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ScienceWeek
CELL BIOLOGY: ON THE NEED FOR A NEW CELL THEORY
The following points are made by Frantiek Baluka et al (Nature 2004 428:371):
1) The concept of the cell as the fundamental structural and functional unit of a multicellular organism stems from the observations of Robert Hooke (1635-1703) in 1665 and Nehemiah Grew (1641-1712) in 1682, both of whom reported on the "cells" of plant tissues. It was 150 years before the universal "German cell theory" -- as it was originally known -- was proposed by Theodore Schwann (1810-1882) and Matthias Schleiden (1804-1881), but even this was not widely accepted for another 100 years. Brain tissues, with their large neurons interconnected by very long thin processes known as axons, had represented a particular hurdle for the early cytologists. But after the cellular basis of neurons and brain tissue had been accepted following the work of Ramon y Cajal (1852-1934) and Camillo Golgi Camillo Golgi (1843-1926), cell theory spread rapidly as a rather dogmatic doctrine.
2) Plants have been notable and important tools in this process of cellular discovery. Not only did they allow the identification of the actual cells, but they also enabled the discovery of nuclei, plasma membrane, microtubules, mitosis, and the cell-division cycle. Paradoxically, recent advances in plant cell biology challenge the dominant position of the current version of cell theory.
3) Cell theory identifies the cell as the elementary unit from which all living organisms are constructed. Although this holds true for all prokaryotic and unicellular eukaryotic organisms, the supracellular structure of higher plants presents a problem. Cell-to-cell channels, called plasmodesmata, connect each plant cell to its neighbors, facilitating the exchange of large molecules (proteins and RNAs) and allowing the mass flow of smaller molecules. Thus, in contradiction with the cell theory, plant cells are neither physically separated nor structurally independent. The challenge to the cell theory does not stop with plants. There are numerous examples of supracellular assemblages known as coenocytes (which are formed by mitosis with the absence of subsequent cell division) and syncytia (which are formed by cells fusing together), both of which are found throughout the eukaryotic superkingdom. These giant cells contain multiple nuclei.
4) In 1853, Thomas Henry Huxley (1825-1895) noted a similar inconsistency between the structure of some tissues and the German cell theory of his day. Huxley was convinced that cells of multicellular organisms are not anatomically isolated. Accordingly, he could not accept that cells were the elementary units of living organisms. Huxley's conceptual problem with cell theory is still relevant, because supracellularity is nowadays an accepted feature of higher plants. Moreover, this identity crisis of the "cell" is not simply a problem confined to plants, as nanotubular intercellular bridges are also generated de novo between animal cells. These cell-to-cell connections can create complex networks of cytoplasmic continuity that facilitate cell-cell transport, of, for example, endosomal-like vesicles. It seems that algae, fungi, plants and animals have all independently developed both supracellularity and multicellularity.(1-5)
References:
1. Baluka, F., Volkmann, D. & Barlow, P. W. Trends Plant Sci. 6, 104-111 (2001)
2. Mazzarello, P. Nature Cell Biol. 1, E13-E15 (1999)
3. Oparka, K. J. Trends Plant Sci. 9, 33-41 (2004)
4. Richmond, M. L. Nature Rev. Mol. Cell Biol. 3, 61-65 (2001)
5. Rustom, A., Saffrich, R., Markovic, I., Walther, P. & Gerdes, H-H. Science 303, 1007-1010 (2004)
Nature http://www.nature.com/nature
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ON THE HISTORY OF CELL THEORY
The term "cell theory" refers to the proposition that all forms of life (on Earth) are composed of cells, and that cells are the simplest units to exhibit the functions characteristic of living systems. Cell theory is a "theory" in the same sense that atomic theory is a theory in the physical sciences -- the proposition is universally accepted by biologists, and the extensions flowing from the theory embrace all of modern biology with widely diverse and ramifying implications. (A similar statement can be made concerning so-called evolutionary "theory".) Cell theory is a unifying concept that has provided direction for the analysis of fundamental biological problems such as reproduction, sexuality, development, heredity, evolution, metabolism, coordination, growth, and numerous equally basic biological phenomena.
Although cursory treatments of cell theory often focus on cell theory as discussed in the 19th century, in actuality the theory originated in the 17th century, when researchers first noted the existence of cells, and the theory passed through various stages of understanding during the following 300 years. The various forms of cell theory have included an elementary conception of basic microscopic units in the mid-17th century, a more fully articulated cell theory in the early 19th century, which held that cells are the basic building blocks of living organisms, a later 19th century conviction that the actions of cellular material actually bring about organic development and differentiation, and the idea that cells hold the key to evolutionary development as well. Controversies and disagreements about detail have occurred, but some version of cell theory and the fundamental role of cells in development has persisted, and the general idea of cells as the basic functional units of living systems remains the foundation of modern biology.
