The ovarian cycle is divided into a follicular and a luteal phase (Figure 9–4).
The follicular phase begins on day 1 of the cycle, the first day of menses, and corresponds to the growth and development of a dominant follicle. Throughout the reproductive life span of the woman (from puberty to menopause), a single mature oocyte is produced each month. Most of the human oocytes (germ cells) present during uterine development are destined to undergo apoptosis, or programmed cell death. Only follicles that are responsive to FSH stimulation (approximately 350) will enter the final stage of development and progress to ovulation. At mid cycle (day 14), the rising levels of estrogen stimulate a surge in LH release, which stimulates ovulation 24–36 hours later.
The luteal phase begins after ovulation, with the reorganization of the remnants of the ovulatory follicle and the formation of the corpus luteum (see Figure 9–4). The corpus luteum is a transient endocrine organ that produces progesterone, and to a lesser extent estradiol and inhibin A. The corpus luteum is under LH regulation. The luteal phase ends when there is no fertilization and placental-mediated survival of the corpus luteum discussed below.
Ovarian Regulation of Gonadotropin Release
Gonadotropin release is under negative and positive feedback regulation by estradiol, progesterone, and inhibins A and B. Progesterone and 17β-estradiol act both in the hypothalamus and the pituitary, and inhibins act at the level of the pituitary (see Figure 9–2). The contributions of these ovarian hormones vary according to the stage of the ovarian cycle.
During this phase, the dominant follicle produces high concentrations of 17β-estradiol and inhibin B. Although initially estradiol exerts negative feedback on FSH and LH release, as concentrations of estradiol increase, toward the end of the follicular phase, a switch from negative to positive feedback occurs. High estradiol levels in the hypothalamus and pituitary lead to low-amplitude, high-frequency pulses (every 90 minutes) of LH, resulting in a midcycle LH surge. The estradiol-mediated stimulation of the LH surge results from an increased responsiveness of gonadotropic cells to GnRH (following exposure to increasing estradiol levels), an increase in GnRH receptor number, and a GnRH surge, triggered by the effect on the hypothalamus of increasing estradiol concentrations (see Figures 9–2 and 9–4). Inhibin B levels rise during the follicular phase and decrease immediately before the LH peak, with a brief surge occurring 2 days after ovulation. Inhibin A levels increase in the late follicular phase to reach a peak concentration on the day of the LH and FSH surge. The concentration then falls briefly before rising to reach a maximum concentration during the midluteal phase.
The midcycle surge in LH levels induces ovulation, resumption of meiosis, and promotes the formation and survival of the corpus luteum during the luteal phase. During the luteal phase, high circulating concentrations of progesterone (produced by the corpus luteum) suppress the frequency and the amplitude of LH release, resulting in an overall decrease in LH by blocking the surges of GnRH, downregulating pituitary GnRH receptor expression, and decreasing gene expression of the α- and β-subunits of both LH and FSH. Thus, negative feedback regulation by progesterone during the luteal phase prevents a second LH surge. The marked suppression of GnRH and LH pulse frequency achieved by high progesterone levels during the luteal phase allows enrichment of gonadotroph FSH levels. Inhibin B levels remain low during the luteal phase. Inhibin A is secreted by the granulosa cells during the luteal phase, and its concentration falls during luteal regression synchronously with estradiol and progesterone, remaining low during the early follicular phase.
Unless the corpus luteum is stimulated by human chorionic gonadotropin (hCG), a placental hormone (described later), it regresses. Regression or lysis of the corpus luteum and the associated decrease in progesterone levels leads to an increase in FSH release toward the beginning of the next ovarian cycle. During the luteal-follicular transition, following the midcycle rise in FSH secretion, the inhibin B concentration rises and peaks 4 days after the peak FSH concentration is reached.
