Structure and function of the amphibian follicular epithelium during ovulation

Low concentrations of cytochalasin B (CCB) are known to inhibit ovulation in the frog, Hyla regilla. Examination of amphibian thecal cell ultrastructure reveals filaments (average diameter 71 A) arranged in bundles parallel to the surface of the oocyte. These filaments are often associated with hemidesmosome-like plaques on the basal plasmalemma, While individual filaments appear unaltered morphologically by CCB (1-5 microgram/ml), their organization into bundles, apparent relationship to the hemidesmosomes, and the highly contorted configuration of the thecal cells after oocyte expulsion, suggest that a nonmuscular contractile system residing within the follicle plays a fundamental role in ovulation. Our data suggest that the flattened epitheloid thecal cells shorten all axes that run parallel to the oocyte surface via filament bundle contractions, while they remain tightly bound together by macular attachment plaques. These cells thus increase in height to become cuboidal-low columnar in shape; the area covered by the base of each is greatly reduced. As this "thecal sac" decreases in size, the compression generated by the contractile mechanism forces the oocyte through the enzymatically weakened apex of the follicle and ovulation results.