Adherens Junctions Modulate Diffusion between Epithelial Cells in Trichoplax adhaerens

Trichoplax adhaerens is the sole named member of Placozoa, an ancient metazoan phylum. This coin-shaped animal glides on ventral cilia to find and digest algae on the substrate. It has only six cell types, all but two of which are incorporated into the epithelium that encloses it. The upper epithelium is thin, composed of a pavement of relatively large polygonal disks, each bearing a cilium. The lower epithelium is thick and composed primarily of narrow ciliated cells that power locomotion. Interspersed among these cells are two different secretory cells: one containing large lipophilic granules that, when released, lyse algae under the animal; the other, less abundant, is replete with smaller secretory granules containing neuropeptides. All cells within both epithelia are joined by adherens junctions that are stabilized by apical actin networks. Cells are held in place during shape changes or under osmotic stress, but dissociate in low calcium. Neither tight, septate, nor gap junctions are evident, leaving only the adherens junction to control the permeability of the epithelium. Small (<4 kDa) fluorescent dextrans introduced into artificial seawater readily penetrate into the animal between the cells. Larger dextrans enter slowly, except in animals treated with reduced calcium, indicating that the adherens junctions form a circumferential belt around each cell that impedes diffusion into the animal. During feeding, the limited permeability of the adherens junctions helps to confine material released from lysed algae within the narrow space under the animal, where it is absorbed by endocytosis.