Oogenesis and Formation of the Dominant Follicle
Unlike the fetal testis, the fetal ovary begins germ cell development (oogenesis) early in fetal life. During early intrauterine development (15 weeks), primordial germ cells (oogonia) proliferate and migrate to the genital ridge. On their arrival in the fetal ovary, some of the oogonia continue mitotic proliferation and some begin to undergo apoptosis (Figure 9–5
). Some of these oogonia begin (but do not complete) meiosis and become oocytes. These cells have 2 X chromosomes. By 6 months postpartum, all oogonia have been converted to oocytes. At or near birth, the meiotic process is arrested at prophase of the first meiotic division. The oocytes remain arrested in the diplotene stage of the first meiotic prophase until they are recruited to grow and mature (by FSH) to produce an ovum or they undergo apoptosis. During the first days of postnatal life, the oocytes recruit somatic follicular cells, which are organized into a finite number of “resting” primordial follicles. Primordial follicles are composed of an outer layer of granulosa cells and a small oocyte, both enveloped in a basal lamina. The pool of primordial follicles in the female ovary reaches its maximum number at approximately 20 weeks of gestational age and then decreases in a logarithmic fashion throughout life until complete depletion occurs during menopause. When reproductive life is initiated, less than 10% of the primordial follicles are left.
Follicle growth and development. Folliculogenesis, or formation of the dominant follicle, consists of 2 stages: the gonadotropin-independent (preantral) period and the gonadotropin-dependent (antral or graafian) period. Primordial follicular growth up to the antral stage occurs during fetal life and infancy and is gonadotropin independent. The final developmental phase of follicular growth, in which antral follicles are protected from apoptosis, begins approximately 85 days before ovulation. One dominant follicle is recruited in the luteal phase of the cycle preceding ovulation. Atr, atretic; FSH, follicle-stimulating hormone; LH, luteinizing hormone.
Folliculogenesis, or formation of the dominant follicle, consists of 2 stages: the gonadotropin-independent (preantral) period and the gonadotropin-dependent (antral or graafian) period. Primordial follicular growth up to the antral stage (up to 0.2 mm) occurs during fetal life and infancy and is gonadotropin independent (see Figure 9–5). Primary follicles are formed when the flattened epithelial cells become cuboidal and undergo mitosis. The antral follicle growth phase is characterized by granulosa cell proliferation, expression of FSH and steroid hormone receptors, and association of the theca cells with the growing follicle and granulosa cells, leading to formation of the secondary follicles. Tertiary follicles are formed following further theca cell hypertrophy and development. Their antrum is filled with estrogen-rich fluid, and the theca interstitial cells start to express FSH and LH receptors. The mechanisms that trigger initiation of follicular growth are still not completely understood, but are thought to involve bidirectional communication between germ and somatic cells through gap junctions and paracrine factors, including cytokines and growth factors (insulin-like growth factor 1 [IGF-1], epidermal and fibroblast growth factors, and interleukin 1β). When follicles reach a size of 2–5 mm, approximately 50% enter the selection growth phase and are rescued from apoptosis. This final developmental phase of follicular growth begins approximately 85 days before ovulation in the luteal phase of the cycle preceding ovulation (see Figure 9–5).
During this gonadotropin-dependent growth phase, follicles grow exponentially, and FSH stimulates estrogen production from granulosa cells, follicular fluid formation, cell proliferation, and LH receptor expression in the dominant follicle. Selection of a dominant follicle is dictated by sensitivity to FSH action, which is locally modulated by antimüllerian hormone (AMH).
AMH or müllerian-inhibiting substance discussed in Chapter 8 as it pertains to sex differentiation in the male embryo, is a peptide growth factor and member of the large transforming growth factor beta family of growth and differentiation factors. AMH is expressed in the granulosa cells of the recruited primordial follicles, and continues to be expressed in the growing follicles that have undergone recruitment from the primordial follicle pool but that have not been selected for dominance. This pattern of expression suggests an important role in the regulation of both the number of growing follicles and their selection for ovulation. Because the number of growing follicles is correlated to the size of the primordial follicle pool size, a marker such as AMH that reflects all follicles that have made the transition from the primordial follicle pool to the growing pool has been proposed to be a good indirect marker of ovarian reserve. FSH and inhibin B levels have also been proposed to serve as predictors of the ovarian reserve.
The average time between primary follicle development and ovulation is 10–14 days (see Figure 9–5).
