Medical Physiology, 3rd Edition

The Ovarian Cycle: Folliculogenesis, Ovulation, and Formation of the Corpus Luteum

Female reproductive life span is determined by the number of primordial follicles established during fetal life

Unlike the male—which produces large numbers of mature gametes (sperm) continuously beginning at puberty and for the remainder of the man's life—the female has a limited total number of gametes, determined by the number of oocytes formed during fetal life (see p. 1078). Oocyte maturation—the production of a haploid female gamete capable of fertilization by a sperm—begins in the fetal ovary. Beginning at around the fourth week of gestation, primordial germ cells migrate from the endoderm of the yolk sac to the gonadal ridge (see Fig. 53-4B and C ), where they develop into oogonia—immature germ cells that proliferate by mitosis.

Primary Oocytes

By ~8 weeks' gestation, ~300,000 oogonia are present in each ovary. At around this time, some oogonia enter prophase of meiosis I and become primary oocytes (Fig. 55-10A ). From this point onward, the number of germ cells is determined by three ongoing processes: mitosis, meiosis, and death by apoptosis (see p. 1241). By 20 weeks, all the mitotic divisions of the female germ cells have been completed, and the total number of germ cells peaks at 6 to 7 million. All oogonia that have not already entered prophase of meiosis I by the 28th to 30th week of gestation die by apoptosis. The oocytes then arrest in the diplotene stage of prophase I. This prolonged state of meiotic arrest is known as the dictyotene state, which lasts until just before ovulation many years later, when the meiosis resumes and the first polar body is extruded. The second meiotic division occurs at syngamy (see p. 1072), at which stage maturation of the haploid oocyte is complete.

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FIGURE 55-10 Maturation of the ovarian follicle.

Primordial Follicles

In the fetal ovary, dictyotene oocytes are surrounded by a single layer of flat, spindle-shaped pregranulosa cells to form a primordial follicle (see Fig. 55-10B ). Each primordial follicle is 30 to 60 µm in diameter and enclosed by a basement membrane. By the 30th week of gestation, the ovaries contain around 5 to 6 million primordial follicles. Unlike male gametes, new oocytes cannot form after this time because all gametogenic stem cells, in this case oogonia, have either died or entered meiosis. Therefore, by midgestation, the female gamete endowment is established. For the remainder of the female's life, the number of primordial follicles gradually decreases. One reason for the decline is that primordial follicles undergo a relentless process of apoptosis that begins at midgestation and ends at menopause when the endowment of primordial follicles is virtually exhausted. This progressive exhaustion is independent of gonadotropic hormones and is unaffected by pregnancy or the use of oral contraceptives. In addition, after puberty, each month a cohort of 10 to 30 primordial follicles is recruited to enter the irreversible process of folliculogenesis, which culminates in either ovulation (rupture of the follicle and expulsion of the ova) or atresia (a coordinated process in which the oocyte and other follicle cells undergo apoptosis, degeneration, and resorption). The mechanism by which some primordial follicles initiate folliculogenesis whereas others remain dormant is not known. Thus, even though the ovaries are invested with ~7 million oogonia at midgestation, the pool of primordial follicles is continually depleted, so that ~1 million exist at birth, ~300,000 remain at puberty, and there are virtually none at menopause. Of the ~300,000 primordial follicles present at puberty, only 400 to 500 are destined for ovulation between puberty and menopause (e.g., 12 per year for 40 years). Another 5000 to 15,000 are part of the monthly cohorts that undergo atresia. However, the vast majority of primordial and primary ovarian follicles are lost as a result of the rapid, continuous process of atresia during the reproductive life of the individual.

The female gametes are stored in the ovarian follicles—the primary functional units of the ovary. Over the course of a female's life, 90% to 95% of all primordial follicles never progress into folliculogenesis. Primordial follicles are dormant for most of their life. At any given time, a small proportion of primordial follicles begins a series of changes in size, morphology, and function referred to as folliculogenesis—the central event in the human female reproductive system. Folliculogenesis—controlled by intrinsic factors within the ovary and by the gonadotropins (FSH and LH)—occurs by three processes: (1) enlargement and maturation of the oocyte, (2) differentiation and proliferation of granulosa and theca cells, and (3) formation and accumulation of a fluid.

