ULTRASTRUCTURE OF THE SOMATIC PORTION OF THE GONADS IN ASTEROIDS, WITH EMPHASIS ON FLAGELLATED-COLLAR CELLS AND NUTRIENT TRANSPORT

Somatic portions of gonads in two phanerozonian sea‐stars, Ctenodiscus crispatus and Hippasteria phrygiana, were similar in all aspects of gross structure and histology seen previously in both forcipulate and spinulosan asteroids. For the first time, detailed ultrastructural observations have been made of cells and tissues that reveal several features believed to be of universal occurrence in the gonads of asteroids. These include flagellated‐collar cells in the visceral peritoneum and other coelomically derived epithelia, muscular‐flagellated‐collar cells in the visceral peritoneum and genital coelomic (perihaemal) sinus, the digestion of collagen fibers by cells in the connective tissue layer, and the intimate relationship of the genital haemal sinus and the entire germinal epithelium. Structural and functional compartmentalization are discussed in relation to major activities of the gonad throughout the annual reproductive cycle. The distinctive ultrastructure and current generation of flagellated‐collar cells found in the visceral peritoneum are analyzed relative to their role in nutrient transport to gonadal tissues. The single flagellum of each flagellated‐collar cell beats in coordination with those on neighboring cells to produce extremely rapid, oriented currents of coelomic fluid. The form of beating in an individual flagellum is planar, and the resulting synchronized activity of many adjacent flagella is non‐metachronal; both of these characteristic aspects of current production have, thus far, been encountered together only in the Echinodermata. Flagellated‐collar cells are efficient in generating currents which mix contents of the coelomic fluid, and they can presumably supply themselves with nutrients. It is concluded that nutrients so obtained are generally not passed through the wall of the gonad to the germinal epithelium and, as a result, have little to do with nutrition of somatic and germinal cells of the germinal epithelium. Alternatively, well‐developed genital portions of the haemal system of the sea‐star are advanced as the major channels supplying nutrients to germinal epithelia during gametogenesis. Copyright © 1979 Wiley‐Liss, Inc.