AOBPreview originally published online on May 20, 2004
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Annals of Botany 94: 9-32, 2004
© 2004 Annals of Botany Company
REVIEW |
Eukaryotic Cells and their Cell Bodies: Cell Theory Revised
EK BALUKA*,1,2
,31 Institute of Cellular and Molecular Botany, University of Bonn, Kirschallee 1, 53175 Bonn, Germany; 2 Institute of Botany, Slovak Academy of Sciences, Dúbravská cesta 14, 842 23 Bratislava, Slovakia; 3 School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, UK
* For correspondence. E-mail baluska{at}uni-bonn.de PWB dedicates his contribution to this paper to his friend and mentor, Professor Paul E. Polani FRS, on the occasion of his 90th birthday, 1 January 2004.
Received: 9 January 2004; Returned for revision: 20 February 2004; Accepted: 2 March 2004. Published electronically: 20 May 2004
• Background Cell Theory, also known as cell doctrine, states that all eukaryotic organisms are composed of cells, and that cells are the smallest independent units of life. This Cell Theory has been influential in shaping the biological sciences ever since, in 1838/1839, the botanist Matthias Schleiden and the zoologist Theodore Schwann stated the principle that cells represent the elements from which all plant and animal tissues are constructed. Some 20 years later, in a famous aphorism Omnis cellula e cellula, Rudolf Virchow annunciated that all cells arise only from pre-existing cells. General acceptance of Cell Theory was finally possible only when the cellular nature of brain tissues was confirmed at the end of the 20th century. Cell Theory then rapidly turned into a more dogmatic cell doctrine, and in this form survives up to the present day. In its current version, however, the generalized Cell Theory developed for both animals and plants is unable to accommodate the supracellular nature of higher plants, which is founded upon a super-symplasm of interconnected cells into which is woven apoplasm, symplasm and super-apoplasm. Furthermore, there are numerous examples of multinucleate coenocytes and syncytia found throughout the eukaryote superkingdom posing serious problems for the current version of Cell Theory.
• Scope To cope with these problems, we here review data which conform to the original proposal of Daniel Mazia that the eukaryotic cell is composed of an elemental Cell Body whose structure is smaller than the cell and which is endowed with all the basic attributes of a living entity. A complement to the Cell Body is the Cell Periphery Apparatus, which consists of the plasma membrane associated with other periphery structures. Importantly, boundary stuctures of the Cell Periphery Apparatus, although capable of some self-assembly, are largely produced and maintained by Cell Body activities and can be produced from it de novo. These boundary structures serve not only as mechanical support for the Cell Bodies but they also protect them from the hostile external environment and from inappropriate interactions with adjacent Cell Bodies within the organism.
• Conclusions From the evolutionary perspective, Cell Bodies of eukaryotes are proposed to represent vestiges of hypothetical, tubulin-based ‘guest’ proto-cells. After penetrating the equally hypothetical actin-based ‘host’ proto-cells, tubulin-based ‘guests’ became specialized for transcribing, storing and partitioning DNA molecules via the organization of microtubules. The Cell Periphery Apparatus, on the other hand, represents vestiges of the actin-based ‘host’ proto-cells which have become specialized for Cell Body protection, shape control, motility and for actin-mediated signalling across the plasma membrane.
Key words: Actin, Cell Body, Cell Periphery Apparatus, Cell Theory, coenocytes, cytoskeleton, nucleus, plasma membrane, plasmodesmata, polarity, syncytia, tubulin.