Primary Follicles

The first step in folliculogenesis is the emergence of a primordial follicle from its quiescent state to become a primary follicle (see Fig. 55-10C ). This process involves proliferation of granulosa cells and their differentiation from flattened pregranulosa cells to cuboidal cells. In addition, the oocyte increases in size and forms the zona pellucida—a glycoprotein shell surrounding the plasma membrane of the oocyte.

Secondary Follicles

The further proliferation of granulosa cells and the appearance of the theca-cell layer converts the primary follicle into a secondary follicle (see Fig. 55-10D ). Secondary follicles contain a primary oocyte surrounded by several layers of cuboidal granulosa cells. In addition, cells in the ovarian stroma surrounding the follicle are induced to differentiate into theca cells that populate the outside of the follicle's basement membrane. The oocyte increases in size to a mean diameter of ~80 µm and the follicular diameter grows to 110 to 120 µm. As the developing follicle increases in size—becoming a late-stage secondary follicle—the number of granulosa cells increases to ~600 and the theca cells show increasing differentiation to form the theca interna layer closest to the granulosa and the theca externa that compresses the surrounding ovarian stroma. Progression to secondary follicles also involves the formation of a blood supply from arterioles that terminate in a wreath-like network of capillaries adjacent to the basement membrane surrounding the granulosa-cell layer, which remains avascular. The theca cells proliferate and acquire LH receptors, as well as the ability to synthesize steroids. Gap junctions also form between the oocyte and the adjacent layer of granulosa cells and between granulosa cells. The oocyte-granulosa junctions may function as thoroughfares to transport nutrients and information from the granulosa cells to the oocyte and vice versa. The granulosa cells in this context are analogous to the Sertoli cells (see pp. 1101–1102) in that they nurse the gamete and act as the barrier between the oocyte and the blood supply.

Tertiary Follicles

The next stage of follicular growth is the maturation of secondary follicles into tertiary follicles (see Fig. 55-10E ) as the increasingly abundant granulosa cells secrete fluid into the center of the follicle to form a fluid-filled space called the antrum. Tertiary follicles represent the first of two antral stages (the second being the graafian follicle, below). FSH induces the transition of preantral secondary follicles to antral tertiary follicles.

Graafian Follicles

As the antrum enlarges, it nearly encircles the oocyte, except for a small mound or cumulus that attaches the oocyte to the rest of the follicle. At this second antral stage, the diameter of the follicle increases to 20 to 33 mm and it is called a preovulatory or graafian follicle (see Fig. 55-10F ).

The granulosa cells of the tertiary and graafian follicles are of three types: (1) Mural granulosa cells, which are the farthest from the center of the follicle, are the most metabolically active and contain large quantities of LH receptors and enzymes that are necessary for the synthesis of steroids. (2) Cumulus granulosa cells are shed with the oocyte at the time of ovulation. (3) Antral granulosa cells, which face the antrum, are left behind within the follicle to become the large luteal cells of the corpus luteum. The capacity of the three types of granulosa cells to generate steroids differs. Cumulus cells contain neither the side-chain-cleavage enzyme (P-450SCC) nor aromatase (P-450arom) and therefore cannot generate estrogens. Moreover, cumulus cells respond less to LH and have a low overall LH receptor content. The exact role of the cumulus layer has not been definitively established, although investigators have postulated that the cumulus layer may function as a feeder layer and may provide stem cells that differentiate into other granulosa-cell types.

