Ovarian folliculogenesis in the southern sea lion Otaria flavescens

In Otaria flavescens, folliculogenesis occurs within ovary cortex. This process attains successively higher level of organization through cell proliferation and differentiation. Here, we identified eight major follicular stages from I to VIII, plus corpus luteum and corpus albicans. Otaria folliculogenesis is divided into (1) primordial follicle recruitment; (2) pre-antral follicle development; (3) growth of antral follicle; and (4) follicle atresia. The primordial follicle is surrounded by thin and flat granulosa cells. As recruitment occurs, granulosa cells organize into cuboidal monolayer marking the transition into pre-antral phase. Pre-antral phase (from stage I to stage V) is characterized by follicular growth and theca layer formation. The typical feature of the antral phase (from stage V to pre-ovulatory Graafian follicle or stage VIII) is the formation of a fluid-filled cavity (antrum) within the follicle. In the Graafian preovulatory follicle, theca cells organize two distinguishable layers surrounding the follicle. In gravid females, the corpus luteum persists. In nongravid females, the corpus luteum transforms into corpus albicans. The corpus albicans is formed by granulosa, thecal cells, and invading blood tissue. Our results provide a detailed histological description of follicular development in O. flavescens and add new knowledge to the reproductive biology of this species. © 2011 The Authors. Acta Zoologica © 2011 The Royal Swedish Academy of Sciences.