are
functionally mature, the spermatids produced by meiosis in males
must undergo further differentiation to create functional sperm.
The process of transforming spermatids into mature sperm cells is
called spermiogenesis. During spermiogenesis the spermatid loses
most of its cytoplasm and the remainder of the cell
differentiates into two structures, the head and the tail."
LJ Kleinsmith and VM Kish: Principles of Cell and Molecular
Biology. HarperCollins 1995, p.680.
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2. OOGENESIS
THE MAMMALIAN OOCYTE ORCHESTRATES THE RATE OF OVARIAN FOLLICULAR
DEVELOPMENT
JJ Eppig et al (Jackson Laboratory Bar Harbor, US) discuss
mammalian oocytes, the authors making the following points:
1) Complex cell-to-cell interactions coordinate the development
of ovarian follicles. The pathways of cellular communication
include endocrine, autocrine, and paracrine regulators, and gap
junctions. Coordination of the development of oocyte and somatic
follicular compartments ensures that the ovulated oocyte is ready
to undergo fertilization and subsequent embryogenesis. Disruption
of this synchrony by inappropriately timed administration of
exogenous gonadotropins can produce oocyte developmental failure
(1). Communication between the oocyte and companion somatic cells
is essential for successful development of both follicular
compartments (2).
2) The oocyte depends on its association with companion somatic
granulosa cells to support its growth and development and to
regulate the progression of meiosis. Likewise, oocytes promote
granulosa cell proliferation, differentiation, and function. The
communication between granulosa cells and oocytes is, therefore,
bidirectional and occurs throughout follicular development (2,3).
In fact, follicular formation itself appears coordinated by a
transcription factor, "factor in the germline" (FIG), expressed
by the oocyte (4). Early follicular development depends on
oocyte-secreted members of the transforming growth factor
family, growth differentiation factor (GDF)-9, and bone
morphogenic protein (BMP)-15 (5). These oocyte-derived paracrine
factors also promote follicular somatic cell proliferation and
steroidogenesis and locally regulate gene expression in granulosa
cells. Thus, communication between the oocyte and companion
somatic cells is crucial for the development of both cell types,
but how this complex interaction is coordinated was not
previously known.
3) In summary: The development of both the mammalian oocyte and
the somatic cell compartments of the ovarian follicle is highly
coordinated; this coordination ensures that the ovulated oocyte
is ready to undergo fertilization and subsequent embryogenesis.
Disruption of this synchrony results in oocyte developmental
failure. Communication between the oocyte and companion somatic
cells is essential for successful development of both follicular
compartments. However, it was not previously known whether one
cell type, either the somatic or the germ cell compartment,
determines the overall rate of follicular development. The
authors report that to test the hypothesis that the oocyte
orchestrates the rate of follicle development, mid-sized oocytes
isolated from secondary follicles were transferred back to
primordial follicles, the earliest stage of follicular
development. This transfer doubled the rate of follicular
development and the differentiation of follicular somatic cells.
Oocyte development in these accelerated follicles appeared
normal; recovered oocytes were competent to undergo fertilization
and embryonic development. The authors suggest these results
demonstrate that oocytes orchestrate and coordinate the
development of mammalian ovarian follicles and that the rate of
follicular development is based on a developmental program
intrinsic to the oocyte.
References (abridged):
1. Hunter, R. H. F. , Cook, B. & Baker, T. G. (1976) Nature
(London) 260, 156-157
2. Eppig, J. J. (1991) BioEssays 13, 569-574
3. Eppig, J. J. (2001) Reproduction 122, 829-838
4. Soyal, S. M. , Amleh, A. & Dean, J. (2000) Development
(Cambridge, U.K.) 127, 4645-4654
5. Dong, J. W. , Albertini, D. F., Nishimori, K., Kumar, T. R.,
Lu, N. F. & Matzuk, M. M. (1996) Nature (London) 383, 531-535
Proc. Nat. Acad. Sci. 2002 99:2890
Related Background:
OOGENESIS: MATURATION OF MOUSE FETAL GERM CELLS IN VITRO
Y OBATA et al (Gunma University, JP) discuss oogenesis, the
authors making the following points:
1) Nuclear reprogramming is essential during gametogenesis for
the production of totipotent zygotes. The authors demonstrate
that premeiotic female germ cells derived from mouse fetuses as
early as 12.5 days post coitum are able to complete meiosis and
genomic imprinting in vitro and that these matured oocytes are
highly competent in supporting development to full term after
nuclear transfer and in vitro fertilization. The authors state
that to their knowledge this is the first time that complete
oogenesis has been successfully accomplished in vitro.
2) Although the ovaries of mammals contain thousands or millions
of immature oocytes, few of these ever mature to the point at
which reproduction in vivo is possible. Ovarian oocytes therefore
constitute a large and potentially valuable resource for clinical
and zoological application. However, although mature oocytes have
been produced in vitro by culturing immature oocytes(1-3),
oogenesis was never complete, and attempts to produce even non-
growing oocytes at the diplotene stage of the first meiosis from
ovaries derived from newborn mice have met with limited success
(only 0.016%; ref. 2).
3) The authors have demonstrated that this poor ability of
oocytes to mature in culture is due to their incompetent
cytoplasm(4), a problem that can be overcome by transferring
their nuclei into enucleated, fully grown oocytes. Although live
pups can eventually be produced from oocytes reconstituted in
this way, this is not possible if nuclei from small, immature
oocytes are used for reconstitution, probably because of defects
in their meiotic chromosomal configuration and/or genomic
imprinting(5).
4) In summary: The authors have demonstrate that the most
primitive murine fetal oocytes can differentiate into competent
oocytes with high efficiency. As well as offering an opportunity
to analyze the mechanisms behind nuclear reprogramming in vitro,
this system might eventually help women undergoing chemotherapy
or radiotherapy become mothers afterwards by prior removal of an
ovary.
References (abridged):
1. Buehr, M. & McLaren, A. Gamete Res. 11, 271-281 (1985)
2. Eppig, J. J. & O'Brien, M. J. Biol. Reprod. 54, 197-207 (1996)
3. Cortvrindt, R., Smitz, J. & Van Steirteghem, A. C. Hum.
Reprod. 11, 2656-2666 (1996)
4. Bao, S., Obata, Y., Carroll, J., Domeki, I. & Kono, T. Biol.
Reprod. 62, 616-621 (2000)
5. Kono, T., Obata, Y., Yoshimzu, T., Nakahara, T. & Carroll, J.
Nature Genet. 13, 91-94 (1996)
Nature 2002 418:497
Related Background:
INTERCELLULAR COMMUNICATION IN THE MAMMALIAN OVARY: OOCYTES CARRY
THE CONVERSATION
MM Matzuk et al (Baylor College of Medicine, US) discuss oocyte-
somatic cell interactions, the authors making the following
points:
1) Germ cells are uniquely specialized to transmit the genome to
succeeding generations. In animal species, sexual reproduction
requires meiotic division to produce haploid gametes (i.e., eggs
and spermatozoa), which upon fertilization give rise to the
totipotent embryo. In both sexes, interactions between the
developing gametes and neighboring somatic cells are crucial for
fertility (1). The importance of this communication in
spermatogenesis is underscored by clinical cases of male
infertility, transgenic mouse models, and xenogenic germ cell
transplantation experiments (1,2). Similarly, in females, complex
intercellular dialogs have evolved to regulate oogenesis in
species as wide-ranging as fruit flies (3) and mice. In the
mammalian perinatal ovary, oocytes arrested in the diplotene
stage of meiosis I become surrounded by a single, squamous layer
of somatic cells to form a finite population of non-growing
primordial follicles (4). Primary follicles are recruited from
the primordial pool as oocytes grow and the surrounding somatic
cells (called granulosa cells) become cuboidal and proliferative.
This transition is associated with a commitment to subsequent
stages of follicular development, and the measured recruitment of
primordial follicles from the resting pool is critical for the
continuity of folliculogenesis throughout the reproductive life-
span of all mammals.
2) In the mouse, an initial synchronous wave of follicular
recruitment occurs within a few days of birth. By 10 to 12 days
of postnatal life, a cohort of secondary-stage follicles
develops, in which oocytes at midgrowth are surrounded by two or
more layers of granulosa cells. Antral-stage follicles form
between 14 and 24 days when fluid-filled cavities develop between
the layers of somatic cells. Antrum formation subdivides the
granulosa cells into two spatially and functionally distinct
populations: Those with closest proximity to the oocyte are
called the "cumulus granulosa cells", and those lining the
follicle wall are "mural granulosa cells". During the preantral
to antral follicle transition, the oocyte acquires the capacity
to resume meiosis (5), and epigenetic modifications essential for
fetal development are progressively established. Meiotic
competence is associated with the accumulation of cell cycle
regulatory factors, as well as reorganization of chromatin and
microtubule configurations. The luteinizing hormone (LH) surge
promotes substantial changes in gene expression in preovulatory
granulosa cells and indirectly stimulates oocyte meiotic
maturation and ovulation of a metaphase II-stage egg that is
competent to undergo fertilization.
3) In summary: The production of functional female gametes is
essential for the propagation of all vertebrate species. The
growth of oocytes within ovarian follicles and their development
to mature eggs have fascinated biologists for centuries, and
scientists have long realized the importance of the ovarian
follicle's somatic cells in nurturing oogenesis and delivering
the oocyte to the oviduct by ovulation. Recent studies have
revealed key roles of the oocyte in folliculogenesis and
established that bidirectional communication between the oocyte
and companion somatic cells is essential for development of an
egg competent to undergo fertilization and embryogenesis. The
challenge for the future is to identify the factors that
participate in this communication and their mechanisms of action.
