A light microscopic study of oogenesis in the brushtail possum Trichosurus vulpecula

Ovaries from young of the brushtail possum, Trichosurus vulpecula, were examined histologically and histochemically to determine stages of oogenesis. Groups of dividing oogonia were first present in a 13-day-old pouch young, with extensive oogonial proliferation after about 23 days of age. Meiosis was initiated in some oogonia by 48 days of age, and by 88 days numerous early primordial follicles were present. The first primary follicles had formed by 103 days of age. In oocytes of quiescent primordial follicles, dark granular material, which stained positively for protein, mainly occupied the perinuclear cytoplasm. Associated with the transition to primary follicles and continuing throughout oocyte growth was the production within the central cytoplasm of large clear vesicles and, to a lesser extent, lipid-like yolk bodies. The former were analogous to similar vesicles found in other species of marsupial, and have a putative role in development of the blastocyst. By contrast, yolk-like bodies are not found in all marsupial oocytes and their importance in development is unknown.