The following points are made by Paul Nurse (Science 2000 289:1711):
1) The discovery of biological cells followed quickly after the invention of the microscope in the 17th century. Robert Hooke (1635-1703) published drawings of sections of cork in 1665, first calling the observed walled cavities "cells". Within a few years, Nehemiah Grew (1641-1712) and Marcello Malpighi (1628-1694) published studies that led to the view that plant tissues are mostly composed of aggregates of cells, and later in the 17th century, Malpighi, Anton van Leeuwenhoek (1632-1723), and Jan Swammerdam (1637-1680) first recognized cells in animals. These microscopists described corpuscles in blood -- no one at that time proposed that solid animal tissues were also made of cells: that proposition was not made until the 19th century, and in particular by Theodor Schwann (1810-1882) and Matthias Schleiden (1804-1881).
2) Although Schleiden and Schwann correctly articulated the cell theory, their ideas concerning the formation of cells were wrong: they believed cells arose by processes similar to precipitation or crystallization. Others, particularly Robert Remak (1815-1865), recognized that cells arose from preexisting cells by a process of binary fission. This view was championed by Rudolf Virchow (1821-1902), who popularized the phrase "all cells come from cells."
3) Most cells contain a single nucleus that reproduces during mitosis and cell division. Elongated chromosomal threads, described by Walther Flemming (1843-1905) and Eduard Strasburger (1844-1912) in the 1880s, were observed to split lengthwise before shortening and thickening as mitosis proceeds. The longitudinal halves then separate into the two daughter nuclei. Approximately the same time as these observations were made, Edouard van Beneden (1846-1910) demonstrated that the chromosomes in a fertilized roundworm (nematode) egg are derived in equal numbers from the egg and the sperm. That led August Weismann (1834-1914) to propose that the hereditary system is based on the chromosomes.
4) The author concludes: "The cell is the simplest unit to exhibit life's functions. We now have both the molecular tools and the conceptual frameworks to undertake a concerted program to understand how cells operate. The genome projects will anchor that foundation by identifying all the genes required for a cell to function, yet researchers will still have to work out how the relevant gene products act and interact to generate cellular organization... The coming years will be exciting ones during which new ideas and theories will help us fully understand cells and thereby life itself."
Science http://www.sciencemag.org
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ON THE BEGINNING OF CELL THEORY IN BIOLOGY
The following points are made by Theodor Schwann (citation below):
1) When organic nature, animals and plants, is regarded as a Whole, in contradistinction to the inorganic kingdom, we do not find that all organisms and all their separate organs are compact masses, but that they are composed of innumerable small particles of a definite form. These elementary particles, however, are subject to the most extraordinary diversity of figure, especially in animals; in plants they are, for the most part exclusively, cells. This variety in the elementary parts seems to hold some relation to their more diversified physiological function in animals, so that it might be established as a principle that every diversity in the physiological signification of an organ requires a difference in its elementary particles; and, on the contrary, the similarity of two elementary particles seems to justify the conclusion that they are physiologically similar...
2) The greater the number of physiologically different elementary parts, which, so far as can be known, originate in a similar manner, and the greater the difference of these parts in form and physiological signification, while they agree in the perceptible phenomena of their mode of formation, the more safely we may assume that all elementary parts have one and the same fundamental principle of development.
3) The elementary parts of all tissues are formed of cells in an analogous, though very diversified manner, so that it may be asserted, that there is one universal principle of development for the elementary parts of organisms, however different, and that this principle is the formation of cells.[*Note #1]
Adapted from: Theodor Schwann: Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants. Sydenham Society, London 1847, transl. Henry Smith, from the original German published in 1839)
Notes by ScienceWeek:
Note #1: The "cell theory" is probably the most important biological generalization of the first half of the 19th century, a generalization that has grown in importance and which serves as a unifying principle in the continued development of modern biology. A number of biologists had been writing about the cellular organization of animals and plants, but it was Theodor Schwann (1810-1882) and Matthias Schleiden (1804-1881) who most clearly stated and summarized the case for the cell theory, Schwann for animals and Schleiden for plants.
Although a major weakness of the theory was its proposition that the formation of cells involved the appearance of a nucleus first and the remainder of the cell afterward, the general idea of cell theory, that of individual physiological entities ("cells") as the fundamental units of biological systems, was a correct and profound conceptual contribution. Schwann also apparently coined the term "metabolism" to represent the overall chemical changes occurring in living systems. He also did important work on digestion, fermentation, and histology. He identified yeast as consisting of tiny plant-like organisms, and he was one of the first to propose that fermentation of sugar and starch was the result of a life process, a proposition that provoked so much scientific criticism in Germany that Schwann left Germany and moved to Belgium. He became professor of anatomy at Louvain in 1838 and at Liege in 1847. It is ironic that in the last 40 years of his life he devoted most of his energies to mysticism and religious meditation, doing nothing to match his earlier intellectual and scientific accomplishments in the one decade of the 1830s.
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