At puberty, the female begins to undergo regular monthly cycles. These sexual cycles are controlled by the hypothalamus. Gonadotropin-releasing hormone (GnRH), produced by the hypothalamus, acts on cells of the anterior lobe (adenohypophysis) of the pituitary gland, which in turn secrete gonadotropins. These hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), stimulate and control cyclic changes in the ovary.
At the beginning of each ovarian cycle, 15 to 20 primary-stage (preantral) follicles are stimulated to grow under the influence of FSH. (The hormone is not necessary to promote development of primordial follicles to the primary follicle stage, but without it, these primary follicles die and become atretic.) Thus, FSH rescues 15 to 20 of these cells from a pool of continuously forming primary follicles (Figs. 3.1 and 3.2). Under normal conditions, only one of these follicles reaches full maturity, and only one oocyte is discharged; the others degenerate and become atretic. In the next cycle, another group of primary follicles is recruited, and again, only one follicle reaches maturity. Consequently, most follicles degenerate without ever reaching full maturity. When a follicle becomes atretic, the oocyte and surrounding follicular cells degenerate and are replaced by connective tissue, forming a corpus atreticum. FSH also stimulates maturation of follicular (granulosa) cells surrounding the oocyte. In turn, proliferation of these cells is mediated by growth differentiation factor 9, a member of the transforming growth factor-? (TGF?) family. In cooperation, theca interna and granulosa cells produce estrogens: theca interna cells produce androstenedione and testosterone, and granular cells convert these hormones to estrone and 17 ?-estradiol. As a result of this estrogen production,
- The uterine endometrium enters the follicular or proliferative phase;
- Thinning of the cervical mucus occurs to allow passage of sperm; and
- The anterior lobe of the pituitary gland is stimulated to secrete LH.
Figure 3.1.Drawing showing the role of the hypothalamus and pituitary gland in regulating the ovarian cycle.
Under the influence of GnRH from the hypothalamus, the pituitary releases the gonadotropins, FSH, and LH. Follicles are stimulated to grow by FSH and to mature by FSH and LH. Ovulation occurs when concentrations of LH surge to high levels. LH also promotes development of the corpus luteum. 1, primordial follicle; 2, growing follicle; 3, vesicular follicle; 4, mature vesicular (graafian) follicle
A. Primordial follicle. B. Growing follicle. C. Vesicular follicle. Every day from the pool of primordial follicles A, some begin to develop into growing follicles B, and this growth is independent of FSH. Then, as the cycle progresses, FSH secretion recruits growing follicles to begin development into vesicular (antral) follicles. C. During the last few days of maturation of vesicular follicles, estrogens, produced by follicular and thecal cells, stimulate increased production of LH by the pituitary gland (Fig. 3.1), and this hormone causes the follicle to enter the mature vesicular (graafian) stage, to complete meiosis I, and to enter meiosis II, where it is arrested in metaphase approximately 3 hours before ovulation.
At midcycle, there is an LH surge that:
- Elevates concentrations of maturation-promoting factor, causing oocytes to complete meiosis I and initiate meiosis II;
- Stimulates production of progesterone by follicular stromal cells (luteinization); and
- Causes follicular rupture and ovulation.
In the days immediately preceding ovulation, under the influence of FSH and LH, the vesicular follicle grows rapidly to a diameter of 25 mm to become a mature vesicular (graafian) follicle. Coincident with final development of the vesicular follicle, there is an abrupt increase in LH that causes the primary oocyte to complete meiosis I and the follicle to enter the preovulatory mature vesicular stage. Meiosis II is also initiated, but the oocyte is arrested in metaphase approximately 3 hours before ovulation. In the meantime, the surface of the ovary begins to bulge locally, and at the apex, an avascular spot, the stigma, appears. The high concentration of LH increases collagenase activity, resulting in digestion of collagen fibers surrounding the follicle. Prostaglandin levels also increase in response to the LH surge and cause local muscular contractions in the ovarian wall. Those contractions extrude the oocyte, which together with its surrounding granulosa cells from the region of the cumulus oophorus breaks free (ovulation) and floats out of the ovary (Fig. 3.3). Some of the cumulus oophorus cells then rearrange themselves around the zona pellucida to form the corona radiata (Figs. 3.2B,3.3,3.4,3.5 to 3.6).
A. Mature vesicular follicle bulging at the ovarian surface. B. Ovulation. The oocyte, in metaphase of meiosis II, is discharged from the ovary together with a large number of cumulus oophorus cells. Follicular cells remaining inside the collapsed follicle differentiate into lutean cells. C. Corpus luteum. Note the large size of the corpus luteum, caused by hypertrophy and accumulation of lipid in granulosa and theca interna cells. The remaining cavity of the follicle is filled with fibrin.
Figure 3.4.Relation of fimbriae and ovary.