References (abridged):
1. K. H. Burns, F. J. DeMayo, M. M. Matzuk, in Molecular Biology
in Reproductive Medicine, B. C. J. M. Fauser, Ed. (Parthenon,
Lancashire, UK, ed. 2, in press)
2. R. L. Brinster, Science 296, 2174 (2002)
3. W. Deng and H. Lin, Int. Rev. Cytol. 203, 93 (2001)
4. H. Peters, Acta Endocrinol. 62, 98 (1969)
5. R. A. Sorensen and P. M. Wassarman, Dev. Biol. 50, 531 (1976)
Science 2002 296:2178
Related Background:
DROSOPHILA OOGENESIS: GENERATING AN AXIS OF POLARITY
Buzz Baum (University College London, UK) discusses oogenesis,
the author making the following points:
1) In eukaryotic cells, a global axis of polarity is established
using two general mechanisms. In one, an oriented cytoskeleton is
assembled which directs polar intracellular transport. In the
other, cells establish stable marks within the actin-rich cortex
which serve as reference points to capture and concentrate
specific polar cell markers as they pass by. Often these systems
work in tandem [1] , so that cells polarize their cytoskeleton
with respect to an existing cortical cue. Determinants of
polarity are then transported along cytoskeletal filaments to
this site, where they become anchored, amplifying the polar
signal.
2) To establish anterior posterior polarity during Drosophila
oogenesis, microtubules direct the transport of oskar mRNA to one
end of the oocyte, which then defines the future posterior pole
of the embryo [2 4]. New studies [5] of the function of the sole
Drosophila member of the ezrin rodexin moesin (ERM) protein
family, Dmoesin, have now shown that an actin-based cortical
anchor is required to maintain this polarized distribution of
oskar mRNA.
3) The large cells of the Drosophila egg chamber make it an
excellent system for identifying genes involved in the generation
of cell polarity. Early on during oogenesis, a microtubule
organizing center (MTOC) forms within the oocyte, which directs
the transport of new synthesized materials along microtubules
from the nurse cells into the developing oocyte. Then, at stages
7 8, the oocyte microtubule cytoskeleton undergoes a dramatic re-
organization in response to a signal from the overlying posterior
follicle cells. The MTOC is disassembled and microtubules become
nucleated at the anterior cortex of the oocyte, generating a
gradient of microtubules with their "plus" ends tightly focused
at the posterior pole. As microtubule inhibitors and mutations
which disrupt microtubule organization or microtubule motor
function block the proper localization of polar markers such as
oskar mRNA, microtubules are thought to play a critical role in
the establishment of oocyte polarity.
References (abridged):
1. Mata J. and Nurse P. (1998) Discovering the poles in yeast.
Trends Cell Biol., 8:163-167
2. Kim-Ha J., Smith J.L. and Macdonald P.M. (1991) oskar mRNA is
localised to the posterior pole of the Drosophila oocyte. Cell,
66:23-35
3. Ephrussi A., Dickinson L.K. and Lehmann R. (1991) Oskar
organizes the germ plasm and directs localization of the
posterior determinant nanos. Cell, 66:37-50
4. Riechmann V. and Ephrussi A. (2001) Axis formation during
Drosophila oogenesis. Curr. Opin. Genet. Dev., 11:374-383
5. Polesello C., Delon I., Valenti P., Ferrer P. and Payre F.
(2002) Dmoesin controls actin-based cell shape and polarity
during Drosophila melanogaster oogenesis. Nat. Cell Biol., 4:782-
789
Current Biology 2002 12:R835
Related Background:
INHIBITION OF XENOPUS OOCYTE MEIOTIC MATURATION BY CATALYTICALLY
INACTIVE PROTEIN KINASE A
A Schmitt and AR Nebreda (European Molecular Biology Laboratory
Heidelberg, DE) discuss oocyte maturation, the authors making the
following points:
1) Oocytes from Xenopus laevis are arrested at the first meiotic
prophase and can be induced to develop into fertilizable eggs by
the steroid hormone progesterone in a process called meiotic
maturation. Progesterone triggers various signal transduction
pathways in the oocyte (1,2), which lead to the activation of
Cdc2/cyclin B and entry into M phase of meiosis.
2) The cAMP-dependent protein kinase (PKA) plays a crucial role
in meiotic maturation. Inactive PKA is a tetrameric holoenzyme
composed of two functionally distinct subunits, a dimeric
regulatory subunit (PKA-R) and two monomeric catalytic subunits
(PKAc) (3). Four PKA-R subunits (RI, RI, RII, and RII) and three
PKAc subunits have been identified in mammals as distinct gene
products (4,5). The PKA-R dimer acts in part as a pseudosubstrate
to inhibit the phosphotransferase activity of the PKAc subunit
and can bind four cAMP molecules in a cooperative manner,
resulting in the release of active PKAc monomers. A second class
of physiological PKA inhibitors is the heat-stable protein kinase
inhibitors PKIs, which bind with high specificity and affinity to
PKA and PKA, whereas PKA is insensitive to inhibition by PKI.
3) It was reported 25 years ago that injection of PKAc purified
from rabbit skeletal muscle blocks progesterone-induced oocyte
maturation, whereas injection of purified PKA-R type II or PKI
induced meiotic maturation in the absence of progesterone. These
results suggested that PKA activity was necessary and sufficient
to maintain the meiotic G2 block of oocytes. Consistent with this
idea, a transient (and modest) decrease in the level of the
second messenger cAMP was measured within minutes after
progesterone stimulation, which correlated with a 50% reduction
in the membrane-bound adenylate cyclase activity. Inhibitors of
phosphodiesterases, such as 3-isobutyl-1-methylxanthine, that
increase intracellular cAMP levels also blocked progesterone-
induced meiotic maturation.
4) In summary: Progesterone induces G2-arrested Xenopus oocytes
to develop into fertilizable eggs in a process called meiotic
maturation. Protein kinase A (PKA), the cAMP-dependent protein
kinase, has long been known to be a potent inhibitor of meiotic
maturation, but little information is available on how PKA
functions. The authors report they have cloned two Xenopus PKA
catalytic subunit isoforms, XPKA and XPKA. These proteins are 89%
identical and both inhibit progesterone-induced meiotic
maturation when overexpressed at low levels, suggesting that PKA
activity is tightly regulated in the oocyte. Unexpectedly,
catalytically inactive XPKA mutants are able to block
progesterone-induced maturation as efficiently as the wild-type
active XPKA. These mutants also block meiotic maturation induced
by Mos, but are less efficient at inhibiting Cdc25C-induced
maturation. The authors suggest their results indicate that PKA
can inhibit meiotic maturation by a novel mechanism which does
not require its kinase activity and is also independent of
binding to the PKA regulatory subunits.
References (abridged):
1. Ferrell, J. E., Jr. (1999) BioEssays 21, 833-842
2. Nebreda, A. R. & Ferby, I. (2000) Curr. Opin. Cell Biol. 12,
666-675
3. Taylor, S. S. , Buechler, J. A. & Yonemoto, W. (1990) Annu.
Rev. Biochem. 59, 971-1005
4. Beebe, S. J. (1994) Semin. Cancer Biol. 5, 285-294
5. Tasken, K. , Skalhegg, B. S. , Tasken, K. A. , Solberg, R. ,
Knutsen, H. K. , Levy, F. O. , Sandberg, M. , Orstavik, S. ,
Larsen, T. , Johansen, A. K. , et al. (1997) Adv. Second
Messenger Phosphoprotein Res. 31, 191-204
Proc. Nat. Acad. Sci. 2002 99:4361
Related Background:
ON GERM CELL MIGRATION
H.-Arno J. Mueller (Heinrich-Heine University, DE) discusses germ
cell migration, the author making the following points:
1) Fertilized eggs of metazoan organisms undergo a series of
rapid cleavage divisions, which give rise to two distinct cell
types: the somatic cells and the germ cells [1] . During
morphogenesis of the gonad, the germ cells migrate through the
interior of the embryo to the somatic gonadal tissues. Germ cells
have to overcome many hurdles during their migration: they need
to cross an epithelial monolayer; they migrate directionally
between tissues; and they have to recognize and firmly attach to
target tissues. Systematic genetic screens in Drosophila are
providing insights into the pathways that control the directional
migration of germ cells [2,3] . An involved novel gene, called
"slow as molasses" (slam), has now been identified which is
exceptional in that this gene is active before germ cell
migration -- during the establishment of the somatic cells [4,5].
2) In Drosophila, fertilization is followed by 13 rapid mitotic
cycles without cytokinesis; most of the resulting nuclei migrate
to the cortical cytoplasm by cycle 9. The germ cells are the
first cells to be formed. During cycle 9, the germ cells pinch
off in a particular posterior cortical region of the egg
containing the germ plasm. At interphase of cycle 14, cytokinesis
of the somatic cells proceeds in a process called
"cellularization". The germ cells are attached to a posterior
region of the blastoderm epithelium, the part that will give rise
to the distal tip of the posterior midgut primordium. When the
midgut primordium has invaginated during gastrulation, the germ
cells begin their migration [1] . The germ cells first migrate
through the posterior midgut epithelium to its basal side, where
they move along the basal surface of the midgut into the lateral
mesoderm. In the mesoderm, the germ cells split up into two
bilateral groups and then associate with the gonadal mesoderm.
Later the gonadal mesoderm and the germ cells coalesce to
establish the embryonic gonad.
