Oogenesis in an Adult Prosimian

Abstract

IN most Eutherian mammals oogenesis, that is, mitotic proliferation of oogonia followed by the formation of oocytes in the arrested diplotene stage of meiosis, occurs only during embryonic life or the first weeks of postnatal life¹. Certain prosimians seem to be exceptions to this general rule. Some fifty years ago Gérard² described germinal cords, containing oogonia and oocytes in the prophase of meiosis, in the ovarian cortex of an adult greater galago (Galago crassicaudatus). Similar histological appearances have been described since then in several other species³. Electron microscopic preparations from the ovarian cortex of an adult Senegal galago (Galago senegalensis senegalensis) show that the germinal cords contain large round cells which have all the ultra-structural features of oogonia seen in mammalian embryos during the period of oogenesis. Some of the cells have synaptinemal cores in their nuclei indicating that they are oocytes in the zygotene/pachytene stage of meiosis⁴. Metaphase figures are rarely seen in the germinal cords, and it is difficult to be certain whether they are in the oogonia or the pre-granulosa cells. It is maintained that the only acceptable histological evidence of neoformation of germ cells in the adult mammalian ovary would be the observation of mitotic divisions in oogonia¹. But if such newly formed oogonia add to the total stock of oocytes it is also necessary to show that some enter the prophase of meiosis and form primordial follicles containing oocytes in arrested diplotene. The only reported attempt to investigate DNA synthesis in the cells of the germinal cords in an adult prosimian is that of Ioannu⁵. Twenty-four hours after the injection of ³H-thymidine into adult specimens of Demidoff's galago (Galagoides demidoffi) labelled cells were found in the germinal cords. They were considered to be either oogonia in mitotic prophase or oocytes in pre-leptotene or leptotene stages of meiosis. This scanty evidence was interpreted as indicating the occurrence of active oogenesis.