Fimbriae collect the oocyte and sweep it into the uterine tube.
A. Scanning electron micrograph of sperm binding to the zona pellucida. B. The three phases of oocyte penetration. In phase 1, spermatozoa pass through the corona radiata barrier; in phase 2, one or more spermatozoa penetrate the zona pellucida; in phase 3, one spermatozoon penetrates the oocyte membrane while losing its own plasma membrane. Inset shows normal spermatocyte with acrosomal head cap.
A. Oocyte immediately after ovulation, showing the spindle of the second meiotic division. B. A spermatozoon has penetrated the oocyte, which has finished its second meiotic division. Chromosomes of the oocyte are arranged in a vesicular nucleus, the female pronucleus. Heads of several sperm are stuck in the zona pellucida. C. Male and female pronuclei. D, E. Chromosomes become arranged on the spindle, split longitudinally, and move to opposite poles. F. Two-cell stage.
During ovulation, some women feel a slight pain, called mittelschmerz (German for “middle pain”) because it normally occurs near the middle of the menstrual cycle. Ovulation is also generally accompanied by a rise in basal temperature, which can be monitored to aid couples in becoming pregnant or preventing pregnancy. Some women fail to ovulate because of a low concentration of gonadotropins. In these cases, administration of an agent to stimulate gonadotropin release, and hence ovulation, can be employed. Although such drugs are effective, they often produce multiple ovulations, so that the likelihood of multiple pregnancies is 10 times higher in these women than in the general population.
After ovulation, granulosa cells remaining in the wall of the ruptured follicle, together with cells from the theca interna, are vascularized by surrounding vessels. Under the influence of LH, these cells develop a yellowish pigment and change into lutein cells, which form the corpus luteum and secrete estrogens and progesterone (Fig. 3.3C). Progesterone, together with some estrogen, causes the uterine mucosa to enter the progestational or secretory stage in preparation for implantation of the embryo.
Shortly before ovulation, fimbriae of the uterine tube sweep over the surface of the ovary, and the tube itself begins to contract rhythmically. It is thought that the oocyte, surrounded by some granulosa cells (Figs. 3.3B and 3.4), is carried into the tube by these sweeping movements of the fimbriae and by motion of cilia on the epithelial lining. Once in the tube, cumulus cells withdraw their cytoplasmic processes from the zona pellucida and lose contact with the oocyte.
Once the oocyte is in the uterine tube, it is propelled by peristaltic muscular contractions of the tube and by cilia in the tubal mucosa with the rate of transport regulated by the endocrine status during and after ovulation. In humans, the fertilized oocyte reaches the uterine lumen in approximately 3 to 4 days.
If fertilization does not occur, the corpus luteum reaches maximum development approximately 9 days after ovulation. It can easily be recognized as a yellowish projection on the surface of the ovary. Subsequently, the corpus luteum shrinks because of degeneration of lutean cells (luteolysis) and forms a mass of fibrotic scar tissue, the corpus albicans. Simultaneously, progesterone production decreases, precipitating menstrual bleeding. If the oocyte is fertilized, degeneration of the corpus luteum is prevented by human chorionic gonadotropin, a hormone secreted by the syncytiotrophoblast of the developing embryo. The corpus luteum continues to grow and forms the corpus luteum of pregnancy (corpus luteum graviditatis). By the end of the third month, this structure may be one third to one half of the total size of the ovary. Yellowish luteal cells continue to secrete progesterone until the end of the fourth month; thereafter, they regress slowly as secretion of progesterone by the trophoblastic component of the placenta becomes adequate for maintenance of pregnancy. Removal of the corpus luteum of pregnancy before the fourth month usually leads to abortion.
Fertilization, the process by which male and female gametes fuse, occurs in the ampullary region of the uterine tube. This is the widest part of the tube and is close to the ovary (Fig. 3.4). Spermatozoa may remain viable in the female reproductive tract for several days.
Only 1% of sperm deposited in the vagina enter the cervix, where they may survive for many hours. Movement of sperm from the cervix to the uterine tube occurs by muscular contractions of the uterus and uterine tube and very little by their own propulsion. The trip from cervix to oviduct can occur as rapidly as 30 minutes or as slow as 6 days. After reaching the isthmus, sperm become less motile and cease their migration. At ovulation, sperm again become motile, perhaps because of chemoattractants produced by cumulus cells surrounding the egg, and swim to the ampulla, where fertilization usually occurs. Spermatozoa are not able to fertilize the oocyte immediately upon arrival in the female genital tract but must undergo (1) capacitation and (2) the acrosome reaction to acquire this capability.
