OVULATION AND THE FINE STRUCTURE OF THE STICHOPUS CALIFORNICUS (ECHINODERMATA: HOLOTHUROIDEA) FECUND OVARIAN TUBULES

The ovary of Stichopus californicus consists of several size classes of tubules, which insert into a central gonad basis. The largest tubules contain the oocytes that will be spawned in the current season. All tubules are composed of three layers. Outermost is a complex peritoneum composed of epithelial cells, axons and muscle cells. The fine structure of the peritoneal neurons suggests their involvement in neurosecretory activity. Between the basal laminae of the peritoneum and the inner epithelium is the ovarian connective tissue compartment, including the genital hemal sinus. This sinus probably conveys nutrients from the periphery of the tubule to oocytes located deep within. The inner epithelium is composed of parietal and follicular epithelial cells and the oocytes. Stichopus oocytes contain three classes of microtubules based upon their location, orientation, and lability during fixation. Microtubules from the apical protuberance encircle the germinal vesicle. Cortical microtubules lie just under the cell surface and run parallel to it. Deep cytoplasmic microtubules run radially from the interior of the oocyte towards the cell surface. Oocytes are held within follicles by junctional complexes until the time of ovulation. Ovulation can be monitored in severed follicles of this species because an oolamina insures follicle integrity after detachment from the ovary. The onset of ovulation is marked by the dissolution of junctional complexes. This is followed by a cytochalasin B sensitive contraction of the follicle cells. The follicle contracts down around the oocyte, to lie collapsed against the ovarian wall while the oocyte is free within the ovarian lumen.