THE LIVING WORLD
Unit Six. Animal Life
In females, eggs develop from cells called oocytes, located in the outer layer of compact masses of cells called ovaries within the abdominal cavity (figure 31.15). Recall that in males the gamete- producing cells are constantly dividing. In females all of the oocytes needed for a lifetime are already present at birth. During each reproductive cycle, one or a few of these oocytes are initiated to continue their development in a process called ovulation; the others remain in developmental holding patterns.
Figure 31.15. The female reproductive system.
The organs of the female reproductive system are specialized to produce gametes and to provide a site for embryonic development if the gamete is fertilized.
Only One Female Gamete Matures Each Month
At birth, a female’s ovaries contain some 2 million oocytes, all of which have begun the first meiotic division. At this stage they are called primary oocytes (1 in figure 31.16). Each primary oocyte waits to receive the proper developmental “go” signal before continuing on with meiosis. Until then, its meiosis remains arrested in prophase of the first meiotic division. Very few ever receive the awaited signal, which turns out to be the pituitary hormones FSH and LH, which you recall were discussed in chapter 30.
Figure 31.16. The ovary and formation of an ovum.
In this figure, the maturation of the ovum through meiosis is shown on the left, and the developmental journey of the ovum is on the right, with corresponding stages numbered on each. At birth, a human female's ovaries contain about 2 million egg-forming cells called oocytes, which have begun the first meiotic division and stopped. At this stage, they are called primary oocytes1, and their further development is halted until they receive the proper developmental signals, which is the hormones FSH and LH. At puberty, an adult monthly cycle of hormone secretion is established. When the hormones FSH and LH are released, meiosis resumes in a few oocytes, but only one oocyte usually continues to mature while the others regress. The primary oocyte (diploid) completes the first meiotic division, and one division product becomes a nonfunctional polar body. The other product, the secondary oocyte, is released during ovulation 2, along with the polar body. The secondary oocyte does not complete the second meiotic division until fertilization 3; that division yields two more nonfunctional polar bodies and a single haploid egg, or ovum. Fusion of the haploid egg with a haploid sperm during fertilization produces a diploid zygote.
With the onset of puberty, females mature sexually. At this time, the release of FSH and LH initiates the resumption of the first meiotic division in a few oocytes. The first meiotic division produces the secondary oocyte and a nonfunctional polar body 2. In humans, usually only a single oocyte is ovulated and the others regress. In some instances more than one oocyte develops; if both are fertilized, they become fraternal twins. Approximately every 28 days after that, another oocyte matures and is ovulated, although the exact timing may vary from month to month. Only about 400 of the approximately 2 million oocytes a woman is born with mature and are ovulated during her lifetime.
Fertilization Occurs in the Oviducts
The oviducts (also called fallopian tubes or uterine tubes) transport eggs from the ovaries to the uterus. In humans, the uterus is a muscular, pear-shaped organ about the size of a fist that narrows to a muscular ring called the cervix, which leads to the vagina (figure 31.17a). Mammals other than primates have more complex female reproductive tracts, where part of the uterus divides to form uterine “horns.”
The uterus is lined with a stratified epithelial membrane, the endometrium. In humans, the surface of the endometrium is shed approximately once a month during menstruation, while the underlying portion remains to generate a new surface during the next cycle. After ovulation, smooth muscles lining the fallopian tubes contract rhythmically, moving the egg down the tube to the uterus in much the same way that food is moved down through your intestines, pushing it along by squeezing the tube behind it. The journey of the egg through the fallopian tube is a slow one, taking from five to seven days to complete. However, if the egg is not fertilized within 24 hours of ovulation, it loses its capacity to develop.
During sexual intercourse, sperm are deposited within the vagina, a thin-walled muscular tube about 7 centimeters long that leads to the mouth of the uterus. Using their flagella, sperm entering the uterus swim up to and enter the fallopian tubes. Sperm can remain viable within the female reproductive tract for up to 6 days. If sexual intercourse takes places 5 days before ovulation or 1 day after, a viable egg will be present high up in the fallopian tubes. Of the several hundred million sperm that are ejaculated, only a few dozen make it to the egg. Once they reach the egg, the sperm must penetrate through two protective layers that surround the secondary oocyte (figure 31.17b 1): a layer of granulosa cells and a protein layer called the zona pellucida. Enzymes within the acrosome cap of the sperm (see figure 31.13) help digest the second of these layers 2. The first sperm to make it through the second layer stimulates the oocyte to block the entry of other sperm 3 and to complete meiosis II. Meiosis II produces the ovum (plural, ova) and two more nonfunctional polar bodies (see figure 31.16 3). When the female haploid nucleus within the ovum meets the male haploid nucleus, the egg is fertilized and becomes a zygote. The zygote then begins a series of cell divisions while traveling down the fallopian tube. After about 6 days, it reaches the uterus, attaches itself to the endometrial lining, and continues the long developmental journey that eventually leads to the birth of a child.
Figure 31.17. Fertilization occurs in the oviducts.
(a) The oviducts extend out from the uterus. Sperm are deposited in the vagina and travel to the oviducts. (b) Fertilization occurs in the oviduct when a sperm penetrates the outer layers of the egg cell.
Key Learning Outcome 31.4. In human females, hormones trigger the development of one or a few oocytes each 28 days. After ovulation, an egg cell travels down the fallopian tube and, if fertilized during its journey, implants in the wall of the uterus.