Plasmodesmata (singular, plasmodesma) are small tubes that connect plant cells to each other, establishing living bridges between cells. Similar to the gap junction found in animal cells, the plasmodesmata penetrate both the primary and secondary cell walls, allowing certain molecules to pass directly from one cell to another.
The biological sciences are dominated by the idea that cells are the functionally autonomous, physically separated, discrete units of life. This concept was propounded in the 19th century by discoveries of the cellular structuring of both plants and animals. Moreover, the apparent autonomy of unicellular eukaryotes, as well as the cellular basis of the mammalian brain (an organ whose anatomy for a long while defied attempts to validate the idea of the cellular nature of its neurons), seemed to provide the final conclusive evidence for the completeness of cell theory, a theory which has persisted in an almost dogmatic form up to the present day. However, it is very obvious that there are numerous observations which indicate that it is not the cells which serve as the basic units of biological life but that this property falls to some other, subcellular assemblage. To deal with this intricate problem concerning the fundamental unit of living matter, we proposed the so-called Cell Body concept which, in fact, develops an exceedingly original idea proposed by Julius Sachs at the end of the 19th century.
In the case of eukaryotic cells, DNA-enriched nuclei are intimately associated with a microtubular cytoskeleton. In this configuration—as a Cell Body —these two items comprise the fundamental functional and structural unit of eukaryotic living matter. The Cell Body seems to be inherent to all cells in all organisms. In effect, the cell is an elaboration of the Cell Body: the cell’s periphery is a result of a Cell Body-organized secretion which can also produce an extracellular matrix within which cells are embedded to form tissues. From an evolutionary perspective, it is proposed that the nuclear component of the Cell Body is descended from the first endosymbionts which were the invaders of an ancient host structure, and which later became specialized for the storage and distribution of DNA molecules to daughter nuclei after a binary nuclear division. It follows that the eukaryotic nucleus corresponds to the entire cell of a prokaryote. Although prokaryotes have a different organization, these organisms can also be accomodated within our present version of Cell Body theory. This is because, in prokaryotic cells, just as in eukaryotes, both the organization and the partitioningof DNA molecules are dependent upon a physical linkage between DNAand cytoskeletal elements.
Contemporary prokaryotes and eukaryotes have an inherent property of being joined together via temporary or stable cell-cell channels. Sometimes these channels even lead to the complete fusion of cells. In bacteria, these channels serve for conjugative transfer of DNA while in ciliates they permit the conjugative transfer of whole nuclei. Similarly, whole nuclei can pass through cell-cell channels in fungal cells. In the case of plants, cells are linked via permanent or semi-permanent channels known as plasmodesmata. These sometimes enlarge into pores which can then transfer whole nuclei from cell to cell. Also, animal cells can be linked with membraneous cell-cell channels which can allow the intercellular passage of whole organelles. Intriguingly, plasmodesmata of plant cells are similar in many respects to nuclear pores, suggesting that nuclear pores are representatives of a prototypic cell-cell channels that already existed, in their primordial form, between the original guest and host cells which merged together in the early formation of the eukaryotic cell. All these considerations call for an update of the traditional ‘cell theory’.This book covers the topic of cell-cell channels at all levels of biological organization, starting with bacteria and unicellular ciliates, via algal,fungal and plant cells, up to and including the diverse cell types of animals.We hope that this book will help update the traditional cell theory and will also stimulate new discussions concerning the basic units of life.
— Preface toCell‒Cell Channels, Baluska, Volkmann and Barlow (Springer Molecular Biology Intelligence Unit, 2006)
A very interesting title which I look forward to reading soon, especially due to the comparisons drawn with nuclear pore complexes. While the trio’s proposal sounds more than a little extreme, it certainly seems to have a good deal of explanation behind it which I’m interested in looking at (i.e. I’m not saying I’ll agree with their conclusions but am interested to see what leads them to draw such radical ones).
I found it quite remarkable this semester to learn that ribonucleoproteins (mRNAs ready for translation, repressed from doing so bound in a protein complex) are stored and transported en masse in mRNP granules.
Further, plants send their mRNAs over long distances via the phloem vascular system (little discussed so far on my course in favour of the more familiar cardiovascular circulatory system).
Likewise, Drosophila fruit flies, a model organism in genetics and cell biology at large, synthesise maternal mRNPs in “nurse cells”, delivering them to the developing oocyte through cytoplasmic canals.