Sex differentiation in the hagfish Eptatretus stouti

A detailed study was made of sex differentiation in three anteroposteriorly distributed body regions of the long gonadal fold in young Pacific hagfish (Eptatretus stouti) from 12.4 to 45 cm in length. In general, the posterior region, adjacent to the cloaca, remained undifferentiated, and only in specimens more than 28 cm long was there development of this region into a differentiated testis. On the other hand, the anterior gonadal region of all specimens under 20 cm developed continually and progressively through stages that included cystic nests of cells derived from the germinal epithelium, acquisition of follicular investments around these cells, growth of these young oocytes and break-up of their original follicular groupings, and, finally, in larger specimens, vitellogenesis. Although the earliest oocytes formed in cyst-like nests of cells, later oocytes were derived individually directly from the germinal epithelium. In larger specimens, in which testicular development occurred in the posterior part of the gonad, the anterior gonad involuted, leaving a thin structure that lacks germ cells. There were numerous exceptions to this general description of events, indicating a certain degree of lability of determination of a particular gonadal region to either male or female differentiation. Some earlier stages of female differentiation could occur in the posterior gonadal region of a few smaller animals. Some posteriorly located ovotestes (differentiated elements of both sexes) developed in a few specimens between 16 and 21 cm in length, but were not seen in larger animals. Some testicular follicles appeared in the anterior gonadal regions of three specimens between 20 and 25 cm in length, but not in specimens larger than that. Three hermaphroditic specimens between 29 and 33 cm in length were found in which there was a well-differentiated testis posteriorly and a well-differentiated ovary (vitellogenic eggs) anteriorly. Questions of hermaphroditism, lability of sex determination, and possible epigenetic influences on these phenomena are taken up in the discussion. Since this study provides the first correlation of stages of sex differentiation with animal length, it forms the basis for experimental approaches in E. stouti to analysis of mechanisms of sex determination and differentiation.