ABSTRACT
The presence of the Y chromosome results in production of testicular androgens and antiMiillerian hormone in the embryo which guide sexual differentiation to the male direction. The female phenotype is the default, and no additional factors are required for development of female gametes and sexual characteristics. Primordial germ cells of both sexes undergo mitotic division in the yolk sac until the time of gonadal differentiation in fetal life. The primordial germ cells appear in the wall of the yolk sac at the third week of fetal life. About 2 weeks later, the gonadal ridge appears, which is the earliest sign of gonadal development. The primordial germ cells progress into the gonadal ridge by ameboid movement at about 6 weeks of fetal life. The gonadal ridge grows and surrounds the primordial germ cells, and together they form three histologic components: the surface epithelium, the primitive cords, and the gonadal blastema, made up of primordial germ cells and mesenchymal cells. After the gonads start to differentiate, the primordial germ cells continue to divide; these are called oogonia in females and spermatogonia in males. The oogonia are transformed to oocytes by the initiation of meiotic division during the third to ninth months of fetal life. The oocytes are arrested in the diplotene stage of the first meiotic prophase until puberty. The spermatogo nia do not undergo meiotic division until puberty.
