The Hypothalamic-Pituitary-Ovarian Axis and Control of the Menstrual Cycle

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Chapter 1 The Hypothalamic-Pituitary-Ovarian Axis and Control of the Menstrual Cycle

Neal Gregory Mahutte, Sophia Ouhilal

INTRODUCTION

The hypothalamus and pituitary gland form a unit that exerts control over a wide range of endocrine organs, including the gonads. This chapter describes the hypothalamic-pituitary-ovarian axis and control of the menstrual cycle, which is modulated by the central nervous system, other endocrine systems, and the environment. Key hormones in the hypothalamic-pituitary-ovarian axis include gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol, and progesterone (Table 1-1). Supporting roles are also played by inhibin, activin, follistatin, and endorphins.

Table 1-1 Major Hormones of the Hypothalamic-pituitary-ovarian Axis

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THE HYPOTHALAMUS

The hypothalamus forms the lower part of the lateral wall and the floor of the third ventricle, and weighs approximately 10 grams. The hypothalamus is typically divided into eight specific nuclei (consistently clustered groups of neurons) and three areas (less clustered, less distinctly demarcated neurons), as illustrated in Figure 1-1. From a reproductive standpoint, the most important of these are the arcuate nucleus and the preoptic area, the principal sites of GnRH-producing neurons.1 The arcuate nucleus is located in the medial basal hypothalamus and is the most proximal of all the hypothalamic nuclei to the optic chiasm and the pituitary stalk. The arcuate nucleus is also the site of dopamine-secreting neurons that function to inhibit pituitary prolactin secretion and neurons that secrete growth hormone-releasing hormone.

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Figure 1-1 Illustration of the hypothalamus, pituitary, sella turcica, and portal system. The arcuate nucleus is the primary site of neurons that produce gonadotropin-releasing hormone (GnRH). GnRH is released from the median eminence into the portal system. The blood supply of the pituitary gland derives from the internal carotid arteries. In addition to the arcuate nucleus, the other hypothalamic nuclei are SO, supraoptic nucleus; SC, suprachiasmatic nucleus; PV, paraventricular nucleus; DM, dorsal medial nucleus; VM, ventromedial nucleus; PH, posterior hypothalamic nucleus; PM, premammillary nucleus; LM, lateral mammillary nucleus; MM, medial mammillary nucleus. The three hypothalamic areas are PA, preoptic area; AH, anterior hypothalamic area; DH, dorsal hypothalamic area.

Neurosecretory cell products from the hypothalamus, including GnRH, are released into the portal system from the median eminence, a prominence in the pituitary stalk at the floor of the third ventricle. The portal system serves as the major route of communication between the hypothalamus and the anterior pituitary. In contrast, the pituitary stalk (infundibulum) directly connects neuronal cell bodies in the hypothalamus to the posterior pituitary. The pituitary stalk lies immediately posterior to the optic chiasm.

GnRH

GnRH is the primary hypothalamic regulator of pituitary reproductive function. Two human forms of GnRH (GnRH-I and GnRH-II) have been identified.2,3 Both are decapeptides and are the products of different genes. At least 20 other types of GnRH have been identified in fish, amphibians and protochordates, but none of these are believed to be present in humans.4,5

GnRH-I was first characterized and synthesized in 1971 by Andrew Schally and Roger Guillemin, an accomplishment for which both men ultimately received the Nobel prize.6–9 The structure of GnRH-I is common to all mammals, and its action is similar in males and females (Fig. 1-2). GnRH-I is synthesized from a much larger, 92-amino acid precursor peptide that contains GnRH-associated peptide.10 GnRH-I then travels along an axonal pathway called the tuberoinfundibular tract to the median eminence of the hypothalamus, where it is released into the portal circulation in a pulsatile fashion. The half-life of GnRH-I is very short (2 to 4 minutes) because it is rapidly cleaved between amino acids 5 and 6, 6 and 7, and 9 and 10. Because of its short half-life and rapid dilution in the peripheral circulation, serum GnRH-I levels are difficult to measure and do not correlate with pituitary action.

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Figure 1-2 Structure of GnRH-I.

GnRH-I has three principal actions on anterior pituitary gonadotrophs: (1) synthesis and storage of gonadotropins, (2) movement of gonadotropins from the reserve pool to a point where they can be readily released, and (3) direct secretion of gonadotropins. GnRH-I pulses occur in response to intrinsic rhythmic activity within GnRH neurons in the arcuate nucleus. Pulsatile release of GnRH-I from the median eminence within a critical frequency and amplitude results in normal gonadotropin secretion.11,12 Continuous, rather than pulsatile, exposure to GnRH-I results in suppression of FSH and LH secretion and suppression of gonadotropin gene transcription.13,14

In the absence of gonadal feedback, the GnRH pulse frequency is approximately once per hour.15 During the menstrual cycle the frequency and amplitude of GnRH pulses vary in response to hypothalamic feedback (Table 1-2).16 In general, the follicular phase is characterized by high-frequency, low-amplitude pulses, and the luteal phase is characterized by lower-frequency, higher-amplitude pulses.17,18 However, considerable variability exists both between and within individuals.19 In humans, GnRH-I pulse frequency and amplitude are best approximated by measuring LH pulse frequency and amplitude.

Table 1-2 Menstrual Cycle Variation in LH Pulse Frequency and Amplitude

Cycle Phase Mean Frequency (minutes) Mean Amplitude (mIU/mL)
Early follicular 90 6.5
Mid-follicular 50 5
Late follicular 60–70 7
Early luteal 100 15
Mid-luteal 150 12
Late luteal 200 8

GnRH-II is most highly expressed outside of the brain, in tissues that include kidneys, bone marrow, and prostate. This is in contrast to GnRH-I, which is not expressed in high levels outside the brain. Although GnRH-II can induce release of both FSH and LH, it appears to have a wide array of physiologic functions outside the brain including regulation of cellular proliferation and mediation of ovarian and placental hormone secretion.20

Initial attempts in the mid-1990s to identify estrogen receptors in GnRH neurons were unsuccessful.21,22 However, subsequent use of more sophisticated techniques identified estrogen receptors α and β in the arcuate nucleus.23–26 Both receptors mediate estrogen action on GnRH neurons in vivo.27,28 The GnRH gene contains a hormone response element for the estrogen–estrogen receptor complex.29 Transcription of GnRH-I and GnRH-II is differentially regulated by estrogen.30 The regulatory role of estradiol on GnRH is complex. Estrogen inhibits GnRH gene expression/biosynthesis, but secretion of GnRH may be increased, decreased, or unaffected.31,32

The activity of the hypothalamus is further modulated by nervous stimuli from higher brain centers. GnRH neurons exhibit many connections to each other and to other neurons. Some of the neurotransmitters that modulate GnRH secretion are outlined in Table 1-3. The effects of these neurotransmitters help explain the mechanism by which certain physical or clinical conditions may affect the menstrual cycle (Table 1-4).

