LAUSR

Abstract The microanatomy of the Fluvitrygon signifer ovary was investigated. The females had a single left ovary containing various stages of germ cells and follicles. Both immature and mature female ovaries contained oogonia initially observable in the germinal epithelium as solitary cells or oogonial nests. Herein, three developmental pathways for the oogonia to become primary oocytes are proposed. (1) The single oogonia developed into individual primary oocytes and, in association with follicular cells, became follicles. (2) Oogonial nests constituted sex cords, which released single follicles. (3) The nests detached from the germinal epithelium as isolated nests, which transformed into nests of primary oocytes and subsequently nests of previtellogenic follicles. We point out a possible mechanism for de novo formation of nests of primary follicles from a single follicle. The presence of follicle nests in adults appeared to be a specific characteristic of F. signifer. Atretic preovulatory follicles were divided into atretic previtellogenic and atretic vitellogenic follicles; the latter were subcategorized into four stages and postovulatory follicles were classified into six stages. In general, F. signifer revealed similar microscopic structures of the ovary to those in other elasmobranchs, with the exception of specific features during early folliculogenesis and potential indeterminate oogenesis.