Morphology of coral desmocytes, cells that anchor the calicoblastic epithelium to the skeleton

We used light, scanning, and electron microscopy to investigate the ultrastructure of desmocytes in the scleractinian Stylophora pistillata from the Red Sea. Desmocytes are abundant on the calicoblastic epithelium, numbering up to 150 per mm super(2) in the coenosarc. The surface of the skeleton bears shallow pits which may represent desmocyte attachment scars. Previously described as cell remnants or extracellular products, coral desmocytes appear to be bona fide cells as they manifest plasma membranes, organelles, and nuclei. Desmocytes attach to the mesoglea in mortise and tenon fashion. A field of 40 or more tenons protrude fingerlike from the proximal surface of the desmocyte and interdigitate with the mesoglea. Each tenon is coated extracellularly with short fibers which are joined to fibers of the mesoglea. The arrangement resembles previously described "fascial" hemidesmosomes. The short fibers pass through the plasma membrane and connect with relatively long intracellular fibers which occupy the center of each tenon. The long fibers extend distally and attach to structures resembling vertebrate hemidesmosomes. These, in turn, attach to the skeleton. The fiber arrangement and orientation seems designed to resist tensile forces. The dynamic adhesion potentially provided by the distal hemidesmosomes may enable desmocytes to detach and reattach to the skeleton during episodes of mineral accretion.