Table 1-3 Neurotransmitter Effects on GnRH Release

Neurotransmitter Effect
Dopamine Inhibits GnRH release
Endorphin Inhibits GnRH release
Serotonin Inhibits GnRH release
Norepinephrine, epinephrine Stimulates GnRH release

Table 1-4 Mechanisms for Oligo/amenorrhea in Various Clinical Conditions

Hyperprolactinemia Elevated dopamine suppresses GnRH
Hypothyroidism Elevated TRH increases prolactin, which in turn increases dopamine which, then suppresses GnRH
Stress Increased corticotropin (ACTH) results in increased endorphins (both are derived from the same peptide precursor); endorphins suppress GnRH
Exercise Increased endorphins suppress GnRH

TRH = thyrotropin-releasing hormone; GnRH = gonadotropin-releasing hormone.

Cells that produce GnRH originate embryologically from the olfactory area.33 GnRH neurons, like olfactory epithelial cells of the nasal cavity, have cilia.34 During embryogenesis GnRH neurons migrate from the medial olfactory placode to the arcuate nucleus of the hypothalamus.35 The common origin of GnRH and olfactory neurons is demonstrated by Kallmann’s syndrome, where GnRH deficiency is associated with anosmia. Kallmann’s syndrome is believed to be caused by a variety of gene defects that affect neuronal cell migration.36

The common origin of GnRH and olfactory neurons is also suggestive of the relationship between pheromones and menstrual cyclicity. Pheromones are small airborne chemicals that when secreted externally by one individual may be perceived by other individuals of the same species, producing a change in sexual or social behavior. It is well recognized that women who work or live together often develop synchrony of their menstrual cycles.37 Moreover, it has been shown that odorless compounds from the axillae of cycling women can alter the cycle characteristics of recipient women.38 Presumably, these alterations occur through olfactory GnRH-mediated mechanisms.

The GnRH Receptor

The GnRH-I receptor is a G-protein receptor that utilizes inositol triphosphate and diacylglycerol as second messengers to stimulate protein kinase, release of calcium ions, and cyclic adenosine monophosphate (cAMP) activity. The GnRH-I receptor is encoded by a gene on chromosome 14q21.1 and is expressed in many parts of the body outside of the brain, including ovarian follicles and the placenta. In humans it appears that GnRH-II signaling occurs through the GnRH-I receptor.5 Although a GnRH-II receptor is present in many mammalian species, its functional capacity is limited due to a frame shift and a stop codon. GnRH receptors are regulated by many substances, including GnRH itself, inhibin, activin, estrogen, and progesterone.

Changes to the amino acid sequence of GnRH can extend its half-life to hours or days and can change its biologic activity from an agonist to an antagonist. All of the GnRH agonists currently available extend their half-life by substitutions at amino acid 6 and sometimes amino acid 10 of native GnRH (Table 1-5). Continuous activation of the GnRH receptor results in desensitization due to phosphorylation and conformational change of the receptor, uncoupling from G proteins, internalization of the receptor via endocytosis, and decreased receptor synthesis.39,40 On administration all GnRH agonists increase gonadotropin secretion (the flare effect). However, after 7 to 14 days GnRH receptor desensitization occurs and pituitary suppression is achieved.

Table 1-5 Properties of Commercially Available GnRH Agonists

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In contrast, GnRH antagonists directly inhibit gonadotropin secretion. Structurally, GnRH antagonists are characterized by multiple amino acid substitutions to the natural GnRH decapeptide. The commercially available GnRH antagonists cetrorelix and ganirelix have large amino acid additions to position 1 of native GnRH. GnRH antagonists compete for and occupy pituitary GnRH receptors, thus competitively blocking endogenous GnRH–GnRH receptor binding. In contrast to GnRH agonists, there is no flare effect with GnRH antagonists. Because receptor loss does not occur, a constant supply of antagonist is necessary to ensure that all GnRH receptors are continuously occupied. Thus, the therapeutic dosage range for antagonists is typically higher than that for agonists (mg versus μg).

THE PITUITARY

The pituitary gland measures approximately 15 × 10 × 6 mm and weighs approximately 500 to 900 mg. The pituitary gland lies immediately beneath the third ventricle and just above the sphenoidal sinus in a bony cavity called the sella turcica (Turkish saddle). It consists of anterior and posterior lobes, each having different embryologic origins, functions, and control mechanisms (Fig. 1-3).

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Figure 1-3 X-ray and T1-weighted magnetic resonance images (MRI) of the pituitary gland. A, Lateral skull film with the sphenoidal sinus and sella turcica. B, Sagittal section demonstrating the relationship between the sphenoidal sinus and the pituitary gland. The normal posterior pituitary is brighter on MRI compared to the anterior pituitary. The sella turcica is not well seen on MRI. C, Sagittal section after administration of gadolinium contrast. Because the pituitary lies outside the blood-brain barrier, both the anterior and posterior pituitary are illuminated with the contrast. D, Coronal section demonstrating the relationship of the pituitary to the optic chiasm and the pituitary stalk. E, Coronal section after gadolinium contrast demonstrating the close proximity of the pituitary to the internal carotid arteries.

The secretion of pituitary hormones is controlled primarily by the hypothalamus. However, the activity of the hypothalamus and pituitary are modulated by nervous stimuli from higher brain centers and feedback from circulating hormones. There is no direct nervous connection between the hypothalamus and the anterior pituitary gland. Instead, communication occurs via the hypothalamic-pituitary portal system.

The arterial blood supply of the pituitary is derived from branches of the internal carotid artery. The anterior pituitary gland is the most richly vascularized of all mammalian tissues. The blood supply of the anterior pituitary comes from the superior hypophyseal artery. The superior hypophyseal artery forms a capillary network in the median eminence of the hypothalamus that recombines in long portal veins draining down the pituitary stalk to the anterior pituitary, where they break up into another capillary network. The blood supply of the posterior pituitary (neurohypophysis) comes from the middle and inferior hypophyseal arteries. Veins from these arteries drain into the cavernous sinuses, from which they ultimately reach the petrosal sinuses and then the jugular veins.

