Oogenesis

Oogenesis is the production of egg cells in the ovaries. It differs from spermatogenesis in its timing and in the unequal division of cytoplasm.Before birth, in the ovaries of a female fetus:
• germinal epithelium cells divide by mitosis to form diploid oogonia (2n);
• oogonia grow into primary oocytes (2n), which begin meiosis I but stop at prophase I. Each is surrounded by a layer of follicle cells, forming a primary follicle. A girl is born with all the primary oocytes she will ever have.From puberty, roughly once a month:
• a primary oocyte completes meiosis I, forming a large haploid secondary oocyte and a tiny first polar body, which receives very little cytoplasm;
• the secondary oocyte starts meiosis II but stops at metaphase II, and is released from the ovary at ovulation in this state.Meiosis II is completed only if a sperm enters the secondary oocyte. This produces the ovum and a second polar body. The first polar body may also divide, and all the polar bodies break down.So one primary oocyte gives only one functional egg, whereas one primary spermatocyte gives four sperm. The unequal divisions mean the egg keeps almost all of the cytoplasm, with the food reserves and organelles needed by the early embryo.Worked example: a woman ovulates about 13 times a year for 35 years, so only about 13 × 35 = 455 secondary oocytes are released in her lifetime, out of 1 to 2 million primary oocytes present at birth.