There are periodic preparations for fertilization and pregnancy in reproductive system of women by the means of menstrual cycle. Menstrual cycle is mainly divided into the ovarian and uterine cycle. To understand the physiology of ovarian cycle, it can be further divided into three phases—follicular phase, ovulation and luteal phase. There is a dynamic relationship between pituitary and gonadal hormones. There is also a corelation with morphologic and autocrine events in the ovary.
Ovulation is a process whereby a secondary oocyte becomes available for conception. In relation to menstrual period, ovulation occurs about 14 days prior to the expected menstrual period. Menstrual cycle can occur without ovulation.
Understanding the physiology of ovulation is important to find out etiology. Management of ovulation is the main concern with infertility and abnormal uterine bleeding (AUB).
During follicular phase orderly sequence of cellular, histological, hormonal and functional changes takes place. This occurs due to interaction between gonadotropins, ovarian steroid hormones and autocrine paracrine peptides. This process occurs over a period of 10–14 days.
Main events occurring during maturation of follicle takes place in follicular phase to proceed to ovulation. The primordial follicle goes through stages of preantral, antral and preovulatory follicle (Flow chart 1.1).2
PRIMORDIAL FOLLICLE
During intrauterine life, the primordial germ cells originate in endoderm and by 5–6 weeks of gestation migrate to genital ridge.
Ovarian differentiation starts between 6th and 8th weeks by rapid mitotic multiplication of germ cells to reach a huge number of upto 6–7 million by 16th to 20th weeks of gestation. The germ cells stop mitotic division and start meiosis division. Germ cell passes through early stage and gets arrested in diplotene stage of meiosis prophase and gets surrounded by a layer of perivascular cells originated by mesenchymal and epithelial cells to form primordial follicle.
The process of growth (continuum) and atresia (apoptosis) of primordial follicle continues at all physiological stages (pregnancy, ovulation, period of anovulation and all ages, infancy till around menopause) till unit the number of primordial follicles is exhausted.
Failure of achieving surrounding layer of granulosa cells, leads oocyte to complete meiosis and die, Resulting in a reduction in the number of oocytes. This process (oocyte getting surrounded by granulosa cells) requires the presence of two X-chromosomes, so in 45 X-chromosomal pattern in Turner's syndrome, gonadal dysgenesis occurs.
The number of follicles decreases irretrievably. The rate of decrease is proportional to total number present. Number of follicles at 20 weeks of intrauterine life is 6–7 million which reduces to 2 million at birth. No similar rate of depletion in germ cells number is seen again. At puberty, it reaches to 3,00,000. From this large reservoir, only 400 follicles will ovulate during a woman's reproductive years. These units will be depleted further to a point at menopause where follicles are almost completely depleted. Major mechanism for this, loss is by elimination through the surface of the ovary into peritoneal cavity (Fig. 1.1).3
Recruitment of follicle to ovulate is done in first few days of the cycle. Early growth of follicles takes over several menstrual cycles before but ovulatory follicles are always one of a cohort, recruited at the time of leuteal follicular transition.
Approximately, 85 days are taken to achieve preovulatory stage. In initial stage, for further growth of follicle is without any external stimulation, which occurs in all physiological conditions. Follicle of 2–5 mm size is ready for further growth. Follicular stimulating hormone (FSH) recruits cohort of follicles, 3–11 follicle cohort per ovary is propelled to grow. The reason for selection of cohort is due to decline in steroidogenesis and inhibin—a secretion causing FSH rise. So in absence of FSH, fate of the cohort of follicles is atresia. This process of growth is known as continuum and process of atresia is known as apoptosis. Early follicle seems to have genetic code for the timing at which it may resume growth. The number of follicles that resumes growth at any point of time is dependent on the size of residual pool of follicles in both ovaries. If residual pool is changed, then remaining follicles will reschedule their readiness to resume growth.