3) What tells the germ cells when to start moving, where to move
and when to stop? The analysis of three genes provided strong
evidence that the directional migration of the germ cells is
governed by attractive and repulsive cues.
References (abridged):
1. Starz-Gaiano M. and Lehmann R. (2001) Moving towards the next
generation. Mech. Dev., 105:5-18.
2. Brohier H.T., Moore L.A., vanDoren M., Newman S. and Lehmann
R. (1998) zfh-1 is required for germ cell migration and gonadal
mesoderm development in Drosophila Development, 125:655-666.
3. Moore L.A., Brohier H.T., vanDoren M., Lunsford L.B. and
Lehmann R. (1998) Identification of genes controlling germ cell
migration and embryonic gonad formation in Drosophila
Development, 125:667-678.
4. Stein J.A., Brohier H.T., Moore L.A. and Lehmann R. (2002)
Slow as molasses is required for polarized membrane growth and
germ cell migration in Drosophila Development, 129:3925-3934.
5. Lecuit T., Samanta R. and Wieschaus E. (2002) slam encodes a
developmental regulator of polarized membrane growth during
cleavage of the Drosophila embryo. Dev. Cell, 2:425-436.
Current Biology 2002 12:R612
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3. SPERMATOGENESIS
PREVIOUSLY UNCHARACTERIZED HISTONE ACETYLTRANSFERASES IMPLICATED
IN MAMMALIAN SPERMATOGENESIS
BT Lahn et al (University of Chicago, US) discuss
spermatogenesis, the authors making the following points:
1) The acetylation of N-terminal lysine residues in core histones
has been implicated in three distinct cellular processes. The
first is the deposition of free histones onto newly synthesized
DNA (1). The second is the regulation of gene expression
(reviewed in ref. 2). The third is the displacement of histones
by transition proteins and protamines during vertebrate
spermatogenesis (3-5). Various histone acetyltransferase (HAT)
enzymes involved in the first two processes have been
characterized. HATs involved in the third process have yet to be
identified.
2) In mammals and many other vertebrates, dramatic chromatin
remodeling occurs during spermiogenesis, whereby histones are
displaced from chromatin, first by transition proteins and later
by protamines. With protamines, DNA in mature spermatozoa is
packaged into an extremely condensed, functionally inert
configuration. Several lines of evidence led to the view that
chromatin remodeling during spermiogenesis is facilitated by
hyperacetylation of histone H4. First, studies of numerous
vertebrate species have shown that H4 hyperacetylation correlates
with the occurrence of the histone-to-protamine transition in
spermatogenesis. Extensive H4 hyperacetylation occurs in the
testes of species where histones are replaced by protamines
during spermiogenesis (3-5), but not in species that completely
retain somatic-type histones in mature spermatozoa (4). Second,
the timing of H4 hyperacetylation is consistent with its role in
promoting histone displacement. Studies in the rat, for instance,
demonstrated that H4 hyperacetylation during spermiogenesis
immediately precedes the histone-to-protamine transition (5).
Finally, the postulated role of histone hyperacetylation in
facilitating histone displacement is consistent with the general
perception that histone acetylation results in a more open
chromatin structure and hence easier access by regulatory
proteins. Indeed, it has been observed in the trout that H4
hyperacetylation during spermiogenesis results in a highly
relaxed chromatin configuration (4). An important inroad into
understanding the regulation of chromatin remodeling during
vertebrate spermatogenesis will be the identification of enzymes
responsible for H4 hyperacetylation during the remodeling
process.
3) In summary: During spermiogenesis (the maturation of
spermatids into spermatozoa) in many vertebrate species,
protamines replace histones to become the primary DNA-packaging
protein. It has long been thought that this process is
facilitated by the hyperacetylation of histone H4. However, the
responsible histone acetyltransferase enzymes are yet to be
identified. CDY is a human Y-chromosomal gene family expressed
exclusively in the testis and implicated in male infertility. Its
mouse homolog Cdyl, which is autosomal, is expressed abundantly
in the testis. Proteins encoded by CDY and its homologs bear the
"chromodomain", a motif implicated in chromatin binding. The
authors demonstrate that (i) human CDY and mouse CDYL proteins
exhibit histone acetyltransferase activity in vitro, with a
strong preference for histone H4; (ii) expression of human CDY
and mouse Cdyl genes during spermatogenesis correlates with the
occurrence of H4 hyperacetylation; and (iii) CDY and CDYL
proteins are localized to the nuclei of maturing spermatids where
H4 hyperacetylation takes place. Taken together, these data link
human CDY and mouse CDYL to the histone-to-protamine transition
in mammalian spermiogenesis. The authors suggest this link offers
a plausible mechanism to account for spermatogenic failure in
patients bearing deletions of the CDY genes.
References (abridged):
1. Parthun, M. R. , Widom, J. & Gottschling, D. E. (1996) Cell
87, 85-94
2. Jenuwein, T. & Allis, C. D. (2001) Science 293, 1074-1080
3. Oliva, R. & Mezquita, C. (1982) Nucleic Acids Res. 10, 8049-
8059
4. Christensen, M. E. , Rattner, J. B. & Dixon, G. H. (1984)
Nucleic Acids Res. 12, 4575-4592
5. Grimes, S. R., Jr. & Henderson, N. (1984) Exp. Cell Res. 152,
91-97
Proc. Nat. Acad. Sci. 2002 99:8707
Related Background:
LACK OF ACROSOME FORMATION IN MICE LACKING A GOLGI PROTEIN, GOPC
R Yao et al (Japanese Foundation for Cancer Research Cancer
Institute Tokyo, JP) discuss acrosome formation, the authors
making the following points:
1) The acrosome is a unique structure of the mature spermatozoon,
which plays an important role at the site of sperm-zona pellucida
binding during the fertilization process. The biogenesis of the
acrosome takes place during the initial phase of spermatid
development, when numerous proacrosomic granules are formed from
trans-Golgi stacks and accumulate in the concave region (medulla)
near the trans-Golgi stacks. The small granules fuse with each
other to form a single large acrosomic granule that associates
with the nuclear envelope (1). During the subsequent cap phase,
the acrosome increases its size and begins to spread over the
anterior nuclear pole. The nucleus changes its shape during the
subsequent acrosomic phase, which is followed by caudal migration
of mitochondria during the maturation phase. Although the
morphogenic change of the acrosome in spermatogenesis has been
well documented, its molecular basis is still largely unknown.
2) Globozoospermia (also called round-headed spermatozoa) is a
human infertility syndrome caused by spermatogenesis defects (2-
4). The most prominent feature of globozoospermia is the
malformation of the acrosome, and, in the most severe cases, the
acrosome is totally absent. Globozoospermia is also characterized
by abnormal nuclear shape as well as abnormal arrangement of the
mitochondria of the spermatozoon (5). Although globozoospermia is
thought to be an inherited disorder (2), the etiology of
globozoospermia is not known.
3) In summary: The authors demonstrate that Golgi-associated PDZ-
and coiled-coil motif-containing protein (GOPC), a recently
identified Golgi-associated protein, is predominantly localized
at the trans-Golgi region in round spermatids, and male mice in
which GOPC has been disrupted are infertile with globozoospermia.
The primary defect was the fragmentation of acrosomes in early
round spermatids, and abnormal vesicles that failed to fuse to
developing acrosomes were apparent. In later stages, nuclear
malformation and an abnormal arrangement of mitochondria, which
are also characteristic features of human globozoospermia, were
observed. Interestingly, intracytoplasmic sperm injection (ICSI)
of such malformed sperm into oocytes resulted in cleavage into
blastocysts only when injected oocytes were activated. Thus, GOPC
provides important clues to understanding the mechanisms
underlying spermatogenesis, and the GOPC-deficient mouse may be a
unique and valuable model for human globozoospermia.
References (abridged):
1. Abou, H. A. & Tulsiani, D. R. (2000) Arch. Biochem. Biophys.
379, 173-182
2. Kullander, S. & Rausing, A. (1975) Int. J. Fertil. 20, 33-40
3. Lalonde, L. , Langlais, J. , Antaki, P. , Chapdelaine, A. ,
Roberts, K. D. & Bleau, G. (1988) Fertil. Steril. 49, 316-321
4. Singh, G. (1992) Int. J. Fertil. 37, 99-102
5. Battaglia, D. E. , Koehler, J. K. , Klein, N. A. & Tucker, M.
J. (1997) Fertil. Steril. 68, 118-122
Proc. Nat. Acad. Sci. 2002 99:11211
Related Background:
UNIQUE CHROMATIN REMODELING AND TRANSCRIPTIONAL REGULATION IN
SPERMATOGENESIS
Paolo Sassone-Corsi (Institut de Gntique et de Biologie
Molculaire et Cellulaire, Strasbourg, FR) discusses chromatin
remodeling in spermatogenesis, the author making the following
points:
1) The developmental process of spermatogenesis relies on a
number of distinct regulatory programs involving sophisticated
hormonal control from the hypothalamic-pituitary axis (1). Unique
rules govern postmeiotic transcription in male germ cells. One
very special feature concerns the process of chromatin
remodeling, which involves various steps that are unlike those in
somatic cells (2). Many generally expressed genes use alternative
promoters in male germ cells, and several genes have a homolog
whose expression is specific for the male germ line. Transgenesis
experiments have revealed that various cis-acting regulatory
elements direct expression exclusively to the testis,
demonstrating the presence of germ cell-specific factors (2,3-5).
