Development of the ovary in the brushtail possum Trichosurus vulpecula (Marsupialia).

The urogenital region of 25 fetuses and 75 pouch young, ranging in age from newborn to 103 days (d) in development, was examined in serial histological sections. The rete ridges formed the anterior extensions of the gonadal ridges and gave rise to the rete system and gonads respectively. Sexual differentiation of the ovary commenced 2.5 d after birth, when 2 cell types appeared: the larger of these then clumped together to form the medullary cords, while the smaller cells gave rise to the stroma. Primordial germ cells were still migrating, dividing and populating the peripheral gonadal regions on d 8. Cortex and medulla were distinguishable by d 12, when a thick fibrous zone separated them. The cortex was augmented by cells from the mesothelium. The rete ovarii developed from cell condensations within the rete ridges, made secondary contact with the mesonephroi and penetrated the ovaries but did not contribute to the granulosa cells. It is concluded that, contrary to the situation in most eutherians, in Trichosurus, as in other marsupials examined, the mesonephros does not contribute to rete formation, or to the granulosa cells, which appear to arise from the medullary cords.