The antral fluid provides a unique environment for oocyte growth and development. It facilitates the release of the oocyte-cumulus at the time of ovulation and serves as a medium for nutrient exchange and waste removal in the avascular compartment. The accumulation of antral fluid is a major factor in the formation of the dominant follicle. Between 5 and 6 days before ovulation the dominant follicle undergoes accelerated expansion, forming a cystic bulge on the surface of the ovary. After this final phase of growth, the folliclenow a graafian follicle—is prepared for ovulation (see Fig. 55-10G ).

The oocyte grows and matures during folliculogenesis

The principal role of folliculogenesis is to produce a mature oocyte that is capable of fertilization and formation of an embryo. The oocyte contributes the majority of the cytoplasmic and nuclear factors needed for embryo development, and these factors are not completely established until after the secondary follicle stage (see Fig. 55-10D ). In addition, oocyte growth and maturation benefits from the gap junctions that connect cumulus granulosa cells to the oocyte, permitting the bidirectional exchange of nutrients, growth factors, and other molecules. Oocyte growth and maturation includes formation of the zona pellucida, formation of increased numbers of mitochondria, acquisition of competence to complete meiosis I. During maturation, the oocyte also re-establishes genomic imprints. Genomic imprinting (see p. 94) is the process by which certain genes—about 1% of the genome—are silenced; particular genes are silenced only in female gametes and others, only in male gametes. image N55-6

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Genomic Imprinting

Contributed by Emile Boulpaep, Walter Boron, Sam Mesiano

Genomic imprinting is the process by which certain genes—about 1% of the genome—are silenced; particular genes are silenced only in female gametes and others, only in male gametes. Thus, these genes are expressed in a manner specific to the parent of origin.

Note that a female diploid oogonium has some paternal genes imprinted or silenced (i.e., only the maternal gene is active) and some maternal genes silenced (i.e., only the paternal gene is active). When the 2N genome splits into two 1N genomes, it is important that all genes in the oocyte have the female pattern of imprinting, which occurs during oocyte maturation.

Failure of proper genomic imprinting causes aberrant gene expression and is associated with several human diseases, including Beckwith-Wiedemann, Prader-Willi, and Angelman syndromes. For example, the IGF2 gene is normally maternally imprinted (i.e., silenced). In Beckwith-Wiedemann syndrome, the material IGF2 gene becomes reactivated (by removal of methyl tags) during oocyte formation in the mother or early embryonic development. The result is that the offspring has two (rather than one) active copies of IGF2 and thus excess IGF2 protein. The most obvious sign is macrosomia (large body size) in the newborn.

References

Wikipedia. s.v. Genomic imprinting.  http://en.wikipedia.org/wiki/Genomic_imprinting [Accessed March 20, 2015].

FSH and LH stimulate the growth of a cohort of follicles

As described above, the development of primordial follicles to secondary follicles occurs continually from fetal life until menopause. However, almost all of these follicles undergo atresia (death of the ovum, followed by collapse of the follicle and scarring) at some stage in their development. This gonadotropin-independent folliculogenesis and atresia is thought to be controlled by factors within the ovary, and especially between somatic cells and the oocyte, acting in a paracrine manner. Some key factors in this process are activin A, the forkhead transcription factor FOXO3, basic fibroblast growth factor, and kit ligand.

At the time of puberty, increased levels of FSH and LH stimulate cohorts of secondary follicles to progress to the tertiary and preovulatory stages. Along the course of this development, most follicles undergo atresia until one dominant graafian follicle remains at the time of ovulation. Controversy exists about the length of this developmental process. Some believe that the entire developmental process occurs over three to four monthly cycles, so that the graafian follicle of the present ovulatory cycle was part of a cohort of secondary follicles recruited three to four cycles earlier. An alternative view is that FSH and LH induce the recruitment of a cohort of follicles during the end of one cycle, and one of these follicles develops into the dominant graafian follicle in the next cycle. In any case, FSH is necessary for continued development of follicles beyond the secondary stage, and only a portion of the cohort of follicles continues to develop in response to FSH and LH. The other follicles undergo atresia.