The posterior pituitary is best understood as a continuation of the hypothalamus. The posterior pituitary is a direct downgrowth of nervous tissue from the hypothalamus through the pituitary stalk. The posterior pituitary is composed of glial tissue and axonal termini that secrete oxytocin and arginine vasopressin (AVP, also known as antidiuretic hormone). Oxytocin is synthesized in the paraventricular nucleus of the hypothalamus, whereas AVP is synthesized in the supraoptic nucleus, just above the arcuate nucleus. Oxytocin and AVP pass down the length of the pituitary stalk and are stored in terminal parts of axons in the posterior pituitary. Both oxytocin and AVP consist of nine amino acid residues (nonapeptides). Release of oxytocin and AVP is controlled directly by nervous impulses passing down the axons from the hypothalamus.

The Anterior Pituitary

The anterior pituitary arises from an epithelial upgrowth from the roof of the primitive oral cavity (Rathke’s pouch). The anterior pituitary wraps around the posterior pituitary and constitutes two thirds of the volume of the pituitary gland. The portion of Rathke’s pouch in direct contact with the posterior pituitary develops less extensively than the rest of the anterior pituitary and is termed the pars intermedia.

The major cell types of the anterior pituitary are outlined in Table 1-6. The anterior pituitary is a classic endocrine gland in that it is composed of secretory cells of epithelial origin supported by connective tissue rich in blood and lymphatic capillaries. In accordance with their active synthetic function the endocrine cells are characterized by prominent nuclei and prolific mitochondria, endoplasmic reticulum, Golgi bodies, and secretory vesicles. Synthesis of gonadotropins takes place in the rough endoplasmic reticulum, after which the hormones are packaged within the Golgi apparatus and stored as secretory granules. In response to GnRH the secretory granules are extruded from the cell membrane. The endothelial lining of capillary sinusoids is fenestrated, facilitating the passage of pituitary hormones into the sinusoids.

Table 1-6 Major Cell Types of the Anterior Pituitary Gland

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Gonadotropins

The anterior pituitary secretes two hormones that stimulate the growth and activity of the gonads, FSH and LH. These glycoprotein hormones, termed gonadotropins, work in conjunction to stimulate secretion of steroid hormones from the ovary.

FSH

FSH is a glycoprotein with a molecular weight of approximately 29,000 daltons. It consists of both an α and β subunit. The α subunit consists of 92 amino acids stabilized by 5 disulfide bonds and is identical to the α subunit of LH, thyroid-stimulating hormone (TSH) and human chorionic gonadotropin (hCG). The β subunit contains 118 amino acids and 5 sialic acid residues. Neither subunit has any intrinsic biologic activity by itself.

The sialic acid content of FSH, LH, TSH, and hCG varies, and these differences are largely responsible for variations in half-life of the glycoprotein hormones. The liver is the major site of clearance for gonadotropins. Sialic acid prevents hepatic clearance; thus, the greater the sialic acid content, the longer the half-life.41 hCG, with 20 sialic acid residues, has the longest half-life (about 24 hours), whereas LH (1 to 2 sialic acid residues) has the shortest half-life (20 to 30 minutes). Addition of sialic acid residues in urinary-derived commercially available gonadotropins (e.g., hMG) is responsible for their longer half-life (30 hours).

In gonadotrophs of the anterior pituitary, GnRH signaling leads to transcription of the α and β subunits for both FSH and LH. The GnRH-dependent availability of the β subunits is the rate-limiting step in gonadotropin synthesis. Although both FSH and LH require GnRH stimulation, synthesis of the FSH β subunit also requires the presence of activin.42,43

Follicle-stimulating hormone plays a crucial role in follicle recruitment and selection of the dominant follicle. FSH has a trophic effect on granulosa cells in antral follicles, including the induction of aromatase activity, inhibin synthesis, and expression of LH receptors. A certain amount of FSH (the FSH threshold) is required to induce these changes in a given follicle. FSH must then remain above that threshold for folliculogenesis to continue.

In the normal menstrual cycle serum concentrations of FSH begin to rise a few days before the onset of menstruation. FSH levels plateau in the midfollicular phase and decline in the late follicular phase in response to the rise in estrogen and inhibin B. This decline contributes to the dominance of selected follicles over others. FSH levels then peak briefly during the ovulatory gonadotropin surge, after which they decline to their nadir in the luteal phase.

LH

LH is a glycoprotein with a molecular weight of 29,000 daltons. Like FSH, TSH, and hCG, it consists of both an α and β subunit. The α subunit is identical to the α subunit of FSH, TSH, and hCG. The β subunit of LH has 121 amino acids and 1 to 2 sialic acid residues.

Luteinizing hormone is synthesized in gonadotrophs of the anterior pituitary gland. Because it contains fewer sialic acid residues than FSH, LH is rapidly cleared from the circulation by the liver and kidney. For this reason LH is rapidly biosynthesized, and LH pulses occur at higher amplitude than those of FSH. It is believed that the pituitary content of LH is turned over 1 to 2 times per day.

Serum LH levels begin to rise a few days before the onset of menstruation. They increase very gradually during the follicular phase. Unlike FSH, serum LH values rise in the late follicular phase, surpassing FSH values around cycle day 9 or 10. The LH surge at midcycle is followed by a steady decline to their nadir in the midluteal phase.

FSH and LH Receptors

The actions of both LH and FSH are mediated by G protein receptors on the cell membrane. LH receptors are found exclusively on theca cells in the ovary. Stimulation of these receptors by increased cytochrome P450c17 enzyme activity (17-hydroxylase and 17,20-lyase) in theca cells, resulting in activation of both adenylate cyclase and cAMP-dependent protein kinases and leading to increased production of androstenedione and testosterone.

In contrast, FSH receptors are found exclusively on granulosa cells in the ovary. FSH binds to receptors on the cell surface of granulosa cells in antral follicles. Like LH, FSH acts via the cAMP-dependent protein kinase pathway.44 In response to FSH the androgens produced as a result of LH stimulation are then aromatized to estrogens in granulosa cells.

