Histological and histochemical studies of post‐ovulatory and pre‐ovulatory atretic follicles in Mustelus canis

The atresia of post‐ovulatory and pre‐ovulatory follicles of the viviparous smooth dogfish, Mustelus canis , is compared for approximately the first fourth of an 11 month gestation. A thick collagenous sheath and numerous tubules in the theca identify the large, folded stage A post‐ovulatory follicle. In stage B the tubules have been filled by cells to form “islands.” In stage C the entire structure is greatly diminished, adjacent islands tend to fuse, the collagenous sheath is virtually gone and the granulosa is degenerating. Preovulatory follicles from large, yolky oocytes pass through four stages beginning with yolk phagocytosis by granulosa cells of the villi (stage I), which are long and granular in stage II; villi fuse, theca cells increase greatly, fill with granules (stage III), encroach on the granulosa and disperse it into small groups of cells which finally disappear (stage IV) leaving a mass of thecal cells. A special type of pre‐ovulatory follicle from small non‐yolky oocyte atresia exhibits prominent thecal tubules and an unusual arrangement of granulosa cells. This follicle appearrs to enlarge during the summer, becoming multilobed; few granules are present. The distribution of lipid in frozen sections, stained by Oil red O , is described for all types of follicles. Schultz and Lewis and Lobban tests for steroids were made on frozen sections with corresponding results. Positive green tests indicating the presence of steroids or possible steroidogenesis were limited to: (1) one post‐ovulatory follicle, in the islands; (2) four stage III and seven late stage IV pre‐ovulatory yolky atretic follicles; (3) two special atretic follicles. The special atretic follicle appears to be a unique feature of M. canis and it is suggested tentatively that it may be related to viviparity.