Capacitation is a period of conditioning in the female reproductive tract that in the human lasts approximately 7 hours. Thus, speeding to the ampulla is not an advantage, since capacitation has not yet occurred and such sperm are not capable of fertilizing the egg. Much of this conditioning during capacitation occurs in the uterine tube and involves epithelial interactions between the sperm and the mucosal surface of the tube. During this time, a glycoprotein coat and seminal plasma proteins are removed from the plasma membrane that overlies the acrosomal region of the spermatozoa. Only capacitated sperm can pass through the corona cells and undergo the acrosome reaction.
The acrosome reaction, which occurs after binding to the zona pellucida, is induced by zona proteins. This reaction culminates in the release of enzymes needed to penetrate the zona pellucida, including acrosin- and trypsin-like substances (Fig. 3.5).
The phases of fertilization include
- Phase 1, penetration of the corona radiate
- Phase 2, penetration of the zona pellucida
- Phase 3, fusion of the oocyte and sperm cell membranes
Phase 1: Penetration of the Corona Radiata
Of the 200 to 300 million spermatozoa normally deposited in the female genital tract, only 300 to 500 reach the site of fertilization. Only one of these fertilizes the egg. It is thought that the others aid the fertilizing sperm in penetrating the barriers protecting the female gamete. Capacitated sperm pass freely through corona cells (Fig. 3.5).
Phase 2: Penetration of the Zona Pellucida
The zona is a glycoprotein shell surrounding the egg that facilitates and maintains sperm binding and induces the acrosome reaction. Both binding and the acrosome reaction are mediated by the ligand ZP3, a zona protein. Release of acrosomal enzymes (acrosin) allows sperm to penetrate the zona, thereby coming in contact with the plasma membrane of the oocyte (Fig. 3.5). Permeability of the zona pellucida changes when the head of the sperm comes in contact with the oocyte surface. This contact results in release of lysosomal enzymes from cortical granules lining the plasma membrane of the oocyte. In turn, these enzymes alter properties of the zona pellucida (zona reaction) to prevent sperm penetration and inactivate species-specific receptor sites for spermatozoa on the zona surface. Other spermatozoa have been found embedded in the zona pellucida, but only one seems to be able to penetrate the oocyte (Fig. 3.6).
Phase 3: Fusion of the Oocyte and Sperm Cell Membranes
The initial adhesion of sperm to the oocyte is mediated in part by the interaction of integrins on the oocyte and their ligands, disintegrins, on sperm. After adhesion, the plasma membranes of the sperm and egg fuse (Fig. 3.5). Because the plasma membrane covering the acrosomal head cap disappears during the acrosome reaction, actual fusion is accomplished between the oocyte membrane and the membrane that covers the posterior region of the sperm head (Fig. 3.5). In the human, both the head and the tail of the spermatozoon enter the cytoplasm of the oocyte, but the plasma membrane is left behind on the oocyte surface. As soon as the spermatozoon has entered the oocyte, the egg responds in three ways:
- Cortical and zona reactions. As a result of the release of cortical oocyte granules, which contain lysosomal enzymes, (1) the oocyte membrane becomes impenetrable to other spermatozoa, and (2) the zona pellucida alters its structure and composition to prevent sperm binding and penetration. These reactions prevent polyspermy (penetration of more than one spermatozoon into the oocyte).
- Resumption of the second meiotic division. The oocyte finishes its second meiotic division immediately after entry of the spermatozoon. One of the daughter cells, which receives hardly any cytoplasm, is known as the second polar body; the other daughter cell is the definitive oocyte. Its chromosomes (22 plus X) arrange themselves in a vesicular nucleus known as the female pronucleus (Figs. 3.6 and 3.7).
- Metabolic activation of the egg. The activating factor is probably carried by the spermatozoon. Activation encompasses the initial cellular and molecular events associated with early embryogenesis.
The spermatozoon, meanwhile, moves forward until it lies close to the female pronucleus. Its nucleus becomes swollen and forms the male pronucleus (Fig. 3.6); the tail detaches and degenerates. Morphologically, the male and female pronuclei are indistinguishable, and eventually, they come into close contact and lose their nuclear envelopes (Fig. 3.7A). During growth of male and female pronuclei (both haploid), each pronucleus must replicate its DNA. If it does not, each cell of the two-cell zygote has only half of the normal amount of DNA. Immediately after DNA synthesis, chromosomes organize on the spindle in preparation for a normal mitotic division. The 23 maternal and 23 paternal (double) chromosomes split longitudinally at the centromere, and sister chromatids move to opposite poles, providing each cell of the zygote with the normal diploid number of chromosomes and DNA (Fig. 3.6D,E). As sister chromatids move to opposite poles, a deep furrow appears on the surface of the cell, gradually dividing the cytoplasm into two parts (Figs. 3.6F and 3.7B).
A. Phase contrast view of the pronuclear stage of a fertilized human oocyte with male and female pronuclei. B. Two-cell stage of human zygote.