Trichoplax Adhaerens - Epitheloid

Epitheloid

Both structurally and functionally, it is possible to distinguish a back or dorsal side from a belly or ventral side in Trichoplax adhaerens. Both consist of a single layer of cells coated on the outside with slime and are reminiscent of epithelial tissue, primarily due to the junctions—belt desmosomes—between the cells. In contrast to true epithelium, however, the cell layers of the Placozoa possess no basal lamina, which refers to a thin layer of extracellular material underlying epithelium that stiffens it and separates it from the body's interior. The absence of this structure, which is otherwise to be found in all animals except the sponges, can be explained in terms of function: a rigid separating layer would make the amoeboid changes in the shape of Trichoplax adhaerens impossible. Instead of an epithelium, therefore, we speak of an epitheloid in the Placozoa.

A mature individual consists of up to a thousand cells that can be divided into four different cell types. The monociliated cells of the dorsal epitheloid are flattened and contain lipid bodies. The cells on the ventral side likewise beat just a single cilium, but their elongated columnar form of small cross section at the surface packs them very close together, causing the cilia to be very closely spaced on the ventral side and to form a ciliated "crawling sole". Between them are found unciliated gland cells thought to be capable of synthesizing digestive enzymes.