Each month, one follicle achieves dominance

Although the mechanism of selection of the dominant follicle is not completely understood, it is thought to be caused by estrogen-induced events within the follicles. As estrogen levels rise during the follicular phase of the cycle, the pituitary gradually lowers its secretion of FSH (see Fig. 55-6). Rising inhibin levels also feed back on the anterior pituitary to decrease FSH secretion. Peak inhibin levels correlate with the number of follicles present and rise in parallel with circulating estradiol levels.

Decreased levels of FSH cause a decline in FSH-dependent aromatase activity in granulosa cells (see p. 1117), which results in a decrease in estradiol production in the less-mature follicles (see Fig. 55-8). Conversely, estrogen increases the effectiveness of FSH in the more mature follicles by increasing the number of FSH receptors. The dominant follicle therefore has more FSH receptors, a greater rate of granulosa-cell proliferation, more FSH-dependent aromatase activity, and more estrogen production than the less dominant follicles. Because the less dominant follicles have less aromatase activity, they convert less androstenedione to estradiol. Thus, the weak androgen androstenedione either builds up or is converted to other androgens. As a result, the less dominant follicles have a lower estrogen/androgen ratio than the dominant follicle, and they undergo atresia under the influence of androgens in their local environment. In contrast, the production of estradiol and inhibins allows the dominant follicle to become prominent and to gain an even greater edge over its competitors. The vascular supply to the theca of the dominant follicle also increases rapidly, which may allow greater FSH delivery to the dominant follicle and thus help to maintain dominance of the follicle selected for ovulation.

Estradiol secretion by the dominant follicle triggers the LH surge and thus ovulation

Ovulation occurs at the midpoint of every normal menstrual cycle, triggered by the LH surge, which in turn is stimulated by rapidly rising levels of estradiol. Estradiol secretion by the dominant follicle increases rapidly near the end of the late follicular phase (see Fig. 55-6). This dramatic rise in circulating estradiol switches the negative-feedback response of estradiol on the hypothalamus and anterior pituitary to a positive-feedback response and also sensitizes the anterior pituitary to GnRH. The result is the LH surge, which generally begins 24 to 36 hours after peak estradiol secretion. Ovulation usually occurs ~36 hours after onset of the LH surge, and ~12 hours after its peak. Thus, it appears that the developing follicle, through its increased estradiol secretion, signals the hypothalamic-pituitary system that follicular maturation is complete and that the hypothalamic-pituitary axis can now release a bolus of gonadotropin to induce ovulation. The LH surge appears to terminate in part as a result of rising levels of progesterone, through negative feedback, and in part as a result of loss of the positive feedback that is derived from estradiol. Depletion of gonadotropin stores in the anterior pituitary gland may also contribute to termination of the LH surge.

At the time of the LH surge, the primary oocyte (4N DNA), which had been arrested in the prophase of its first meiotic division since fetal life (see Fig. 53-2C ), now resumes meiosis and completes its first meiotic division several hours before ovulation. The result of this first meiotic division is a small first polar body, which degenerates or divides to form nonfunctional cells, and a much larger secondary oocyte. Both the first polar body and the secondary oocyte, like secondary spermatocytes (see p. 1100), have a haploid number of duplicated chromosomes (2N DNA): 22 duplicated somatic chromosomes and 1 duplicated X chromosome. This secondary oocyte begins its second meiotic division, but it becomes arrested in metaphase until the time of fertilization (see pp. 1131–1132). The secondary oocyte is surrounded by the zona pellucida and one or more layers of follicular cells, the corona radiata. Before ovulation, the cumulus oophorus expands under the influence of LH, and eventually the oocyte and its surrounding cells break free from the inner follicular-cell layer and, with their “stalk,” float inside the antrum, surrounded by follicular fluid. Breaking away of the oocyte-cumulus complex is probably facilitated by increased hyaluronidase synthesis that is stimulated by FSH.

