The growth of the oocyte and follicle in the ovaries of monotremes and marsupials

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Abstract
Oocyte growth in relation to follicular growth in marsupials conforms to the uniform biphasic pattern characteristic of eutherian mammals. However, the oocyte of the marsupial is larger and completes growth in a correspondingly larger follicle. At the completion of oocyte growth the follicle, like that of eutherians, shows no appreciable accumulation of follicular fluid. The ratio of oocyte/follicular diameter in marsupials at this stage is 0.59, a value close to that of 0.55 for eutherian mammals. The oocyte nucleus of the marsupial is also significantly larger than that of the eutherian, but the oocyte nucleolus is approximately the same diameter. By contrast, the growth of the oocyte in relation to the follicle in monotremes is linear throughout the whole period of follicular growth. However, like eutherians and marsupials oocyte nuclear and oocyte nucleolar growth are arrested in the early stages of follicular growth. The diameters of the oocyte nucleus and nucleolus at the completion of growth are approximately twice those found in marsupials. It is concluded that the growth patterns of the oocyte nucleus and nucleolus are common to all mammals but that the biphasic pattern of oocyte and follicular growth which is characteristic of eutherian and marsupial ovaries is not found in the monotremes.