Oogenesis in the marsupial stripe-faced dunnart, Sminthopsis macroura

The timetable of oogenesis in Sminthopsis macroura is accelerated like in other marsupials showing relatively early maturation of the female. On the day of parturition (day 0) migration of primordial germ cells to the indifferent gonads has been completed. Follicular growth seems not to correspond to the biphasic pattern, in which oocyte and follicle grow synchronously until antral stages when only the follicle increases in size, but shows a continuous growth of the oocyte and the follicle up to the time of ovulation. During primordial and early primary follicle stage a paranuclear complex is present in the oocyte, consisting mainly of smooth tubules of endoplasmic reticulum. Cortical granules appear early in oocytes in secondary follicles. The conspicuous inclusions in the antral follicle are the clusters of electron-lucent vesicles in the oocyte. These inclusions grow from multivesicular bodies (MVB), which are formed from Golgi and endoplasmic reticulum vesicles. Further increase in the size of MVB involves the incorporation of endocytic vesicles and the coalescence of larger vesicles. The polarized nature of the oocyte at ovulation is due in part to the to accumulation of these vesicles in the cytoplasm opposite the eccentrically placed nuclear material.