The first visible sign of follicular growth is increase in size of the oocyte and presence of squamous shape granulosa cells than cuboidal granulosa cells. This indicates maturation of follicle than growth. Size of primordial follicle is 50 µm and oocyte is 20 µm. There are small gap junctions present between granulosa cells and oocytes which permit exchange of nutrition, metabolites and signal interchange between granulosa cells and oocyte. In one direction, inhibition of final maturation of oocyte [until the luteinization hormone (LH) surge] is maintained by factors derived from the granulosa cells. In the other direction, the process of follicular growth is influenced by regulatory factors that originate 4in the oocyte. Locally produced transforming growth factor-β (TGF-β) and neurotrophins (neurotrophins are growth factors that are known to have a role in promoting cell survival and differentiation) are responsible for molecular regulation of primordial follicle. Activin, inhibin, anti-mullerian hormone (AMH) and bone morphogenetic proteins (BMPs) are members of TGF-β family.
BMPs, activin promote and inhibin, AMH inhibit growth of primordial follicle. Oocyte is dependent on adjacent granulosa cells to metabolize glucose, transport of essential amino acids, synthesis and transfer of cholesterol.
Growth differentiation factor-g (GDF-g) is a peptide synthesized only in oocyte after primordial follicle becomes a preantral follicle. GDF-g and BMP 15 are required for normal ovarian follicle development.
The gap junctions are composed of channels formed by a arrangement of proteins known as connexins. They are needed for growth and multiplication of granulosa cells, nutrition and regulation of oocyte development. Connexin expression in follicle is up-regulated by FSH and down-regulated by LH.
Multiplication of cuboidal granulosa cells causes primordial follicle to become primary follicle. This graulosa layer get separated from stromal cells by basement member called as basal lamina. The surrounding stromal cells differentiate into concentric layers designated as the theca interna (near to basal lamina) and theca externa (outer portion of cells). The theca layers appears when granulosa proliferation produces 3–6 layers of its own.
Summary of Events
- Formation of primordial follicle from intrauterine life 6th–8th weeks of gestation.
- Maximum number of follicles 6–7 million at 16–20 weeks of gestation reduces to 3,00,000 at puberty out of which of 300 undergo ovulation.
- Size of primordial follicle is 50 µm and size of oocyte is 20 µm.
- Oocyte arrested in diplotene stage of meiosis prophase.
- Process of growth (continuum) and atresia (apoptosis) is continuous in all physiological phases.
- Cohort of follicles (primordial) recruited in initial phase with out external stimulation.
- Further growth depends or is governed by FSH at lateal–follicular transition phase.
- Granulosa cells differentiate and forms theca interna and theca externa in relationship with basal lining.
- TGF-β and neurotrophins are responsible for molecular regulation of primordial follicle.
PREANTRAL FOLLICLE
Appearance of FSH receptors in the primordial follicle leads it to preantral stage. In preantral follicle, oocyte enlarges to reach size of 80 µm, which is limited by zona pellucida.5
The granulosa cells undergo multilayer proliferation as theca layer from stroma. Growth is totally dependent on gonadotropin and locally secreted estrogen (Fig. 1.2). Size of follicle at antral stage in 200 µm.
Granulosa cells produce estrogen, androgen and progestins. FSH initiates steroidogenesis in granulosa cells and stimulates granulosa cell growth.
Aromatase enzyme causes conversion of androgen to estrogen. At this stage, steroidogenesis in ovarian follicle is not regulated only by gonadotropins but also influenced by other factors such as growth factor, nitric oxide, prostaglandins, peptides such as GnRH, angiotensin II, tissue necrosis factor-α, vasoactive intestinal peptide.
Androgen not only undergo FSH-induced aromatization, but can also increase aromatase activity. When androgen is in excessive amounts, it causes conversion of androgen to potent 5α androgen which further inhibits aromatase activity along with reduction in FSH-induced LH receptor formation which are essential for follicular development. If such a process continues, then fate of preantral follicle is to proceed to atresia. So, success of follicle depends upon its ability to convert an androgen dominated microenvironment to an estrogen-dominated microenvironment.