Opioid Modulation of Pituitary Hormones

Opioids (i.e., endogenous opiates) are natural occurring sedative narcotics produced in the brain whose structure and function are similar to opium. Opioids include enkephalins, endorphins, and dynorphins; they modulate every pituitary hormone by acting on the hypothalamus. An important action of opioids is to inhibit gonadotropin secretion by suppressing GnRH release.45

Opioid tone is an important regulator of menstrual cyclicity.46–49 Endorphins are at a nadir in the early follicular phase (menstruation) and gradually rise to peak levels in the luteal phase in response to the rise in estrogen and progesterone. It is believed that opioids mediate the negative feedback of ovarian steroids on gonadotropin release, particularly in the luteal phase.50

Endogenous opioids appear to play a central role in hypothalamic amenorrhea. Treatment of women suffering from this condition with an opioid receptor antagonist (e.g., naltrexone) results in the return to ovulatory menstrual patterns and even conception in some cases.51,52 Women with stress-related amenorrhea demonstrate increased hypothalamic corticotropin-releasing hormone and pituitary corticotropin, which manifests as hypercortisolism.53 The corticotropin precursor peptide, pro-opiomelanocortin, is also the precursor for endorphin synthesis. It is hypothesized that stress-related amenorrhea is the result of GnRH inhibition secondary to increased production of endogenous opioids. Opioids also rise during exercise (“runners’ high”), and this may contribute to hypothalamic amenorrhea in athletes.54,55

Ovarian Peptide Hormone Feedback on Gonadotropin Secretion

The ovary secrets two polypeptide hormones that inhibit or stimulate FSH secretion by the anterior pituitary. Inhibin and activin act as opposing nonsteroidal gonadal hormones that regulate FSH synthesis and secretion by the pituitary. They also have paracrine effects within the ovary, where they modulate follicle growth and steroidogenesis. Follistatin is a binding protein that modulates the effects of activin but not inhibin.

Inhibin and Activin

Inhibin and activin are members of the transforming growth factor-β (TGF-β) superfamily of ligands, which includes müllerian inhibiting substance (MIS; see Chapter 2). Like the gonadotropins, inhibin and activin are comprised of two subunits. Inhibin is comprised of an α and β subunit and has been isolated in two forms containing different β subunits, Inhibin-A and Inhibin-B. Activin is comprised of two beta subunits identical to those found in inhibin.

Inhibin is secreted by granulosa cells in response to FSH.56 However, mRNA for inhibin has also been found in pituitary gonadotrophs. Inhibin selectively inhibits FSH but not LH secretion.57 Thus, a negative feedback loop is created where FSH stimulates inhibin and in turn inhibin suppresses FSH.

Inhibin B is predominantly secreted in the follicular phase of the menstrual cycle, whereas inhibin A is predominantly secreted in the luteal phase.58 Peak levels of inhibin B in the follicular phase are in the range of 50 to 100 pg/mL. Peak levels of inhibin A in the luteal phase are between 40 and 60 pg/mL.

Activin is also secreted by granulosa cells. Activin augments the secretion of FSH by the pituitary by enhancing GnRH receptor formation. Activin is also required for synthesis of the FSH β subunit.

In the ovary, activin augments FSH action and stimulates production of follistatin by a paracrine effect. These effects of activin are blocked by both inhibin and follistatin. With increased GnRH stimulation, activin is increasingly antagonized by inhibin and bound by follistatin.

Follistatin

Follistatin is an activin-binding protein that sequesters activin. Follistatin is produced in the same tissues as activin, including the pituitary, and regulates activin’s local paracrine and autocrine actions. In the circulation, follistatin binds the majority of activin, thereby inhibiting FSH secretion. This suggests that activin’s primary actions are paracrine in nature. Follistatin does not bind inhibin, thus allowing it to act as a conventional endocrine hormone between the ovary and the pituitary in addition to its local ovarian effects.

OVARIAN STEROIDOGENESIS DURING THE MENSTRUAL CYCLE

Ovarian steroidogenesis during the menstrual cycle occurs in granulosa and theca cells (Table 1-7 and Fig. 1-4). Before ovulation, theca cells are separated from granulosa cells in the same follicle by a basal membrane. Thus, granulosa cells of preovulatory follicles do not have a blood supply. However, at the time of the LH surge the preovulatory follicle undergoes luteinization with disappearance of the basal membrane and capillary invasion of the granulosa cells. Theca cells become theca-lutein cells, and granulosa cells become granulosa-lutein cells.

Table 1-7 Site of Synthesis of Major Steroidogenic Products of the Ovary

Cell Type Major Steroid Hormone Products
Theca cells Androgens (androstenedione, DHEA, testosterone)*
Granulosa cells Estrogens (estradiol, estrone)
Theca-lutein cells Progestogens (progesterone, 17-hydroxyprogesterone)**
Granulosa-lutein cells Estrogens (estradiol, estrone)

* Mostly via Δ5 pathway

** Via Δ4 pathway

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Figure 1-4 The Δ5 and Δ4 pathways. The rate-limiting step in steroidogenesis is the conversion of cholesterol to pregnenolone via side chain cleavage (P450scc). In the follicular phase pregnenolone is preferentially converted to androstenedione via the Δ5 pathway involving 17-hydroxypregnenolone and dehydroepiandrosterone (DHEA). In contrast, the corpus luteum preferentially converts pregnenolone to progesterone (Δ4 pathway) via 3β-hydroxysteroid dehydrogenase (3βHSD).

If pregnancy does not occur, the lifespan of the corpus luteum is fixed at approximately 14 days. After 12 to 14 days luteolysis and apoptosis are initiated. The corpus luteum involutes, and menstruation occurs. Ovarian steroidogenesis then shifts to a new cohort of follicles with their granulosa and theca cells.

Two-Cell Theory

The two-cell, two-gonadotropin theory of ovarian steroidogenesis holds that follicular estrogen/androgen production is compartmentalized.59 Ovarian theca cells produce androgens in response to LH. These androgens may then be aromatized to estrogens in granulosa cells appropriately stimulated by FSH. FSH receptors are present only on granulosa cells, and early in the follicular phase LH receptors are present only on theca cells.60 The enzyme P450c17 (17-hydroxylase and 17,20-lyase) is only present in theca cells. Thus, only theca cells have the ability to convert 21-carbon steroids to 19-carbon steroids. In contrast, aromatase is only present in granulosa cells. Thus, in the ovary only granulosa cells have the ability to aromatize androgens to estrogens (Table 1-8 and Fig. 1-5). Supporting evidence for the two-cell theory includes the fact that women with hypogonadotropic hypogonadism may develop follicles in response to treatment with recombinant FSH, but do not significantly elevate androgen or estrogen levels unless LH is added to the stimulation regimen.61

Table 1-8 Location Specificity of P450c17 and Aromatase

Enzyme Location Function
P450c17 Theca cells only Converts 21-carbon steroids (progesterone/pregnenolone) to 19-carbon steroids (androstenedione, DHEA)
Aromatase Granulosa cells only Converts 19-carbon steroids (androstenedione/testosterone) to 18-carbon steroids (estrone, estradiol)
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Figure 1-5 The two-cell theory and follicular phase steroidogenesis. Binding of luteinizing hormone (LH) to LH receptors on ovarian theca cells stimulates conversion of cholesterol to androstenedione. Binding of follicle-stimulating hormone (FSH) to FSH receptors on granulosa cells then stimulates the aromatization of androgens to estrogens. StAR, steroidogenic acute regulatory protein; scc, side chain cleavage; HSD, hydroxysteroid dehydrogenase.