2) In somatic cells, specific chromatin remodeling events have
been directly coupled to transcriptional activation and
silencing. Are the same events operating in male germ cells?
During spermatogenesis, the haploid genome undergoes extensive
reorganization through meiosis and DNA compaction. Meiosis
involves homologous chromosome pairing at synapsis and meiotic
recombination. After desynapsis and completion of meiosis, gene
transcription increases, but then the haploid genome is compacted
within the sperm head to a volume of approximately 5% of that of
a somatic cell nucleus. This remarkable repackaging event is
achieved by replacing histones with protamines, arginine- and
cysteine-rich proteins that organize the haploid male genome into
a highly specialized, doughnut-shaped chromatinic structure that
is fundamentally different from the classical nucleosomal
architecture. The reason for the histone-protamine transition is
probably related to the high compaction potential of
nucleoprotamines and the requirement for a unique chromatin
architecture that would enable a specific transcription schedule
after fertilization.
3) In mammals, histones are not replaced directly by protamines.
Transition proteins (TP1 and TP2) are small, basic nuclear
proteins that appear when histones are displaced and chromatin
condensation initiates. Targeted mutation of each transition
protein suggests a redundant role for the transition proteins.
Both TP1- and TP2-mutant mice are fertile and display only minor
spermiogenesis abnormalities, indicating that histone replacement
and chromatin compaction are transition protein-independent
processes. Indeed, precocious chromatin condensation occurs in
transgenic mice with premature protamine-1 translation.
4) In summary: Most of our knowledge of transcriptional
regulation comes from studies in somatic cells. However,
increasing evidence reveals that gene regulation mechanisms are
different in haploid germ cells. A number of highly specialized
strategies operate during spermatogenesis. These include a unique
chromatin reorganization program and the use of distinct promoter
elements and specific transcription factors. Deciphering the
rules governing transcriptional control during spermatogenesis
will provide valuable insights of biomedical importance.
References (abridged):
1. W. F. Crowley, et al., Rec. Progr. Horm. Res. 47, 27
2. K. C. Kleene, Mech. Dev. 106, 3 (2001)
3. P. Sassone-Corsi, Cell 88, 163 (1997)
4. E. M. Eddy, et al., Curr. Top. Dev. Biol. 37, 140 (1998)
5. N. B. Hecht, Bioessays 20, 555 (1998)
Science 2002 296:2176
Related Background:
CARBOHYDRATE RECOGNITION IN SPERMATOGENESIS
Takashi Muramatsu (Nagoya University, JP) discusses
spermatogenesis, the author making the following points:
1) Carbohydrates linked to proteins or lipids in the plasma
membrane are involved in cellular processes as varied as
embryonic development and the recruitment of white blood cells to
sites of inflammation (1-3). Akama et al.(4) have identified a
unique carbohydrate, required for spermatogenesis in the mouse,
that enables male germ cells to adhere to Sertoli cells in the
testis.
2) Chains of sugar molecules (oligosaccharides) linked to
proteins through an asparagine residue are described as N-linked.
To form complex N-linked carbohydrate, a long sugar chain is
transferred to the protein, is "trimmed" (1,5) and then further
embellished by the addition of sugars, such as N-
acetylglucosamine (GlcNAc), galactose (Gal), fucose, and sialic
acid. The enzyme a-mannosidase II is important for the final
processing step in the formation of complex carbohydrate. When
mice are engineered to lack the gene encoding a-mannosidase II,
their red blood cells (but not other cells and tissues) lose the
ability to make complex carbohydrate. The mice also develop
dyserythropoietic anemia, which resembles human congenital
dyserythropoietic anemia type II. The other cells and tissues of
the mutant mouse retain their ability to make complex
carbohydrate, probably because they produce an alternative
enzyme, a-mannosidase IIx.
3) According to Akama et al. (4), male mice lacking the a-
mannosidase IIx gene, which is predominantly expressed by male
germ cells, exhibit almost complete suppression of fertility.
This result complements the finding that the chemical
swainsonine, which inhibits both a-mannosidase II and IIx, causes
male sterility. By staining with labeled lectin, Akima et al
discovered that a sugar structure present on male wild-type germ
cells was almost undetectable on the germ cells of mice lacking
a-mannosidase IIx.
4) Germ cells must adhere to Sertoli cells to survive, and the
Akama et al. work has now identified the key molecule involved in
binding. Their finding opens up new avenues that may benefit
research into the cause of human male sterility. The Akama et al.
study illustrates the importance of cell-specific or even
protein-specific glycosylation machinery for cellular
recognition. An interesting subject for future research will be
identification of the germ cell target glycoprotein to which the
new oligosaccharide is attached.
References (abridged):
1. A. Varki et al., Eds., Essentials of Glycobiology (Cold Spring
Harbor Laboratory Press, Cold Spring Harbor, NY, 1999)
2. J. Alper, Science 291, 2338 (2001)
3. T. Muramatsu, J. Biochem. 127, 171 (2000) [Medline]
4. T. O. Akama et al., Science 295, 124 (2002)
5. A. Helenius, M. Aebi, Science 291, 2364 (2001)
Science 2002 295:53
ScienceWeek http://www.scienceweek.com
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4. FERTILIZATION
ON THE VULNERABILITY OF THE HUMAN SPERMATOZOON
J. Aitken and J.A. Graves (University of Newcastle, UK) discuss
the human spermatozoon, the authors making the following points:
1) The impact of environmental toxicants and the innate
inadequacy of human spermatozoa are compounded by the advent of
effective contraception and the introduction of assisted-
conception technologies. This lifting of the selection pressure
on fertility means that those endowed with genes for high
fecundity have lost their advantage over those without. As a
result, future generations are bound to experience a further
decline in semen quality and, ultimately, human fertility.
2) The authors consider the mechanisms responsible for the poor
fertilizing potential and genetic damage shown by human
spermatozoa. Two main causes of germ-cell dysfunction have
recently been discovered: gene deletions on the long arm of the
male sex-determining Y chromosome, and oxidative stress. The
authors suggest these etiologies may be associated. The Y
chromosome is particularly vulnerable to gene deletions because
it is not a matching partner for the X chromosome, so it cannot
retrieve lost genetic information by homologous recombination.
Over the past 300 million years, the mammalian Y chromosome has
been reduced from a pairing partner to the X chromosome to a
shadow of its former self, rescued only by a large addition from
a non-sex-determining chromosome in "placental" mammals. Many of
the remaining genes have acquired functions essential for sex
determination and spermatogenesis.
3) The original Y chromosome contained approximately 1500 genes,
but during the ensuing 300 million years all but about 50 were
inactivated or lost. Overall, this gives an inactivation rate of
five genes per million years. The presence of many genes that
have lost their function (pseudogenes) on the Y chromosome
indicates that this process of attrition is continuing, so that
even these key genes will be lost. At the present rate of decay,
the Y chromosome will self-destruct in approximately 10 million
years. This has already occurred in the mole vole, in which the Y
chromosome (together with all of its genes) has been completely
lost from the genome.
References (abridged):
1. Aitken, R. J. J. Reprod. Fertil. 115, 1 7 (1999).
2. Marshall Graves, J. A. Biol. Reprod. 63, 667 676 (2000).
3. Just, W. et al. Nature Genet. 11, 117 118 (1995).
4. Kuroda-Kawaguchi, T. et al. Nature Genet. 29, 279 286 (2001).
5. Kamp, C. et al. Mol. Hum. Reprod. 7, 987 994 (2001).
Nature 2002 415:963
Related Background:
PENETRATION, ADHESION, AND FUSION IN MAMMALIAN SPERM-EGG
INTERACTION
P Primakoff and DG Myles (University of California Davis, US)
discuss fertilization, the authors making the following points:
1) In mammals, fertilization is completed by the direct
interaction of sperm and egg, a process mediated primarily by
gamete surface proteins. Therefore, an essential task in the
study of sperm-egg interaction is an exploration of the
capabilities of a distinct set of surface proteins, some gamete
specific and others more widely expressed. On gametes, these
proteins act in a sequential pattern to orchestrate the close
approach and ultimate fusion of the two cells.
2) To penetrate the substantial cumulus cell barrier surrounding
ovulated eggs of most mammalian species, sperm use hyperactivated
motility (1) and a glycosylphosphatidylinositol (GPI) anchored
surface hyaluronidase, named PH-20 (2). The motility and surface
hyaluronidase are necessary, and perhaps sufficient, to digest a
path through the extracellular matrix of the cumulus cells; no
proteases have yet been implicated in this process.
3) The egg's zona pellucida is a cell type-specific extracellular
matrix or coat composed of three glycoproteins termed ZP1, ZP2,
and ZP3. Sperm that reach and bind to the zona pellucida receive
a signal to acrosome-react, i.e., release by exocytosis the
contents of their large secretory granule, the acrosome.
4) The currently favored model is that sperm bind to O-linked
carbohydrate on ZP3. Sperm preincubation with ZP3 strongly
inhibits sperm binding to the zona, whereas preincubation with
ZP1 or ZP2 has no effect (3). Other studies show that sperm
binding can be blocked by O-linked oligosaccharides of ZP3,
present on Ser332 and Ser334 near the ZP3 COOH-terminus (4, 5).
Thus, sperm adhesion to the zona is a carbohydrate-mediated
event.
