oogenesis

Our Living Language  : The details of the exact nature of oogenesis vary by species, since the females of some species produce thousands of eggs at a time, while in others, females produce relatively few mature eggs. The human female, for example, ovulates only about 400 times during her lifetime. Oogenesis in humans begins in embryonic and fetal development, when diploid germ cells called oogonia divide by mitosis to produce cells called primary oocytes. The primary oocytes of the female fetus enlarge and begin to undergo meiosis (in which the number of chromosomes will be reduced by half). But they are suspended in an early phase of meiosis called prophase until the female reaches puberty. The human female has about 700,000 such primary oocytes at birth. After puberty, one of the oocytes resumes development each month in response to changes initiated by follicle-stimulating hormone. The primary oocyte undergoes the first meiotic division, producing a cell called a secondary oocyte and another called the first polar body. During cytokinesis, most of the cytoplasm of the primary oocyte moves to the secondary oocyte. The first polar body undergoes the second meiotic division and its daughter cells degenerate. The secondary oocyte is released from the ovary during ovulation. If it encounters a spermatozoon and fertilization is initiated, the secondary oocyte undergoes the second meiotic division, producing the ovum and a second polar body that degenerates. The spermatozoon then fertilizes the ovum. If the secondary oocyte does not encounter a spermatozoon, it does not undergo the second meiotic division and simply degenerates.