The Egg

Abstract

The mammalian egg develops from primordial germ cells characterized by their relatively large size and pronounced staining for alkaline phosphatase. These appear in the yolk sac of the developing embryo and migrate by ameboid movement to genital ridges located near the Wolffian ducts (Zamboni, 1972b; Baker, 1972). There they multiply to form oogonia, which degenerate in the medulla of the developing gonad, but they continue to multiply in the outer (cortical) regions, losing their centrioles. After several more mitotic divisions the oogonia enter meiotic prophase and differentiate into oocytes, which are characterized by prominent Golgi complexes. Cytoplasmic bridges connect the developing oocytes, and this may account for the relatively high degree of synchronization observed in mammalian oogenesis (Zamboni, 1972b). Shortly after birth the oocytes enter the diplotene stage of the first meiotic division, where they remain until the onset of puberty. In some rodents the chromosomes become so diffuse that the oocyte is spoken of as being in the dictyate stage, from the Greek word for net, diktyon. The diplotene and dictyate chromosomes bear faint loops, resembling the lampbrush chromosomes of lower vertebrates and invertebrates (Baker and Franchi, 1967). These are probably sites of active gene transcription.