5) In summary: Fertilization is the sum of the cellular
mechanisms that pass the genome from one generation to the next
and initiate development of a new organism. A typical, ovulated
mammalian egg is enclosed by two layers: an outer layer of
approximately 5000 cumulus cells and an inner, thick
extracellular matrix, the zona pellucida. To reach the egg plasma
membrane, sperm must penetrate both layers in steps requiring
sperm motility, sperm surface enzymes, and probably sperm-
secreted enzymes. Sperm also bind transiently to the egg zona
pellucida and the egg plasma membrane and then fuse. Signaling in
the sperm is induced by sperm adhesion to the zona pellucida, and
signaling in the egg by gamete fusion. The gamete molecules and
molecular interactions with essential roles in these events are
gradually being discovered.
References (abridged):
1. R. Yanagimachi, in The Physiology of Reproduction, E. Knobil,
J. D. Neill, Eds. (Raven, New York, 1994), pp. 152-162
2. Y. Lin, et al., J. Cell Biol. 125, 1157 (1994)
3. J. D. Bleil, et al., Cell 20, 873 (1980)
4. J. Chen, et al., Proc. Natl. Acad. Sci. U.S.A. 95, 6193 (1998)
5. R. A. Kinloch, et al., Proc. Natl. Acad. Sci. U.S.A. 92, 263
(1995)
Science 2002 296:2183
Related Background:
Related Background:
ON SPERM COOPERATION IN THE WOOD MOUSE
RV Short (University of Melbourne, AU) discusses sperm
cooperation, the author making the following points:
1) After animals have mated, sperm in the female reproductive
tract race to produce fertilization of an egg. Because sperm
express little of their genetic make-up in their outward
appearance, it is difficult to select the good from the bad. So
it might be better for an individual's sperm to cooperate rather
than compete with one another. For species in which females mate
with multiple partners, this will be particularly true if the
sperm of one male could unite to defeat those of its rivals.
2) Moore et al (2002) describe an amazing example of such
altruistic behavior in the sperm of the common European wood
mouse, Apodemus sylvaticus (Nature 418, 174 177; 2002). They find
that hundreds or thousands of sperm link hooked structures on
their heads and swim en masse in a train, which enables them to
progress at almost twice the speed of a single sperm. These
trains must break up before fertilization, so many of the
component sperm commit genetic hara-kiri by undergoing a
premature "acrosome reaction". This involves the release of
enzymes that break down cell adhesion molecules, which also makes
it impossible for the sperm concerned to fertilize the egg.
Somewhere on the train -- perhaps the locomotive driver up front
-- there must be one acrosome-intact sperm that has retained its
capacity to perform fertilization.
3) Sperm motility is ultimately driven by the engine of
mitochondrial DNA in the sperm's midpiece. M Anderson and A
Dixson (Nature 416, 496; 2002) have shown that in primates the
volume of the sperm midpiece is highly correlated with relative
testicular size and mating behavior, the most sexually athletic
species having the largest mitochondrial midpieces to power their
sperm. So we can look forward to further work to see whether the
wood mice have vast mitochondrial midpieces to power their sperm
trains, and whether the sperm of especially promiscuous primates
such as chimpanzees would leave their human counterparts for dead
in the Olympic swimming pool.
Nature 2002 418:137
Related Background:
EXCEPTIONAL SPERM COOPERATION IN THE WOOD MOUSE
In this context, the term "Hamilton's rule" refers to a rule put
forth by WD Hamilton (1964) that in general states that a social
act is favored by natural selection if it increases the inclusive
fitness of the performer. Hamilton's theory is often referred to
as "kin selection".
The wood mouse, A. sylvaticus, is a murid rodent common
throughout Western Europe, with a breeding season from February
to October.
H. Moore et al (University of Sheffield, UK) discuss sperm
cooperation, the authors making the following points:
1) Spermatozoa from a single male will compete for fertilization
of ova with spermatozoa from another male when present in the
female reproductive tract at the same time(1). Among small
mammals, multiple matings resulting in sperm competition and
mixed paternity in littermates are believed to be widespread(1).
Close genetic relatedness predisposes individuals towards
altruism, and as haploid germ cells of an ejaculate will have
genotypic similarity of 50%, it is predicted that spermatozoa may
display cooperation and altruism to gain an advantage when inter-
male sperm competition is intense(2).
2) Several examples of sperm cooperation have been reported
mainly in molluscs and insects(3,4). A possible exception in
Mammalia is the spermatozoa of opossums that conjugate to form
pairs during sperm maturation and disengage immediately before
fertilization(5). Sperm will benefit from cooperation if
"Hamilton's rule" is fulfilled. This depends on the probability
of sperm survival in terms of reaching the site of fertilization
and the difference in relatedness of cooperating sperm and other
sperm competing for fertilization. For true altruism, the
fertilizing capacity of one spermatozoon is compromised or
sacrificed to benefit another; however, evidence in Eutheria has
been largely lacking. Spermatozoa of some rodents (for example,
guinea-pig) stack in rouleaux formation or agglutinate, but these
cell associations do not appear particularly advantageous.
Conversely, it has been suggested that a primary function of some
spermatozoa in the rat and human ejaculate is to incapacitate
spermatozoa of another male, so-called kamikaze spermatozoa;
however, this hypothesis is not supported by experimental
evidence.
3) In summary: The authors report the probable altruistic
behavior of spermatozoa in an eutherian mammal. Spermatozoa of
the common wood mouse, Apodemus sylvaticus, displayed a unique
morphological transformation resulting in cooperation in
distinctive aggregations or "trains" of hundreds or thousands of
cells, which significantly increased sperm progressive motility.
Eventual dispersal of sperm trains was associated with most of
the spermatozoa undergoing a premature acrosome reaction. The
authors propose that cells undergoing an acrosome reaction in
aggregations remote from the egg are altruistic in that they help
sperm transport to the egg but compromise their own fertilizing
ability.
References (abridged):
1. Birkhead, T. R. & Moller, A. P. Sperm Competition and Sexual
Selection (Academic, London, 1996)
2. Trivers, R. Social Evolution (Benjamin Cummings, California,
1985)
3. Sivinski, J. in Sperm Competition and the Evolution of Animal
Mating Systems (ed. Smith, R. L.) 223-249 (Academic, Orlando,
1984)
4. Hayashi, F. Insemination through an externally attached
spermatophore: bundled sperm and post-copulatory mate guarding by
male fishflies (Megaloptera: corydalidae). J. Insect Physiol. 42,
859-866 (1996)
5. Moore, H. D. Gamete biology of the new world marsupial, the
grey short-tailed opossum, Monodelphis domestica. Reprod. Fertil.
Dev. 8, 605-615 (1996)
Nature 2002 418:174
Related Background:
SPERM VIABILITY AND SPERM COMPETITION IN INSECTS
FM Hunter and TR Birkhead (University of Sheffield, UK) discuss
sperm competititon, the authors making the following points:
1) Sperm quality plays an important role in vertebrates in
determining which male has the advantage when two or more males
compete to fertilize a female's ova [1,2]. In insects, however,
the importance of sperm quality has never been considered,
despite sperm competition being widespread and well studied in
this group [3,4].
2) The authors report they tested the hypothesis that sperm
viability, measured as the proportion of live sperm, covaried
with the intensity of sperm competition in insects. In a pairwise
comparison of seven closely related species pairs, each
comprising a monandrous and a polyandrous species (i.e., with and
without sperm competition, respectively), the authors found that
in all cases the polyandrous species had a higher proportion of
live sperm in their sperm stores. The distribution of the
percentage of live sperm showed considerable inter- and
intraspecific variation, suggesting that, all else being equal,
males will vary in their ability to fertilize ova on the basis of
sperm viability alone. The authors suggest their results indicate
that sperm viability is one of a suite of male adaptations to
sperm competition in insects.
3) The authors suggest their results also indicate that
considerable intraspecific variation exists in the proportion of
dead sperm. To evaluate this properly, one should establish
whether individual males show consistent differences in ejaculate
viability. Such information is potentially available for a number
of vertebrates where it is possible to obtain replicate semen
samples from males, and it is already well established that
consistent differences in fertilizing capacity exist among males.
The consistency of sperm viability in individual male insects
remains to be tested.[5]
References (abridged):
1. Dziuk P.J. (1996) Factors that influence the proportion of
offspring sired by a male following heterospermic insemination.
Anim. Reprod. Sci., 43:65-88
2. Birkhead T.R., Martinez J.G., Burke T. and Froman D.P. (1999)
Sperm mobility determines the outcome of sperm competition in the
domestic fowl. Proc. R Soc. Lond. B Biol. Sci., 266:1759-1764
3. Parker G.A. (1970) Sperm competition and its evolutionary
consequences in the insects. Biol. Rev., 45:525-567
4. Simmons L.W. and Siva-Jothy M.T. (1998) Sperm competition in
insects: mechanisms and the potential for selection. In: Birkhead
T.R. and Mller A.P. (Eds.) Sperm Competition and Sexual
Selection. London: Academic Press
5. Wishart G.J. and Palmer F.H. (1986) Correlation of the
fertilizing ability of semen from individual male fowls with
sperm motility and ATP content. Br. Poult. Sci., 27:97-102
Current Biology 2002 12:121
Related Background:
A CHEMOATTRACTANT FOR ASCIDIAN SPERMATOZOA IS A SULFATED STEROID
M. Yoshida et al (University of Tokyo, JP) discuss
chemoattractants for sperm, the authors making the following
points:
1) Chemotactic behavior is an important communication system
among cells. Chemotaxis of spermatozoa toward eggs during
fertilization is known in most animals and lower plants (1,2).