Estrogens

Estrogens are 18-carbon steroids that include estradiol (i.e., 17β-estradiol), estrone, and estriol. The most potent estrogen, estradiol, is predominantly secreted by the ovary. Estrone, which is one twelfth as potent as estradiol, is also secreted by the ovary. However, the principal source of estrone is from peripheral conversion from androstenedione. Estriol, which is one eightieth as potent as estradiol, is the principal estrogen formed by the placenta during pregnancy. Estriol is also formed by metabolism of estradiol and estrone by the liver and is the most abundant estrogen found in urine.

Estrogen is largely bound to carrier proteins in serum. Approximately 60% of estradiol is bound to albumin, 38% is bound to sex hormone-binding globulin (SHBG), and 2% to 3% is free. It had previously been thought that only the free hormone was active and could enter cells, but recent evidence suggests that hormone transport and hormone availability may be more complex.62

Estrogen Receptors

There are two known estrogen receptors, ERα and ERβ. Both contain a steroid-binding domain, a DNA-binding domain, a hinge region, and a transcription-activation functional domain. The negative feedback of estradiol on FSH secretion is a direct effect of estradiol coupled to its receptor repressing transcription of the FSH β subunit.63 Negative feedback of estradiol on FSH may also be modulated by the estrogen-associated decline in pituitary expression of activin.64

Estrogens can enter any cell, but only cells with estrogen receptors will respond to its presence. When estrogens enter susceptible cells, estrogen associates with its receptor in the cell nucleus. This binding then activates the receptor. The DNA-binding domain of the estrogen–ER complex then associates with specific promoter sequences (DNA response elements) and activates gene transcription.

Estrogen Metabolism

Serum concentrations of estradiol are less than 50 pg/mL in the early follicular phase. In response to follicle development estradiol levels rise, typically peaking at 200 to 300 pg/mL just before ovulation. Estradiol levels drop at the time of ovulation, then rise again, with a second peak in the midluteal phase reflecting estrogen secretion from the corpus luteum.

The liver conjugates estrogens to form glucuronides and sulfates, about 80% of which are then excreted into the urine and 20% of which are excreted in the bile. In the liver, circulating estradiol is rapidly converted to estrone by 17β-hydroxysteroid dehydrogenase. Estrone may then be further metabolized in the liver to 16α-hydroxyestrone and then estriol. Estriol is then converted to estriol 3-sulfate-16-glucuronide before excretion by the kidney.

Some 16α-hydroxyestrone may also be converted to catechol estrogens (i.e., 2-hydroxy or 4-hydroxyestrone). Biologically active catechol estrogens may then be converted to 2-methoxy and 4-methoxy compounds. Catechol estrogens are important because they have been implicated in carcinogenesis. Metabolism of catechol estrogens may generate oxygen free radicals.

Progesterone

Progesterone, a 21-carbon steroid, is the principle secretory steroid of the corpus luteum. Progesterone is responsible for the induction of secretory changes that prepare estrogen-primed endometrium for implantation. If implantation occurs, continued progesterone production is necessary for maintenance of the pregnancy.

The release of FSH and LH require the continuous pulsatile release of GnRH. The coordinated secretion of FSH and LH control follicle growth, ovulation, and maintenance of the corpus luteum. The release of FSH and LH is both positively and negatively influenced by estrogen and progesterone. Whether estrogen and progesterone stimulate or inhibit gonadotropin release depends on the quantity and duration of exposure to the steroid.

At high concentrations progesterone inhibits both FSH and LH secretion by negative feedback on both the hypothalamus and pituitary.65 Progesterone also slows the GnRH pulse generator; hence the decline in GnRH pulse frequency in the luteal phase. However, at low concentrations, and only after previous exposure to estrogen, progesterone stimulates LH release.66

Progesterone Receptors

Progesterone receptors are similar to estrogen receptors in that they contain a steroid-binding domain, a DNA-binding domain, a hinge region, and a transcription-activation functional domain. There are two progesterone receptors, A and B. Receptor B is the positive regulator of progesterone-responsive genes. Binding of progesterone to progesterone receptor A inhibits activity of receptor B. Progesterone causes a depletion of estrogen receptors. This is the mechanism by which progestins protect against endometrial hyperplasia.

Progesterone Metabolism

Throughout the follicular phase serum progesterone concentrations are less than 1 ng/mL. Much of this progesterone is believed to result from adrenal production of pregnenolone and progesterone as a by-product of cortisol and aldosterone synthesis. In the late follicular phase progesterone levels begin to rise as the ovary starts to contribute to progesterone production. The rise in progesterone accelerates markedly after ovulation, and progesterone levels reach 10 to 20 ng/mL in the midluteal phase. If pregnancy does not occur, progesterone levels fall in the late luteal phase and return to levels below 1 ng/mL just before the onset of menstruation.

In the circulation, approximately 80% of progesterone is bound to albumin. Another 18% is bound to corticosteroid-binding globulin. About 2.5% is freely circulating, and only about 0.5% is bound to SHBG.

Progesterone is rapidly cleared from the circulation by the liver. The liver converts progesterone to pregnanediol. Pregnanediol is then conjugated to glucuronic acid, and pregnanediol glucuronide is excreted in the urine. Measurement of urinary pregnanediol glucuronide can be used as an index of progesterone production.

Androgens

Ovarian theca cells secrete a variety of androgens (i.e., 19-carbon steroids), including androstenedione, testosterone, and dehydroepiandrosterone (DHEA). Androstenedione is the principal androgen secreted by ovarian theca cells. Theca cells also possess the enzyme 17β-hydroxysteroid dehydrogenase, which may convert androstenedione to testosterone. In premenopausal women, at least 60% of circulating testosterone is derived from the ovary, by either this conversion or direct secretion.

