Gametogenesis and ovarin development in the pig as determined by histologic, cytologic and radio isotopic techniques

Gametogenesis and ovarian development have been characterized in a number of species and follow much the same pattern in all. From their extragonadal origin, the germ cells migrate, apparently actively and selectively, up the gut mesentery to the future site of the germinal ridge (Witschi, 1948; Mintz, 1957 and 1960), Germinal divisions occur frequently during this migration and continue once the germ cells are in the definitive gonad to produce an excessive number of germ cells in relation to the number needed for future ovulations (Beaumont and Mandl, 1962; Hughes, 1963; Baker, 1963; Ioannou, 1964; Erickson, 1966a; Baker, 1966). Meiosis begins either during or shortly after the period of high mitotic activity, but chromosomal reduction is not completed until fertilization; instead, a condition of "rest" is achieved during late prophase I and follicular development begins. Prior to birth, germinal mitosis cease and all oogonia eventually enter meiosis and achieve the resting stage. These processes are, however, quite tenuous and as many as two-thirds of the germ cells produced in most species fail to survive to birth (Beaumont and Mandl, 1962; Baker, 1963; Ioannou, 1961; Erickson, 1966a; Baker, 1966). Those that do survive represent the maximum number of germ cells possible in the adult since germinal mitosis has ceased and the number is fixed, but many more of these are lost between birth and puberty as follicular development continues (Arai, 1920a). Finally, a few hours prior to ovulation, the germ cell to be ovulated continues the processes which will lead to the haploid condition. Even though the pig is a major farm animal and has become increasingly utile in biomedical research because of its many similarities to man, little is known regarding gametogenesis and ovarian development in this animal. In view of this, a study was undertaken to characterize its gametogenesis and ovarian development from conception to near puberty (130 days post partum). More specifically, this study encompassed the following aspects of the problem: (1) a quantitative and qualitative appraisal of meiosis and mitosis; (2) a quantitative and qualitative assessment of germ-cell numbers, follicular development and necrosis; and (3) the determination of whether prenatal oocytes contribute to the oocyte stock of the neonatal pig and, if so, the period of prenatal development during which they arise.