The chemical nature of the sperm attractants has been determined
in the bracken fern to be a bimalate ion (3) and in brown algae
to be unsaturated hydrocarbons (4-5). In animal species also,
some candidates of sperm attractants have been reported, and the
chemical structures of the sperm chemoattractants in three
species, the sea urchin Arbacia punctulata, the coral Montipora
digitata, and Xenopus laevis, have been identified. Despite much
effort having been devoted to clarification of the mechanism
underlying the chemotaxis, the absence of reliable bioassay
methods has hampered the quantitative evaluation of sperm
chemotaxis.
2) Spermatozoa of the ascidian Ciona intestinalis are either
immotile or only slightly motile when suspended in seawater.
However, when an unfertilized egg is set in the sperm suspension,
the spermatozoa near the egg are intensely activated and begin to
show chemotactic behavior toward the egg. The authors showed in a
previous study that the eggs probably release a sperm-activating
and -attracting factor (SAAF) from their vegetal pole. SAAF
induces entry of extracellular Ca2+ and an increase in
intracellular cAMP in the sperm, which induces protein kinase A-
dependent phosphorylation of 21- and 26-kDa axonemal proteins and
activation of sperm motility. On the other hand, the chemotactic
behavior of the ascidian sperm also requires extracellular Ca2+,
but theophylline-activated sperm, in which the drug induces
increase in the [cAMP]i by virtue of being a phosphodiesterase
inhibitor, show similar chemotactic behavior to that of normal
sperm. Therefore, changes in the [cAMP]i probably are not
required for sperm chemotaxis, and sperm chemotaxis is probably
induced by a mechanism different from that inducing sperm
activation, although SAAF induces both phenomena.
3) In summary: Sperm chemotaxis toward eggs before fertilization
has been demonstrated in many animals and plants, and several
peptides and small organic compounds acting as chemoattractants
have been identified. The authors previously showed that sperm of
the ascidians Ciona intestinalis and Ciona savignyi are activated
and then attracted toward the egg by a common factor released
from the egg. In this study, the authors purified sperm-
activating and -attracting factor (SAAF) from the egg-
conditioning medium of C. intestinalis by using several steps of
column chromatography. Determination of the molecular structure
by NMR and MS/MS analysis revealed that SAAF is a previously
uncharacterized sulfated steroid: 3,4,7,26-
tetrahydroxycholestane-3,26-disulfate. Furthermore, it was shown
that the SAAF of C. savignyi was indistinguishable from that of
C. intestinalis in terms of the chromatographic behavior and
molecular weight, indicating that the same compound might be
responsible for sperm activation and chemotaxis in both the
species. Furthermore, the authors established a method for
quantitative analysis of sperm chemotaxis and showed that the
chemotactic behavior of Ciona sperm is controlled by the
"chemotactic turn" associated with decrease in the concentration
of SAAF.
References (abridged):
1. Miller, R. L. (1985) in Biology of Fertilization, eds. Metz,
C. B. & Monroy, A. (Academic, New York), Vol. 2, pp. 275-337
2. Cosson, M. P. (1990) in Controls of Sperm Motility: Biological
and Clinical Aspects, ed. Gagnon, C. (CRC, Boca Raton, FL), pp.
104-135
3. Brokaw, C. J. (1958) J. Exp. Biol. 35, 192-196
4. Mller, D. G. , Jaenicke, L. , Donike, M. & Akintobi, T.
(1971) Science 171, 815-817
5. Maier, I. & Mller, D. G. (1986) Biol. Bull. (Woods Hole,
Mass.) 170, 145-175
Proc. Nat. Acad. Sci. 2002 99:14831
Related Background:
TOPPING OFF: A MECHANISM OF FIRST-MALE SPERM PRECEDENCE IN A
VERTEBRATE
AG Jones et al (Oregon State University, US) discuss sperm
precedence in fertilization, the authors making the following
points:
1) Sexual selection is an important facet of the evolutionary
process. Although many of the key aspects of sexual selection
occur before mating, research over the last three decades (1-3)
has led to the understanding that competition among sperm within
a female's reproductive tract can be vital to the ultimate
outcome of mating competition. A central goal of sperm
competition research in recent years has been to quantify the
proportion of offspring sired by the second of two males mated
sequentially to a female, a value known as P2 (4). Unfortunately,
such an approach to the problem often says very little about the
actual mechanism of sperm competition, because multiple distinct
mechanisms can lead to the same value of P2 (5).
2) The authors describe a scheme of sampling and analysis,
involving the temporal sampling of eggs as they are laid, by
which a molecular study of P2 can lead to additional insights
regarding the mechanisms of sperm dynamics within the female. The
focal organism for this study was the rough-skinned newt, Taricha
granulosa. This species provides a useful model for studies of
sperm competition for several reasons. First, females receive
sperm during a short receptive period at the beginning of the
reproductive season. Females then lay eggs singly over the course
of several weeks to months, fertilizing them with stored sperm.
Second, females lay large numbers of eggs, a characteristic that
permits a description of the change in P2 over time. Such data
are relevant to the pattern of sperm stratification within a
female's spermathecae. Third, the transfer of sperm in newts is
indirect. Before insemination, the male must unclasp the female
to deposit a spermatophore on the substrate in front of her. The
unrestrained female then has the option of either picking up the
spermatophore (using her cloaca) or moving away and ending
courtship. This indirect transfer of sperm, which is typical of
many urodeles, gives female newts greater control over sperm
acquisition than females of other vertebrate taxa with internal
fertilization, and may be a major factor in the evolution of
sperm usage patterns.
3) In summary: Competition among the sperm of rival males is an
important evolutionary phenomenon in many organisms. Yet, despite
extensive research on sperm competition in some vertebrate taxa,
very little progress has been made on this topic in amphibians.
Urodele amphibians (newts and salamanders) are of particular
interest to theories of sperm competition because most urodele
females -- in contrast to other vertebrate females -- control the
transfer of sperm from the male. The authors present a molecular
study of sperm precedence and storage patterns in the rough-
skinned newt (Taricha granulosa). First, the authors used
microsatellite markers to show that female newts typically use
sperm from 1-3 males under natural and seminatural conditions.
Second, the authors mated experimental females sequentially to
two males and collected fertilized eggs in a temporal series.
Patterns of paternity were consistent with first-male sperm
precedence and complete mixing of sperm within the female. This
simple pattern of sperm usage, best described as "topping off,"
is consistent with the expectation from sexual conflict theory
that free female choice before insemination eliminates selective
pressures for the evolution of complex patterns of paternity
manipulation involving cryptic female choice.
References (abridged):
1. Parker, G. A. (1970) Biol. Rev. 45, 525-567
2. Birkhead, T. R. & Mller, A. P. (1992) Sperm Competition in
Birds: Evolutionary Causes and Consequences (Academic, London)
3. Birkhead, T. R. & Mller, A. P. (1998) Sperm Competition and
Sexual Selection (Academic, London)
4. Boorman, E. & Parker, G. A. (1976) Ecol. Entomol. 1, 145-155
5. Simmons, L. W. & Siva-Jothy, M. T. (1998) in Sperm
Competition and Sexual Selection, eds. Birkhead, T. R. & Mller,
A. P. (Academic, London), pp. 341-434
Proc. Nat. Acad. Sci. 2002 99:2078
Related Background:
INDIVIDUAL ADJUSTMENT OF SPERM EXPENDITURE ACCORDS WITH SPERM
COMPETITION THEORY
A. Pilastro et al (University of Padova, IT) discuss sperm
competition, the authors making the following points:
1) In many animal taxa females mate multiply and sperm
competition is therefore an important evolutionary force in
sexually reproducing animals (1). The most common adaptation to
high levels of sperm competition in males is represented by an
increase in sperm expenditure at mating to increase their
probability of fertilizing the eggs (2,3). However, ejaculates
can be energetically costly to produce (4-5), and males are
expected to allocate sperm strategically in response to varying
levels of sperm competition (2). Specifically, theory predicts
that ejaculate expenditure should depend on the number of males
competing for fertilization. When the probability of competition
between a maximum of two ejaculates is low, male gametic
expenditure is predicted to increase with sperm competition risk
(the so-called "risk model"). Support for this prediction comes
from several within-species studies. However, in other instances,
such as the group-spawning fishes, several ejaculates compete
simultaneously for the same set of eggs. Under such
circumstances, theoretical models predict that an individual male
that faces variable levels of sperm competition among successive
spawns or matings should release fewer sperm as the estimated
number of competitors at a given spawning increases above two,
because returns are diminishing for providing more sperm (the so-
called "intensity model"). This counterintuitive prediction is
due to the fact that in spawns with several competitors the
chances of encountering an unfertilized egg are too low to favor
the release of additional sperm. In other words, if a male can
strategically allocate sperm among spawns, an increase in output
will profit more when the intensity of sperm competition is low.
2) In summary: Sperm competition theory predicts that males
should strategically allocate their sperm reserves according to
the level of sperm competition, defined as the probability that
the sperm of two males compete for fertilizing a given set of
ova. Substantial evidence from numerous animal taxa suggests that
at the individual level sperm expenditure increases when the risk
of sperm competition is greater. In contrast, according to the
"intensity model" of sperm competition (G. Parker et al: 1996
Proc. R. Soc. London Ser. B 263:1291), when more than two
ejaculates compete during a given mating event, sperm expenditure
should decrease as the number of competing males increases.
Empirical evidence supporting this prediction, however, is still
lacking. The authors report they measured sperm expenditure in
two gobiid fishes, the grass (Zosterisessor ophiocephalus) and
black goby (Gobius niger), in which up to six sneakers can
congregate around the nest of territorial males and release their
sperm when females spawn. The authors demonstrate that in
accordance with theory sneaker males of both species release
fewer sperm as the number of competitors increases.
