Trichoplax Adhaerens - Fiber Syncytium

Fiber Syncytium

Between the two layers of cells is a liquid-filled interior space, which, except for the immediate zones of contact with the central and dorsal sides, is pervaded by a star-shaped fiber syncytium: a fibrous network that consists essentially of a single cell but contains numerous nuclei that, while separated by internal crosswalls (septa), do not have true cell membranes between them. Similar structures are also found in the sponges (Porifera) and many fungi.

On both sides of the septa are liquid-filled capsules that cause the mentioned separating structures to resemble synapses, i.e. nerve-cell junctions that occur in fully expressed form only in animals with tissues (Eumetazoa). Striking accumulations of calcium ions, which may have a function related to the propagation of stimuli, likewise suggest a possible role as protosynapses. This view is supported by the fact that fluorescent antibodies against cnidarian neurotransmitters, i.e. precisely those signal carriers that are transferred in synapses, bind in high concentrations in certain cells of Trichoplax adhaerens and thus indicate the existence of comparable substances in the Placozoa. In addition, the fiber syncytium contains molecules of actin and probably also of myosin, which occur in the muscle cells of eumetazoans. In the placozoans, they ensure that the individual fibers can relax or contract and thus help determine the animals' shape.

In this way, the fiber syncytium assumes the functions of nerve and muscle tissues. Moreover, at least a portion of digestion occurs here. On the other hand, no gelatinous extracellular matrix exists of the kind observed, as mesoglea, in cnidarians and ctenophores.

Pluripotent cells, which can differentiate into other cell types, have not yet been demonstrated unambiguously in T. adhaerens, in contrast to the case of the Eumetazoa. The conventional view is that dorsal and ventral epithelium cells arise only from their own kind.

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