Androstenedione and testosterone can then be aromatized to estrogens in granulosa cells under the influence of FSH. Androstenedione can also be converted to estrone or testosterone in peripheral tissues. Unlike testosterone and dihydrotestosterone, androstenedione does not have high affinity for the androgen receptor.

Side chain cleavage of cholesterol to pregnenolone is the starting point and rate-limiting step in steroidogenesis. In the ovary cholesterol side chain cleavage is regulated by LH. Low-density lipoprotein (LDL) cholesterol is the principal source of cholesterol for steroidogenesis in the human ovary.67 Increased cAMP production due to LH stimulation of adenylate cyclase increases transcription of LDL receptor mRNA and LDL uptake. cAMP-activated steroidogenic acute regulatory protein then increases the intracellular transport of cholesterol to the inner mitochondrial membrane, where side chain cleavage occurs.68

In the preovulatory follicle the preferred pathway for androgen/estrogen synthesis involves conversion of pregnenolone to 17-hydroxypregnenolone, the so-called Δ5 pathway (see Fig. 1-4). Ovarian theca cells have the enzymatic capability to convert pregnenolone to androgens, but lack the ability to aromatize androstenedione or testosterone into estrogens. Only granulosa cells, under the influence of FSH, can aromatize androgens to estrogens. In contrast to the preovulatory follicle, the corpus luteum prefers the Δ4 pathway, the initial conversion of pregnenolone to progesterone.

Androgen Receptors

The androgen receptor is similar to the estrogen receptor. It activates target gene expression via a similar sequence of ligand binding, nuclear translocation, DNA binding, and complex formation with coregulators and general transcription factors. Although the androgen receptor is known to play a central role in the development of male sex organs and secondary sexual characteristics, its physiologic roles in female reproduction remain unclear. Recent studies in androgen-deficient animals suggest that androgen receptors play an important role in granulosa cell development.69 These animals also exhibit reduced fertility with defective folliculogenesis, reduced corpus luteum formation, and reduced uterine response to gonadotropins.

Androgen Metabolism

Androstenedione is produced in equal amounts by the ovaries and adrenal glands. The serum concentrations mirror estradiol levels and range from 0.5 to 3 ng/mL. Androstenedione levels increase in the mid- to late follicular phase and are maximal just before the LH surge. They decline at the time of ovulation, reach a second peak in the midluteal phase, and are lowest around the time of menstruation. Serum testosterone concentrations, in contrast, vary to a much lesser extent during the menstrual cycle and exhibit only a transient periovulatory increase. Testosterone concentrations range from 1.5 to 60 ng/dL.

CONTROL OF THE MENSTRUAL CYCLE

Normal menstrual cycles, termed eumenorrhea, normally range in length between 24 to 35 days, with menstrual bleeding lasting 3 to 7 days. The average amount of blood loss is approximately 30 mL.70 Heavy, prolonged, or irregular menses are referred to as abnormal uterine bleeding and are considered in length in Chapter 21.

Normal menstrual cycles result from a relatively precise interaction of the hypothalamus, pituitary, and ovaries. Under the influence of pituitary gonadotropins, the ovary undergoes cyclic changes providing for the development and release of a mature oocyte and production of ovarian hormones that prepare the endometrial lining for implantation. LH stimulates androgen production in theca cells; FSH promotes follicle development and aromatization of androgens to estrogens in granulosa cells (see Fig. 1-5). In turn, estrogens lead to proliferation of the endometrial lining and the induction of endometrial receptors for both estrogen and progesterone.71

To understand the menstrual cycle it is helpful to divide it into four phases: the follicular phase, the ovulatory phase, the luteal phase, and the luteal–follicular transition. We concentrate on changes in pituitary and ovarian hormones and the effects that these hormones have on the hypothalamus, pituitary, and ovary.

Follicular Phase

The purpose of the follicular phase is to develop a single mature follicle to release a mature oocyte at ovulation. The presence of sufficient FSH also leads to expression of LH receptors on mature granulosa cells of preovulatory follicles. Thus, in the late follicular phase LH can sustain follicular endocrine activity, even in the absence of FSH.72 The follicular phase is variable in duration, but the other three phases are relatively constant, averaging 14 ± 2 days.

GnRH

Throughout most of the follicular phase GnRH pulses occur every 60 to 90 minutes. Just before the onset of the LH surge both GnRH pulse frequency and amplitude increase.

Gonadotropins

FSH levels rise in the early follicular phase due to the lack of negative inhibition from estradiol and inhibin (Fig. 1-6). FSH stimulates follicle growth and estrogen production.73 Through binding of FSH to its receptor, granulosa cells in developing follicles attain the ability to aromatize androstenedione to estrone and testosterone to estradiol. Importantly, receptors for FSH are not detected on granulosa cells until the preantral stage.74 Moreover, both in vitro and in vivo administration of FSH to granulosa cells can cause upregulation or downregulation of granulosa cell FSH receptors.75 Without functional FSH follicle growth and ovarian estrogen production cannot occur.76

image

Figure 1-6 Hormone fluctuations during the menstrual cycle. A, Mean values of follicle-stimulating hormone and luteinizing hormone throughout the cycle. B, Mean values of estradiol and inhibin. C, Mean values of progesterone during the menstrual cycle.

The steady decline in FSH beginning in the midfollicular phase serves to inhibit development of all but the dominant follicle. The dominant follicle remains dependent on FSH and must complete its development in spite of declining FSH levels. Because it has the largest cohort of granulosa cells, it has the largest cohort of FSH receptors and thus is able to grow in the face of insufficient FSH for smaller follicles.

Luteinizing hormone levels are stable in the first half of the follicular phase. However, in the second half LH levels rise in response to positive feedback from increasing estrogen.