References (abridged):
1. Birkhead, T. R. & Mller, A. P., eds. (1998) Sperm Competition
and Sexual Selection (Academic, London)
2. Parker, G. A. (1990) Proc. R. Soc. London Ser. B 242, 120-126
3. Parker, G. A. (1998) in Sperm Competition and Sexual
Selection, eds. Birkhead, T. R. & Mller, A. P. (Academic,
London), pp. 1-54
4. Dewsbury, D. A. (1982) Am. Nat. 119, 601-610
5. Nakatsuru, K. & Kramer, D. L. (1982) Science 216, 753-755
Proc. Nat. Acad. Sci. 2002 99:9913
Related Background:
BINDING OF RAINBOW TROUT SPERM TO EGG IS MEDIATED BY STRONG
CARBOHYDRATE-TO-CARBOHYDRATE INTERACTION BETWEEN (KDN)GM3
(DEAMINATED NEURAMINYL GANGLIOSIDE) AND GG3-LIKE EPITOPE
S Yu et al (University of Washington Seattle, US) discuss sperm-
egg binding, the authors making the following points:
1) Glycoconjugates are highly expressed at the cell surface,
displaying a morphologically distinctive form of plasma membrane
termed glycocalyx (1). Some glycans with extensive structural
variation, such as polysialic acid of neural cell adhesion
molecule, may be required for "coarse and fine tuning" of
adhesive behavior of cells (2). Numerous studies have suggested
that the glycocalyx may be involved in cell adhesion, although it
was only recently that endogenous lectins, particularly selectins
(3-5) and siglecs, were assigned as binding sites of defined
glycoconjugates to mediate cell-to-cell adhesion among blood
cells, endothelial cells, and immunocytes. Adhesion of cells in
tissues may be mediated by a group of endogenous lectins termed
galectins, although unambiguous evidence for this concept has not
been presented.
2) Prominent changes in glycosylation and glycosylation-dependent
cell adhesion are typically observed during ontogenic and
oncogenic processes. Compaction of morula, the first cell
adhesion event during embryogenesis, and its model process,
displayed as autoaggregation of teratocarcinoma, are mediated by
carbohydrate (chydr)-to-chydr interaction. Adhesion of mouse
melanoma B16 to mouse endothelial cells is mediated by GM3-to-
Gal4Glc1Cer (LacCer) interaction, and B16 metastasis to lung may
be mediated by GM3-to-Gg3 interaction.
3) A series of studies indicate that sponge (Microciona
prolifera) cell aggregation factor is a macromolecular
proteoglycan having repetitive glycan. Antibodies to the glycan
inhibited the aggregation, suggesting that it takes place between
glycans, although this may not completely exclude the possibility
of glycan-protein interaction. The fact that autoaggregation of
sponge cells is mediated by chydr-to-chydr interaction was
verified by determination of specific structures involved, i.e.,
pyruvated Gal4GlcNAc3Fuc, and recently by self-recognition of 3-
sulfated GlcNac3-L-Fuc1R. Specific interaction of various other
chydr with chydr was verified by quantitative determination using
surface plasmon resonance spectroscopy or molecular force
microscopy.
4) In summary: The authors report that KDN23Gal4Glc1Cer
[(KDN)GM3] is a major (90%) component of total gangliosides found
in sperm of rainbow trout (Oncorhynchus mykiss) and was shown to
be present prominently at the sperm head by immunochemical
staining with its specific mAb kdn3G. Liposomes containing
(KDN)GM3 adhere specifically to GalNAc4Gal4Glc1Cer (Gg3Cer)-
coated plastic plates. Interaction between (KDN)GM3 and Gg3Cer
was much stronger than that previously observed between
Neu5Ac23Gal4Glc1Cer and Gg3Cer. (KDN)GM3-Gg3Cer interaction did
not require the presence of Ca2+ and Mg2+, but was enhanced in
the presence of Mn2+. Fresh trout sperm adhered specifically to
Gg3Cer-coated plates under physiological conditions, and the
binding was inhibited by pretreatment of sperm with mAb kdn3G.
The presence of Gg3 or Gg3-related epitope structure in the
specific area surrounding the micropyle, through which sperm
enter the egg, was confirmed by immunogold labeling under
electron microscopy. The authors suggest these findings indicate
that initial sperm-egg adhesion during the process of
fertilization occurs when sperm adhere to the area surrounding
the micropyle through specific interaction between (KDN)GM3 on
the sperm head and Gg3 epitope (GalNAc4Gal1) expressed at a
defined region of the egg surface membrane.
References (abridged):
1. Varki, A. , Cummings, R. , Esko, J. , Freeze, H. , Hart, G. &
Marth, J. (1999) Essentials of Glycobiology (Cold Spring Harbor
Lab. Press, Plainview, NY)
2. Inoue, S. & Inoue, Y. (2001) J. Biol. Chem. 276, 31863-31870
3. Varki, A. (1994) Proc. Natl. Acad. Sci. USA 91, 7390-7397
4. Lowe, J. B. (1997) Kidney Int. 51, 1418-1426
5. Rosen, S. D. (1999) Am. J. Pathol. 155, 1013-1020
Proc. Nat. Acad. Sci. 2002 99:2854
Related Background:
ON MEMBRANE FUSION
L. Yang and H.W. Huang (Brookhaven National Laboratory, US)
discuss membrane fusion, the authors making the following points:
1) Membrane fusion takes place during many cellular processes,
including membrane traffic, fertilization, and infection by
enveloped viruses. Fusion allows the exchange of contents between
different membrane compartments. In order to maintain the
individuality of each of the intracellular compartments and of
the cell itself, membranes do not fuse easily under normal
circumstances. Thus, the process requires special proteins and is
subject to selective control. Understanding the fusion mechanism
is important not only for fundamental biology but also for
medical applications such as drug delivery and gene therapy.
2) Substantial progress has been made in the elucidation of the
structures of membrane fusion proteins (1,2) and in the
estimations of the free energies for the rearrangement of lipid
bilayers during fusion (3-5). Theoretical studies have identified
the free energy barriers that suggest a requirement for
mechanical work by fusion proteins, providing guidance for
identifying the functions of protein structures. However, how
proteins induce membrane fusion remains speculative, even in the
best-studied case of viral fusion proteins (1), because key
intermediate structures have not been observed.
3) Membrane fusion between phospholipid bilayers can be induced
by a variety of chemicals, perhaps most simply by multivalent
ions. The apparent role of multivalent ions is to bring two
apposing lipid bilayers into contact. The authors report the
observation of a phase of phospholipid that contains a structure
similar to the commonly postulated interbilayer state that is
crucial to membrane fusion. The widely accepted model for
membrane fusion suggests that there is an intermediate state in
which the two contacting monolayers become continuous via an
hourglass-shaped structure called a "stalk". Many efforts have
been made to estimate the free energy for such a state in order
to understand the functionality of membrane fusion proteins and
to define key parameters in energy estimates. This observation of
the stalk structure supports the stalk hypothesis for membrane
fusion and enables the measurement of these parameters
experimentally.
References (abridged):
1. J. M. White, Science 258, 917 (1992)
2. A. T. Brnger, Curr. Opin. Struct. Biol. 11, 163 (2001)
3. D. P. Siegel, Biophys. J. 65, 2124 (1993)
4. Y. A. Chizmadzhev, F. S. Cohen, A. Shcherbakov, J. Zimmerberg,
Biophys. J. 69, 2489 (1995)
5. P. I. Kuzmin, J. Zimmerberg, Y. A. Chizmadzhev, F. S. Cohen,
Proc. Natl. Acad. Sci. U.S.A. 98, 7235 (2001)
Science 2002 297:1877
Related Background:
ROLE OF SPERM IN MAMMALIAN EMBRYONIC PATTERNING
In many animal species, the point of entry of a sperm into an egg
cell during fertilization apparently determines the orientation
of the division of the one-celled embryo into two cells. The
entry point also apparently determines the orientation of one of
the 3 embryonic "axes", which form later in development and which
mark the 3 dimensions of the embryo: front to back, head to tail,
and left to right. Until now, this determining effect of the
sperm entry point was believed not to be true for mammals.
K. Piotrowska and M. Zernicka-Goetz (University of Cambridge, UK)
present a study of the role of sperm in spatial patterning of the
early mouse embryo, the authors making the following points:
1) The authors point out that despite a lack of known
determinants of cell fate in the mouse embryo, the spatial
patterning of the embryo is evident early in development. The
vertical axis of the embryo after transplantation in the uterus
can be traced back to organization of the pre-implantation
*blastocyst, and this in turn reflects the organization of the
*cleavage stage embryo and the *animal-vegetal axis of the
*zygote. These findings suggest that the cleavage pattern of
normal development may be involved in specifying the future
embryonic axis, but how and when this pattern becomes established
is unclear. In many animal eggs, the sperm entry position
provides a cue for embryonic patterning, but until now no such
role has been found in mammals.
2) The authors report that in the mouse sperm entry position
predicts the plane of initial cleavage of the mouse egg cell and
can define embryonic and *abembryonic halves of the future
blastocyst. In addition, the embryonic cell inheriting the sperm
entry position acquires a division advantage and tends to cleave
ahead of its sister. Since cell identity reflects the timing of
the early cleavages, these events together shape the blastocyst,
the organization of which will become translated into axial
patterning after uterine implantation.