During follicular recruitment, the oocyte enters a growth phase that leads to the completion of the first meiotic division. The resumption of meiosis is mediated by the midcycle surge in LH. LH acts on mature follicles to terminate the program of gene expression associated with folliculogenesis. The transcription of genes that control granulosa cell proliferation (ie, IGF-1, FSH and estrogen receptors, cyclin D2) and those that encode steroidogenic enzymes is rapidly turned off by LH-mediated increases in intracellular cAMP. In addition, LH induces genes involved in ovulation (ie, progesterone receptor, cyclooxygenase-2) and luteinization (ie, cell cycle inhibitors, steroidogenic enzymes, transcription factors, and protein kinases). At this stage, mRNA synthesis virtually ceases and does not resume until 1–3 days after the egg has been fertilized, when the final phases of meiosis are completed. One preovulatory follicle is selected every cycle, approximately 350 times during the female reproductive life span.
The surge of LH induces follicular rupture and ovulation, releasing the oocyte and corona radiata into the peritoneal cavity, close to the opening of the fallopian tubes (Figure 9–6
). Follicle rupture is an inflammatory process that involves cyclooxygenase-2, plasminogen activator, and metalloproteinases. Ciliary movement on the mucous membrane of the fimbria aids movement of the ovum into the fallopian tubes. Throughout the preovulatory stage, the oocyte, granulosa cells, and theca cells acquire specific functional characteristics: The oocyte becomes competent to undergo meiosis; granulosa cells acquire the ability to produce estrogens and respond to LH via the LH receptor; and theca cells begin to synthesize increasing amounts of androgens that serve as substrates for the aromatase enzyme in the granulosa cells.
Fertilization and embryo migration. Sperm binds to the zona pellucida and undergoes the acrosome reaction, releasing its enzymatic contents, which are necessary for penetration of the zona pellucida. In addition, cortical granules in the ovum release their contents, preventing multiple sperm from fertilizing 1 ovum. Once the sperm penetrates the zona pellucida and begins entry into the perivitelline space, the sperm repositions itself from its original orientation with binding at the tip of the head to binding in an equatorial or sideways position, leading to fusion with the egg plasma membrane and formation of the zygote. This leads to completion of the meiotic division and initiation of mitotic divisions while the zygote is being propelled through the fallopian tubes through both ciliary movements by the epithelium and rhythmic contractions of the smooth muscle walls. The embryo enters the uterine cavity (where implantation occurs) as a blastocyst on day 4 following fertilization.
Formation of the Corpus Luteum
Following ovulation, the reorganization of the follicle leads to formation of the corpus luteum, composed of small (theca) and large (granulosa) cells, fibroblasts, endothelial cells, and immune cells. Small amounts of bleeding into the antral cavity occurring during ovulation lead to the formation of the corpus hemorrhagicum and the invasion by macrophages and mesenchymal cells, leading to revascularization of the corpus luteum. The corpus luteum, a temporary endocrine gland, continues to produce and secrete progesterone and estradiol, playing a key role in regulating the length of the ovarian cycle, maintaining gestation in its early stages, and suppressing LH and FSH release through the inhibition of GnRH release.
If fertilization occurs, the corpus luteum continues to grow and function for the first 2–3 months of pregnancy. Regression of the corpus luteum is prevented by placental production of hCG during the initial gestational period. hCG stimulates the granulosa-lutein cells to produce progesterone, 17-hydroxyprogesterone, estrogen, inhibin A, and relaxin, a polypeptide hormone from the insulin/IGF hormone family. Relaxin regulates the synthesis and release of metalloproteinases, mediators of tissue (uterus, mammary gland, fetal membranes, birth canal) growth and remodeling, in preparation for birth and lactation. After the first trimester of pregnancy, the corpus luteum slowly regresses as the placenta assumes the role of hormone biosynthesis for the maintenance of pregnancy.
Luteolysis is the process of lysis or regression of the corpus luteum if fertilization does not occur within 1–2 days of ovulation. Luteolysis marks the end of the female reproductive cycle and involves an initial decline in progesterone secretion (functional luteolysis), followed by changes in the cellular structure leading to gradual corpus luteum involution (structural or morphologic luteolysis) to form a small scar of connective tissue known as the corpus albicans.