Ovarian Hormones

Estradiol levels rise as the dominant follicle emerges. FSH and estrogen synergistically exert a mitogenic effect on granulosa cells, stimulating their proliferation. This in turn increases the FSH receptor content of the follicle, enhancing the ability of the follicle to respond to FSH and produce estrogen. Curiously, not every granulosa cell must express FSH receptors to respond to the gonadotropin signal. Gap junctions between cells allow cells with receptors to transmit protein kinase activation to their neighbors.77

Within the follicular fluid of follicles greater than 8 mm in diameter, the concentration of FSH, estradiol, and progesterone are all extremely high. Within smaller antral follicles, androgens predominate in the follicular fluid. The role of androgens in the follicle is dose-dependent. At low levels androgens provide a substrate for aromatization. However, at higher levels androgens are converted in granulosa cells by 5α-reductase to more active forms such as dihydrotestosterone (DHT) that cannot be aromatized to estrogens.78,79 Granulosa cells have androgen receptors.80 Activation of granulosa cell androgen receptors inhibits aromatase activity and also inhibits FSH induction of granulosa LH receptors.81 Follicles exposed to excessive androgens eventually become atretic.82,83 In contrast, follicles with the highest estrogen-to-androgen ratios and the highest estrogen concentrations are most likely to contain a competent oocyte.84

Rising estradiol levels have a dual role for the follicle. Within the follicle, estradiol promotes granulosa cell growth, aromatization of androgens to estrogens, and, in combination with FSH, induction of development of LH receptors on granulosa cells. However, outside the follicle rising serum estradiol levels have an inhibitory effect on FSH secretion. The resulting decline in FSH levels in the mid- to late follicular phase limits aromatase activity in smaller follicles, leading to higher androgen levels and follicular atresia. Indeed, a reduction in granulosa cell expression of FSH receptors is one of the first signs of follicular atresia.

Inhibin B levels begin to rise almost immediately after the rise in FSH levels. By the midfollicular phase the rise in estradiol and inhibin B levels causes FSH levels to decline. Inhibin B levels peak approximately 4 days after the FSH peak.47 In the late follicular phase inhibin B levels fall, mirroring the decline in FSH levels.

Progesterone and inhibin A levels are low throughout most of the follicular phase, but both begin to rise in the days immediately preceding ovulation. The rise in progesterone in the late follicular phase mirrors the rise in LH.

Ovulatory Phase

The ovulatory phase begins with the midcycle LH surge, which disrupts contacts between granulosa cells and the cumulus oophorus (specialized granulosa cells surrounding the oocyte), causing the oocyte to detach from the follicle wall. The LH surge also induces the resumption of meiosis within the oocyte and release of the oocyte–cumulus complex from the follicle (ovulation).

GnRH

GnRH plays a supporting role for the midcycle gonadotropin surge, but it does not trigger the surge.85 There is no change in GnRH pulse frequency during the midcycle gonadotropin surge.86 Rather, ovarian steroid feedback to the primed anterior pituitary triggers the LH surge.87 Whereas estrogen inhibits the secretion of pituitary gonadotropins, it facilitates their synthesis and storage. Estrogen also increases the expression of GnRH receptors.88,89 Thus, in the mid- to late follicular phase each pulse of GnRH is met with a greater gonadotropin response.90,91 When the estradiol level in the circulation meets a critical level for a sufficiently long period of time, the inhibitory action of estradiol on LH secretion changes to a stimulatory one. The LH surge is accompanied by a surge of GnRH in both portal and peripheral blood.92 However, as demonstrated by women with hypogonadotropic hypogonadism treated with a pulsatile GnRH pump, ovulation and pregnancy may occur in the absence of any change in GnRH pulse frequency or amplitude.93 Moreover, the LH surge ends before there is a decline in the GnRH signal.94

Gonadotropins

Both FSH and LH levels peak just before ovulation. The initiation of the gonadotropin surge is dependent on attaining serum estradiol levels of at least 200 pg/mL for at least 2 days.95 In natural cycles this level of estradiol is typically not attained until the dominant follicle reaches a mean diameter of 15 mm.96

During the gonadotropin surge serum LH levels increase tenfold over a period of 2 to 3 days, while FSH levels increase fourfold. Within the dominant follicle the LH surge induces detachment of the oocyte–cumulus complex from the follicle wall. The release of the oocyte–cumulus complex from its follicular wall attachments is accompanied by the resumption of meiosis and release of the first polar body.

The LH surge also induces luteinization of the periovulatory follicle. Luteinization refers to functional and morphologic changes within the theca–granulosa cell complex associated with accumulation of a yellow pigment called lutein. The function of the FSH surge is less clearly known, but it is believed to ensure an adequate number of LH receptors on the granulosa layer and to increase production of plasminogen activator, thus increasing the concentration of the proteolytic enzyme plasmin.

Ovulation typically occurs from mature follicles 34 to 36 hours after the onset of the LH surge.97 The peak of LH and FSH occurs 10 to 12 hours before ovulation.98 The LH surge usually lasts 48 to 50 hours and must be maintained for at least 14 hours for full maturation of the oocyte to occur.99 The mechanism that turns off the LH surge is unknown. It may simply reflect the depletion in pituitary LH content.

Ovarian Hormones

Just prior to ovulation the follicle becomes vascularized.100 Angiogenesis is mediated by LH and a variety of other factors, including vascular endothelial growth factor.101–103 Prostaglandins reach peak levels in the follicular fluid.104 Proteolytic enzymes digest collagen in the follicular wall, resulting in distensibility and thinning just before ovulation.105 Progesterone rises in the follicular fluid. FSH, LH, and progesterone all serve to increase the activity of proteolytic enzymes. There is a rapid increase in follicular fluid volume, but due to increased elasticity there is little to no change in intrafollicular pressure. Finally, a protrusion (stigma) appears on the follicular wall, and it is at this site that ovulation ultimately occurs.

Interestingly, spontaneous lueteinization occurs in the absence of LH when granulosa cells are removed from follicles and cultured in vitro. Similarly, cumulus-enclosed oocytes removed from developing follicles before the LH surge will spontaneously resume meiosis.106,107 These findings have led to speculation that substances functioning as oocyte maturation inhibitors or luteinization inhibitors must exist within each follicle. Further support for this hypothesis lies in the fact that cumulus cells lack LH receptors.

Estradiol levels fall beginning with the onset of the LH surge. Progesterone and inhibin A levels continue to rise at the time of ovulation. Inhibin B levels surge at the time of ovulation. Luteinization of the follicle begins.

Luteal Phase

The luteal phase is the time when the ovary secrets progesterone, resulting in endometrial receptivity for embryo implantation. LH receptors on granulosa cells result in luteinization, driving the postovulatory follicle to become a corpus luteum. Granulosa cells become granulosa-lutein cells with their own blood supply and begin secretion of both estrogen and progesterone. Release of large amounts of progesterone from the corpus luteum results in secretory endometrial changes and the final development of endometrial receptivity.