Release of the oocyte from the follicle—ovulation—follows thinning and weakening of the follicular wall, probably under the influence of LH, progesterone, and prostaglandins (particularly those in the E and F series). These agents enhance the activity of proteolytic enzymes (e.g., collagenase) within the follicle, which leads to the digestion of connective tissue in the follicular wall. Ultimately, a stigma—or spot—forms on the surface of the dominant follicle, in an area devoid of blood vessels. As this stigma balloons out under the influence of increased follicular pressure and forms a vesicle, it ruptures and the oocyte is expelled.

The expelled oocyte, with its investment of follicular cells, is guided toward the fallopian tube by the fimbriae that cover the surface of the nearby ovary (see Fig. 55-1). The oocyte is then transported through the infundibulum into the ampulla by ciliary movement of the tubal epithelium, as well as by muscular contractions of the tube. Fertilization, if it occurs, takes place in the ampullary portion of the fallopian tube. The resulting zygote resides in the ampulla for ~72 hours, followed by rapid transport through the isthmus to the uterine cavity, where it floats free for an additional 2 to 3 days before attaching to the endometrium.

After ovulation, theca and granulosa cells of the follicle differentiate into theca-lutein and granulosa-lutein cells of the corpus luteum

After expulsion of the oocyte, the remaining follicular granulosa and theca cells coalesce into folds that occupy the follicular cavity and, under the influence of LH, undergo a phenotypic transformation to form the corpus luteum—a temporary endocrine organ whose major product is progesterone. The mature corpus luteum is composed of two cell types, granulosa-lutein cells (also known as large luteal cells) derived from the granulosa cells, and theca-lutein cells (also known as small luteal cells) derived from the theca cell. The corpus luteum is highly vascularized, consistent with its primary function as an endocrine organ. During the early luteal phase, progesterone and estradiol produced by the corpus luteum exert negative feedback on the hypothalamic-pituitary axis to suppress gonadotropin secretion and thus inhibit folliculogenesis. If pregnancy is not established, the corpus luteum regresses ~11 days after ovulation. One possible mechanism for this regression—or luteolysis—is that withdrawal of trophic support results in demise of the corpus luteum. A second possibility is that local factors, such as prostaglandin F produced by the endometrium, inhibit luteal function and terminate the life of the corpus luteum.

Growth and involution of the corpus luteum produce the rise and fall in estradiol and progesterone during the luteal phase

Although the corpus luteum produces both estradiol and progesterone, the luteal phase is primarily dominated by progesterone secretion. Estradiol production by the corpus luteum is largely a function of the theca-lutein cells, which also produce androgens. Progesterone production in the corpus luteum is primarily a function of the granulosa-lutein cells (see Fig. 55-9), which also produce estradiol.

Progesterone production rises modestly before follicular rupture but increases sharply after ovulation, peaking in ~7 days. Progesterone acts locally to inhibit follicular growth during the luteal phase. In addition, progesterone may act centrally by inhibiting gonadotropin secretion. Progesterone is also an antiestrogen in that it inhibits expression of ERs, thereby reducing estrogen responsiveness. The net effect is that increasing progesterone production suppresses folliculogenesis.

Estradiol levels also rise during the luteal phase (see Fig. 55-6), which reflects production by the corpus luteum. Because estrogens induce expression of PRs in target cells, the estradiol produced during the luteal phase is necessary for progesterone-induced changes in the endometrium.

Unless rescued by hCG—produced by the syncytial trophoblasts of the blastocyst (see p. 1136)—luteal production of progesterone ceases toward the end of the menstrual cycle. hCG produced by the developing conceptus maintains steroidogenic function of the corpus luteum until approximately the ninth week of gestation, at which time placental function is well established. If not rescued by pregnancy, the hormone-producing cells of the corpus luteum degenerate and leave behind a fibrotic corpus albicans.

LESION

REFLEXOGENIC ERECTION

PSYCHOGENIC ERECTION

EFFECT ON EJACULATION

Upper motor neuron

Present

Absent

Significantly impaired

Lower motor neuron

Absent

Present

Less impaired