Summary of Events
- FSH stimulation propels follicles to preantral stage.
- Size of antral follicle is 200 µm and size of oocyte is 80 µm which continues until ovulation.
- FSH-induced aromatization of androgen in granulosa cells causes production of estrogen.
ANTRAL FOLLICLE
Follicular fluid provides endocrine environment for nurturing oocyte and surrounding granulosa cells.
The oocyte now get surrounded by a layer of granulosa cells called as cumulus oophorus (Fig. 1.3) which attaches the oocyte to the rest of granulosa cells. The differentiation is believed to be a response to the signal originated from oocyte. The follicular fluid rich in hormones, growth factors and cytokines, provides the milieu that is required for the orderly maturation and development of the oocyte and surrounding cells. In presence of FSH, estrogen becomes the dominant substance in follicular fluid. Conversely, in the absence of FSH, androgen predominates. The dominance of estrogen and FSH is essential for sustained accumulation of granulosa cells and continued follicular growth. An androgenic milieu antagonizes estrogen-induced granulosa proliferation, and if such a situation is sustained, this will promote degenerative changes in the oocyte.
The synthesis of steroid hormones is functionally compartmentalized within the follicle. ‘The two cell system, ‘ Two-Gonadotropin system’ in human preantral and antral follicles, LH receptors are present only on theca cells and FSH receptors only on the granulosa cells. The acceleration of production of estrogen is done by interaction between granulosa and theca compartments.
The theca cells are characterized by their expression for LH receptors to P450 C causing regulated entry of cholesterol in mitochondria, utilizing internalized LDL cholesterol, which get converted to androgens. Under influence of P450 C 17, theca cells limit conversion of LDL to androgen.
Granulosa cells do not express this enzyme and thus dependent on androgen from theca to make estrogen. Granulosa cells begin to express genes to FSH receptor P450 aromatase and converts androgen to estrogen. This phenomenon is known as two cells (theca and granulosa) two gonadotropins (FSH and LH).
FSH causing folliculogenesis and final stages of maturation are optimized by LH, increasing the amount of androgen substance for estrogen production and promoting the growth of dominant follicle while simultaneously hastening the regression of smaller follicles.
So, selection of a follicle to become dominant depends on estrogen levels. The selection process is a result of two estrogen actions: (1) Local interaction between estrogen and FSH within follicle (2) The effect of estrogen on pituitary secretion of FSH.
Estrogen causes negative feedback on FSH at hypothalamus pituitary level, causing withdrawal of gonadotropins to other less developed follicles (Fig. 1.4). Reduction in FSH receptors in granulosa cells causes interruption, which promotes conversion to androgen microenvironment resulting in irreversible atretic change. Atresia is called apoptosis, programmed cell death, and β-heralded by alterations in mRNAs required for cell proteins that maintain follicle intergrity. This type of ‘natural death’ is a physiological process. Once process of apoptosis starts, TNF produced in granulosa cells inhibit FSH stimulation for formation of estrogen except in dominant follicle.
Anti-mullerian hormone (AMH) responsible for mullerian duct regression during sexual differentiation is detected in primordial follicle which reaches to peak concentration in small antral follicle. AMH is a excellent marker for prediction of ovarian reserve and ovarian function as it reflects the number of growing follicles. AMH, by its paracrine activity causes inhibition of FSH-stimulated follicle growth, thus suppressing the growth of lesser follicles and allowing the dominant follicle to emerge.
AMH level can be measured at any day of menstruation cycle as does not get affected by gonadotropins or sex steroids.
The dominant follicle has two significant advantages, a great content of FSH receptors acquired because of a rate of granulosa proliferation that surpasses that of its cohorts and enhancement of FSH action because of its high intrafollicular estrogen concentration a consequence of local autocrine-paracrine molecules.
By day 9, theca vascularity in dominant follicle is twice than of other antral follicles. This allows preferential delivery of gonadotropins to dominant follicle to retain FSH responsiveness and sustain continued development and function despite waning gonadotropins levels.