GnRH and Gonadotropins

GnRH pulse frequency declines but pulse amplitude increases in the luteal phase. Changes in GnRH pulse frequency correlate with the duration of exposure to progesterone; changes in pulse amplitude correlate with progesterone levels.17

Luteinizing hormone and FSH levels reach a nadir during the luteal phase in response to the elevation in estrogen, progesterone, and inhibin A. Nevertheless, function of the corpus luteum is dependent on continued low-level pituitary gonadotropin secretion throughout the luteal phase.108 LH pulses stimulate pulses of progesterone secretion from the corpus luteum.17,109 Moreover, reducing LH pulse frequency and amplitude with a GnRH agonist in the luteal phase shortens the luteal phase itself.110

Ovarian Hormones

After ovulation granulosa and theca cells of the dominant follicle are luteinized. Luteinization involves both chemical and morphologic changes. Granulosa and theca cells hypertrophy and increase steroidogenesis. Moreover, breakdown of the basal membrane that previously separated granulosa and theca cells leads to capillary invasion around granulosa-lutein cells.

Granulosa-lutein cells can now make progesterone directly from LDL via side chain cleavage and 3β-hydroxysteroid dehydrogenase. Levels of mRNA for side chain cleavage and 3β-hydroxysteroid dehydrogenase are maximal at the time of ovulation and in the early luteal phase.111 The induction of LDL receptor expression in granulosa cells occurs in response to the LH surge and is an early feature of luteinization.112 Progesterone secretion correlates with the number of LH receptors and adenylate cyclase activity.113 Progesterone levels peak during the midluteal phase.

Granulosa-lutein cells cannot make estrogens directly from cholesterol because they lack the enzyme P450c17. However, granulosa-lutein cells continue to be able to aromatize theca-lutein produced androgens to estrogens, and estrogen levels remain high throughout most of the luteal phase.

In granulosa-lutein cells inhibin production switches from inhibin B to inhibin A. Thus inhibin B levels decline to their nadir during the luteal phase, whereas inhibin A levels reach their peak. Secretion of inhibin A by granulosa-lutein cells is controlled by LH.114 Inhibin A, like inhibin B, suppresses FSH levels.115

Luteal–Follicular Transition

As the luteal phase continues, progesterone inhibits LH release via a negative feedback loop on the anterior pituitary. During the luteal–follicular transition, subsequent decline in LH causes the corpus luteum to involute unless the corpus luteum is rescued by production of hCG from an implanting embryo.

Involution of the corpus luteum leads to a fall in both estrogen and progesterone production. The endometrium can no longer be maintained and menstruation occurs. The fall of estrogen production then reactivates FSH secretion, initiating a new cycle of follicular development with estrogen secretion and renewed proliferation of the endometrial lining.

GnRH

A progressive and rapid increase in GnRH pulse frequency occurs during the luteal–follicular transition. GnRH pulse frequency, as estimated by LH pulse frequency, increases from 3 pulses per 24 hours (midluteal phase) to 14 pulses per 24 hours.18

Gonadotropins

FSH and LH levels rise from their nadir due to the decline in negative feedback from estradiol and inhibin and the rise in activin.116 The rise in FSH initiates recruitment of gonadotropin-responsive follicles for the next menstrual cycle. This recruitment of antral follicles actually begins at least 2 days before the onset of menstrual bleeding. In fact, an increase in FSH bioactivity can be measured back to the midluteal phase.117 During the luteal–follicular transition both inhibin A and B levels are at a nadir.118 In contrast, activin levels begin to increase in the late luteal phase and peak at the time of menstruation.119 Activin plays an important role as gonadotropin responses to GnRH require the presence of activin.43

Ovarian Hormones

In the absence of pregnancy corpus luteum function declines approximately 10 days after ovulation. The exact mechanisms for luteolysis are unclear. Luteolysis involves apoptosis and expression of matrix metalloproteinases.120,121 Luteolysis may also be mediated by nitric oxide.122 Nitric oxide induces apoptosis in the human corpus luteum.123 One of the final signs of luteolysis is ovarian production of prostaglandin F2α, which inhibits luteal steroidogenesis. Thus, unless the corpus luteum is rescued by the hCG of pregnancy, estrogen, progesterone, and inhibin levels fall as the luteal–follicular transition occurs.

PEARLS

• GnRH-secreting neurons are found predominantly in the arcuate nucleus of the hypothalamus. GnRH is released into the portal circulation at the median eminence in a pulsatile fashion and binds to cell membrane receptors in the anterior pituitary.
• GnRH pulse frequency and amplitude vary during the menstrual cycle. The follicular phase is characterized by high-frequency (q60–90 minutes), low-amplitude pulses; the luteal phase is characterized by lower-frequency (q2–4 hours), higher-amplitude pulses.
• The activity of GnRH-releasing neurons is modified by a variety of factors, including neurotransmitters, glycoprotein hormones, and steroid hormones.
• GnRH has an extremely short half-life (2 to 4 minutes). GnRH agonists and antagonists are characterized by modifications to the native GnRH decapeptide structure that extend its half-life.
• The pituitary gland lies in the sella turcica and is connected to the hypothalamus via the pituitary stalk. The blood supply to the pituitary gland comes from the internal carotid artery. Specialized cells within the anterior pituitary (gonadotrophs) secrete FSH and LH in response to GnRH. Gonadotrophs comprise only 5% of all the cells in the anterior pituitary gland.
• FSH and LH are glycoproteins that share the same α subunit. The half-life of LH (20 to 30 minutes) is much shorter than that of FSH (1 to 4 hours), and LH pulse frequency/amplitude correlates closely with GnRH pulse frequency and amplitude.
• FSH and LH bind to cell membrane receptors that activate adenylate cyclase, thus raising intracellular cyclic AMP. FSH plays a crucial role in folliculogenesis, as well as inducing aromatization of androgens to estrogens. LH plays a crucial role in ovulation and steroidogenesis.
• Certain steroidogenic enzymes are specific to certain ovarian cell types. Only theca cells express P450c17, the enzyme that allows conversion of progestins (21-carbon steroids) to androgens (19-carbon steroids). In turn, only granulosa cells, stimulated by FSH, have the ability to aromatize androgens to estrogens (18-carbon steroids).
• Granulosa cells of preovulatory follicles do not have a direct blood supply; they are separated from theca cells by a basal membrane. In contrast, almost immediately after ovulation capillaries invade the granulosa layer.
• In theca cells of the preovulatory follicle pregnenolone is preferentially converted to 17-hydroxypregnenolone (Δ5 pathway) on the way to synthesis of androstenedione.
• In the corpus luteum pregnenolone is preferentially converted to progesterone (Δ4 pathway). Progesterone, 17-hydroxyprogesterone, and inhibin A levels all peak in the luteal phase.
• The lifespan of the corpus luteum is fixed at approximately 14 days. If the corpus luteum is not rescued by hCG, luteolysis and apoptosis are initiated.

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