Ultrastructure of oogenesis in the brushtail possum.

Marsupial ova are slightly larger on average than those of eutherians. They have often been described as "yolky", although the nature and roles of their contents in later development are poorly understood. In the ova of all marsupial species studied, large quantities of electron-lucent vesicles are present. This study describes the ultrastructure of oogenesis in the brushtail possum, Trichosurus vulpecula. Oocytes of primordial follicles contain a variety of organelles of unknown function, some of which have been described previously in bandicoots. Large electron-lucent vesicles begin to accumulate after the transition from primordial follicle to primary follicle and are derived at least partly from the Golgi and endoplasmic reticulum. Other inclusions that are absent in some other marsupial species are present in growing possum oocytes in smaller quantities, including yolklike lipid bodies and electron-dense granular bodies.