Oogenesis and Fertilization - Female Reproductive System

Oogenesis and Fertilization

 The formation of female gametes begins during fetal development, even before the female is born. By the fourth month of develop-ment, the ovaries contain 5 million oogonia (ō-ō-gō′ nē-ă), the cells from which oocytes develop (figure 19.11). By the time of birth, many of the oogonia have degenerated, and the remaining ones have begun meiosis. Also, some data indicate that oogonia can form after birth from stem cells, but the extent to which this occurs, and how long it occurs, is not clear. As in meiosis in males, the genetic material is duplicated, and two cell divisions occur (see “Formation of Gametes” earlier). Meiosis stops, however, during the first meiotic division at prophase I. The cell at this stage is called a primary oocyte, and at birth there are about 2 million of them. From birth to puberty, many primary oocytes degenerate. The number of primary oocytes decreases toaround 300,000 to 400,000; of these, only about 400 will complete development and be released from the ovaries. Nearly all others degenerate after partial development.

 Ovulation is the release of an oocyte from an ovary (figure 19.11,step 8). Just before ovulation, the primary oocyte completes the first meiotic division to produce a secondary oocyte and a polar body. Unlike meiosis in males, cytoplasm is not split evenly between the two cells. Most of the cytoplasm of the primary oocyte remains with the secondary oocyte. The polar body either degenerates or divides to form two polar bodies. The secondary oocyte begins the second meiotic division but stops in metaphase II.

  After ovulation, the secondary oocyte may be fertilized by a sperm cell (figure 19.11, step 9). Fertilization (fer′ til-i-zā′ shŭn) begins when a sperm cell penetrates the cytoplasm of a secondary oocyte. Subsequently, the secondary oocyte completes the second meiotic division to form 2 cells, each containing 23 chromosomes. One of these cells has very little cytoplasm and is another polar body that degenerates. In the other, larger cell, the 23 chromo-somes from the sperm join with the 23 from the female gamete to form a zygote (zı̄ ′ ḡo t) and complete fertilization. 

The zygote has 23 pairs of chromosomes (a total of 46 chromosomes). All cells of the human body contain 23 pairs of chromosomes, except for the male and female gametes. The zygote divides by mitosis to form 2 cells, which divide to form 4 cells, and so on. The mass of cells formed may eventually implant in, or attach to, the uterine wall and develop into a new individual .

Follicle Development

As we discussed, when a female is in her mother’s uterus, her ovaries have already begun oocyte formation. The primary oocytes present at birth are surrounded by a primordial follicle. A primordial follicle is a primary oocyte surrounded by a single layer of flat cells, called granulosa cells (figure 19.11). Once puberty begins, some of the pri-mordial follicles are converted to primary follicles when the oocyte enlarges and the single layer of granulosa cells becomes enlarged and cuboidal. Subsequently, several layers of granulosa cells form, and a layer of clear material called the zona pellucida (z̄o ′ n̆a pel-l̄u ′ sid-d̆a ) is deposited around the primary oocyte.

 Approximately every 28 days, hormonal changes stimulate some of the primary follicles to continue to develop (figure 19.11). The primary follicle becomes a secondary follicle as fluid-filled spaces called vesicles form among the granulosa cells, and a cap-sule called the theca (thē ′ kă ; a box) forms around the follicle.

  The secondary follicle continues to enlarge, and when the fluid-filled vesicles fuse to form a single, fluid-filled chamber called the antrum (an′tr̆um), the follicle is called the mature follicle, orgraaf-ian (graf′̄e-̆an) follicle. The primary oocyte is pushed off to one sideand lies in a mass of granulosa cells called the cumulus cells.

 The mature follicle forms a lump on the surface of the ovary. During ovulation, the mature follicle ruptures, forcing a small amount of blood, follicular fluid, and the secondary oocyte, surround-ed by the cumulus cells, into the peritoneal cavity. In most cases, only one of the follicles that begin to develop forms a mature follicle and undergoes ovulation. The other follicles degenerate. After ovu-lation, the remaining cells of the ruptured follicle are transformed into a glandular structure called the corpus luteum (k̄o r′ p̆u s, body; loo′ t̄e -̆u m, yellow). If pregnancy occurs, the corpus luteum enlarges in response to a hormone secreted by the placenta called humanchorionic gonadotropin hormone (hCG) (k̄o-r̄e-on′ik ḡo′nad-o-tr̄o ′ pin) (see table 19.1). If pregnancy does not occur, the corpus luteum lasts for 10–12 days and then begins to degenerate.