3) The authors conclude that 2 axes of the blastocyst become
specified in the single-cell embryo. One of these axes is defined
by the animal pole, and the second axis, the embryonic-
abembryonic axis, relates to the sperm entry point. These axes
are initially not fixed and can be re-established if development
is perturbed. The authors suggest that the direction of
blastocyst organization by the plane of cleavage and cellular
identity by the order of cleavage may offer an interpretation of
the regulative events that occur following perturbation of
development. In normal development, the orientation and timing of
cleavage are mechanisms that progress together, but one might act
as a failsafe mechanism for the other when development is
perturbed. The authors conclude: "It will be a future challenge
to determine how these axes are initiated by the earliest events
of embryogenesis and how they become transformed into the final
body pattern.
In a commentary on this work, Roger A. Pedersen (University of
California San Francisco, US) states: "This finding represents a
leap forward in our knowledge of how the mammalian embryo
acquires its body pattern. It also suggests that mammals might
share other features of axis formation with species such as
frogs, for which we have a better understanding of the effects of
fertilization on embryonic organization."
Nature 25 Jan 01 409:473,517
Notes:
... ... *cleavage: The term "cleavage" refers to a series of
consecutive cell divisions, with the cells produced during
cleavage called "blastomeres". During cleavage, almost no growth
occurs between consecutive divisions, and the total volume of the
embryo does not substantially change: the size of the cells is
reduced by almost half at each division. At the beginning of
cleavage, cell divisions tend to occur synchronously in all
blastomeres, and the number of cells is doubled at each division.
As cleavage progresses, this synchrony of division is lost. In
most animals, cleavage follows an orderly pattern, with the first
division in the plane of the main axis of the egg cell. This
cleavage plane is arbitrarily called "vertical". The second
cleavage plane is again vertical but at right angles to the
first, producing 4 equal cells arranged around the main axis of
the egg. The third cleavage plane is at right angles to both the
first and second cleavage planes and is horizontal (or
equatorial). Subsequent divisions may alternate between vertical
and horizontal cleavage planes, but ultimately cleavage divisions
become randomly oriented. The above cleavage pattern is typical
of many animal groups, but variations are common in many species.
Although the shape and volume of the embryo do not change during
cleavage, an important change in gross organization occurs: as
the blastomeres are produced, they move outward, leaving a
centrally-located fluid-filled cavity, and the embryo at this
stage approximates a hollow ball and is known as a "blastula".
The formation of the blastula marks the end of the cleavage stage
of embryonic development.
... ... *blastocyst: The "morula" is an early embryonic *cleavage
stage consisting of a solid mass of cells (blastomeres). The next
stage is the "blastula" stage, in which the cells form a hollow
sphere. The "blastocyst" is an early form of the blastula stage,
an egg in the later stages of cleavage. The blastocyst consists
of a hollow fluid-filled ball of cells and an inner cell mass
(embryonic stem cells) from which the embryo develops.
... ... *animal-vegetal axis: After fertilization, the egg cell
acquires polarity, two poles of the egg becoming distinct from
each other. At one pole, known as the "animal pole", the
cytoplasm is apparently more active and contains the nucleus. At
the other pole, called the "vegetal pole", the cytoplasm is
apparently less active and contains most of the nutritive
material ("yolk") of the egg. The general organization of the
future animal is apparently closely related to the polarity of
the egg. (The archaic and confusing terminology "animal" vs.
"vegetal" in this context derives from 19th century microscopy,
but is still in use.)
... ... *zygote: In general, the term "zygote" refers to the cell
formed by the union of male and female gametes (sperm and egg
cells).
... ... *abembryonic [region]: In general, the area of the
mammalian blastocyst opposite the region where the embryo is
formed (opposite the stem cell region).
Related Background:
ON THE MOLECULAR BIOLOGY OF SPERM-EGG FERTILIZATION
V.D. Vacquier (University of California San Diego, US) presents a
review describing the molecular diversity in the steps of sperm-
egg interaction, with examples of the apparent rapid evolution of
fertilization molecules. The author makes the following points:
1) A generalized scheme of animal fertilization involves 5 steps
in sperm-egg interaction. Spermatozoa may be *chemotactically
attracted to swim toward the egg by egg-released molecules (step
A). Depending on the species, sperm bind to the egg envelope
either before or after the opening of the sperm acrosomal vesicle
(steps B and C). Soon after exocytosis of the acrosomal vesicle
occurs (called the "acrosomal reaction"), a hole is created in
the egg envelope through which the sperm passes (step D). Once
the sperm is through the envelope, the two cells fuse (step E)
and the sperm nucleus is incorporated into the egg cytoplasm.
2) One or more of the 5 steps can exhibit species specificity,
meaning that if spermatozoa and eggs are from the same species,
their interaction leading to fusion is more efficient than if two
gametes are from different species.
3) The author presents details of current knowledge of the
molecular events involved in each of the 5 steps. Concerning
chemoattraction, the author notes that although sperm
chemoattraction to egg-derived factors has been
phenomenologically demonstrated in most invertebrate and some
vertebrate groups, the chemical nature of the attractants is
known in but a few species. The structures of known sperm
chemoattractants are chemically unrelated, indicating that they
evolved independently in different phyla. For example, in *brown
algae, female gametes release species-specific 11-carbon cyclo-
olefinic hydrocarbons, which in picomolar concentrations attract
male gametes. In the *ciliate protozoan Euplotes, small mating-
type-specific proteins control the cellular activities of
chemotaxis, *conjugation, and growth. In marine invertebrates,
sperm swim up gradients of an egg-derived peptide... Although
human sperm are attracted to the *follicular fluid surrounding
the human egg cell, the chemoattractant remains unknown.
Science 1998 281:1995
Notes:
... ... *acrosomal vesicle: (acrosome) The acrosome is a
specialized penetrating vesicular organelle at the tip of a
spermatozoon. It contains several enzymes (e.g., hyaluronidase)
that are released when the sperm contacts the egg cell, and which
effectively puncture the egg cell envelope and/or egg cell
membrane.
... ... *exocytosis: In general, any process in which an
intracellular vesicle fuses with the plasma membrane of the cell
with a resultant release of the contents of the vesicle into the
extracellular phase.
... ... *chemotactically attracted: In general, the term
"chemotaxis" refers to any movement of an organism in response to
chemical concentration gradients.
... ... *brown algae: (Phaeophyta) Mostly brown seaweeds of
coastal and marine environments. Brown algae produce *flagellated
motile sperm cells that fertilize egg cells.
... ... *flagellated: A flagellum is a long threadlike extension
providing locomotion for a cell.
... ... *ciliate protozoan: (Ciliophora) Cilia are short
threadlike extensions, hundreds usually present on an individual
ciliated cell, the cilia undergoing synchronized movements to
produce locomotion of the protozoan (e.g., the common
Paramecium).
... ... *conjugation: This is the usual reproductive process in
ciliated protozoa, but it also occurs in some species of
bacteria. The essential process is physical contact of two cells
with transfer of genetic material between them.
... ... *follicular fluid: In this context, the liquid
environment of the mammalian egg-cell containing follicle.
Related Background:
A SPERM ION CHANNEL REQUIRED FOR SPERM MOTILITY
D. Ren et al (Harvard University, US) discuss discovery of a new
sperm ion channel. Male sperm and female eggs interact
reciprocally in mammalian fertilization. To reach the site of
fertilization, sperm must travel long distances and become primed
for fertilization of the eggs through *capacitation and other
processes. Once they arrive at the surface of the egg, sperm
interact with the egg's extracellular matrix glycoproteins, which
include the *zona pellucida proteins. Sperm release acidic
material during the *acrosome reaction, a signaling event that
presumably involves the opening of calcium ion channels and the
influx of calcium ions into the sperm heads. Calcium and cyclic
nucleotides have crucial roles in mammalian fertilization, but
the molecules comprising the calcium ion-permeation pathway in
sperm motility are poorly understood. The authors describe an
apparent sperm cation channel, called "CatSper", whose amino-acid
sequence most closely resembles a single 6-transmembrane-spanning
repeat of the voltage-dependent calcium ion channel 4-repeat
structure. CatSper is located specifically in the principal piece
of the sperm tail. Targeted disruption of the gene of the CatSper
protein results in male sterility in otherwise normal mice. Sperm
motility is decreased markedly in CatSper mice, and CatSper sperm
are unable to fertilize intact eggs. In addition, the cyclic-AMP-
induced calcium ion influx is abolished in the sperm of mutant
mice. The authors suggest that CatSper is thus vital to cyclic
AMP-mediated calcium ion influx in sperm, sperm motility, and
fertilization, and that the gene represents an excellent target
for non-hormonal contraceptives for both men and women.
Nature 2001 413:603
Notes:
... ... *capacitation: Although sperm undergo maturation in the
testes, in humans they are unable to fertilize an oocyte until
they have been in the female reproductive tract approximately 10
hours. The term "capacitation" refers to the functional changes
that sperm undergo in the female reproductive tract that allow
the sperm to fertilize an oocyte. Among the functional changes
are permeabilization of the acrosome membrane, followed by
release of various enzymes necessary for penetration of the
oocyte.
... ... *zona pellucida: A gelatinous glycoprotein layer, one of
the layers surrounding the oocyte.
... ... *acrosome reaction: The acrosome is a dense granule at
the head of the sperm cell, the granule containing various
enzymes (hyaluronidase and proteases) that aid penetration of the
oocyte. In general, the term "acrosome reaction" refers to the
activation of sperm by egg jelly, the details of the process
varying according to species.
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