Ultrastructure and presumed origin of the phagocytic cells involved in the regression of headless stalks of globiferous pedicellariae in the echinoid Sphaerechinus granularis (Echinodermata)

When globiferous pedicellariae of Sphaerechinus granularis inject their venom, their head autotomizes whereas their stalk remains on the test. Following autotomy, the headless stalk shows repair of the apical wound, resorption of the stalk tissues, and production of a collagen-rich fibrous tissue. Three cell types are involved in these processes. The phagocytic cells perform the resorption of both the joint tissues and the intradermal nerve tracts; the skeletoclastic cells are responsible for the resorption of the calcareous rod; and the collagen-associated cells are involved in both the production and the resorption of collagen fibrils. The three cell types share morphological characteristics between each other and with phagocytic coelomocytes. It is suggested that they could either correspond to migrating coelomocytes or share a common origin with them.