Oogenesis

Shortly after arriving at the ovarian anlage, the primordial germ cells (PGC) differentiate into oogonia. These become discernible from the PGC through rapid successive mitoses and formation of clones that are connected to each other through cytoplasmic bridges. During month 5, the total number of oogonia increases from some thousands up to about 7 million cells. After month 7, the number decreases rapidly, falling to about 1 to 2 million by birth. Most oogonia die during this period. Those that survive enter the prophase of the 1st maturation division (meiosis I). These are called primary oocytes. The primary oocytes undergo the prophase up to the diploten stage (dictyoten stage), and remain in this stage until puberty. At the beginning of sexual maturity, groups of oocytes periodically recommence meiosis. Primary oocytes continue to die after birth; during puberty, there are only about 250,000 per ovary. Only about 400 oocytes mature during a female's life.
Meiosis in oogenesis differs from meiosis in spermatogenesis. A particular difference is that in the metaphase, the spindle is shifted towards the cell periphery. In the telophase, one of the daughter cells receives hardly any cytoplasm and becomes the 1st polar body. The other daughter cell receives almost the entire contents of the cell. Thereby, it remains very large and becomes the secondary oocyte. During the 2nd maturation division, cytokinesis occurs in a similarly unequal manner. It results in the formation of the haploid 2nd polar body. This receives only a small part of the cytoplasm, whereas the fertilized oocyte contains a large amount (Fertilization).