Gonadal development in the opossum, Monodelphis domestica: the rete ovarii does not contribute to the steroidogenic tissues.

The gonads of 273 animals, ranging in age from newborn to adult, were examined in serial histological sections. Primordial germ cells were located in the hindgut, dorsal mesentery and gonadal primordia of neonates but were alkaline phosphatase negative. The testis differentiated between d 13 of gestation and birth, about half a day later. Testis cords, composed of pre-Sertoli cells and containing germ cells, were distributed peripherally in the gonad. Ovarian differentiation began on d 6, when an ill defined cortex and medulla became discernible. Meiosis commenced on d 14, medullary cords formed from blastema cells by d 26 and gave rise to granulosa cells around d 29. The rete ovarii was first observed in the hilar region of the gonad primordium. It penetrated maximally two thirds of the ovarian length between d 26 and d 29. It is concluded that, at least postnatally, the mesonephros does not contribute to the rete ovarii and that the granulosa cells are derived not from the rete but from the medullary cords.