There is shifting in action of FSH in dominant follicle in presence of increasing concentration of estrogen. It causes generation of LH receptors rather than its own (FSH) receptor up regulation. Luteinizing hormone (LH) induces its own up regulation in FSH primed granulosa cells.
The LH takes the lead role in late stage of follicle development providing support for final maturation and function of dominant follicle. LH-stimulated production of androgen in theca cell provides large amount of estrogen required at this point of cycle.
Estrogen exerts its inhibitory effects on both hypothalamus and anterior pituitary decreasing both GnRH pulsatile secretion and GnRH pituitary response. Higher level of estrogen in combination with inhibin causes suppression of FSH. At lower level of estrogen, there is a negative inhibitory (stimulatory) effect.
Higher levels of estrogen is capable of a exerting a positive stimulatory feedback effect on LH release. At low levels, estrogen imposes a negative feedback relationship with LH. The transition from suppression to stimulation of LH release occurs as estradiol rises during mid-follicular phase.
There is a need of sustained estrogen stimulus beyond intiation of LH surge, if this fails to occur, LH surge is abbreviated. Luteinizing hormone (LH) secretion is more frequent but smaller in amplitude during follicle phase compared to luteal pahse, with slight increase in frequency observed as follicular phase progresses to ovulation.
GnRH and gonadotropins are released from the inhibitory effects of estradiol, progesterone and inhibin. The frequency of GnRH and LH pulses increases 4.5 fold during convertion of previous luteal phase to next follicular phase, accompanied by 3.5 fold increasing levels of FSH and 2 fold increase in LH levels.
Both estradiol and progesterone are required to achieve low, suppressed seretory pattern of GnRH during luteal phase. Estrogen appears to enhance the stimulating action of progesterone in luteal phase on endogenous opioid peptide, creating high levels of it.
There is a positive feedback action of estrogen causing increasing both quantity and quality (bioactivity) of FSH and LH. As a result, there are more biologically active gonadotropins secreted at midcycle than at any other period of cycle.
There is diurnal rhythm in FSH and LH secretion. There is nocturnal rise seen in ACTH, thyroid-stimulating hormone (TSH) growth hormone and prolactin, in contrast, there is nocturnal decline in FSH and LH. Luteinizing hormone (LH) exhibits this pattern in early follicular phase.9
AUTOCRINE-PARACRINE REGULATION
Assistant peptides granulosa cell in response to FSH secretes these peptides in follicular fluid and ovarian venous effluent. Main peptides are inhibin, activin, follistatin.
- Inhibin consists of two peptides known as alpha- and beta-subunits. Linked by disulfide bond. Beta-subunit has two parts: β-A and β-B. So, two forms of inhibin are inhibin A: Alpha-Beta and inhibin β-Alpha-Beta β. There is a reciprocal relationship between FSH and inhibin. FSH stimulates inhibin in granulosa cells and inhibin causes inhibition of FSH.The secretion of inhibin-β amplifies withdrawal of FSH from other follicles, another mechanism by which emerging follicle secures dominance. Inhibin-β rises slowly and steadily reaching peak levels at early and mid-follicular phase. Then decreases in late follicular phase before ovulation and reach nadir in midluteal phase (Increase in level of inhibin can be seen after ovulation due to its release from ruptured follicle) with appearance of LH receptors in granulusa cells of dominant follicle. LH controls inhibin causing changing of inhibin B to A. So there is a rise in inhibin A in late follicular to mid-luteal phase. Inhibin A causes suppression of FSH to nadir level in luteal phase to luteal-follicle transmission.
- Activin, derived from granulosa cells, but also present in pituitary gonadotropes. It contains two subunits. There are three forms of activin:
- Activin A: Beta A – Beta A
- Activin B: Beta A – Beta B
- Activin B: Beta B – Beta B.
In ovarian follicle activin increases FSH binding in granulosa cell and augments FSH stimulation of aromatization and inhibin production. Inhibin and activin act directly on theca cells to regulate androgen synthesis. Prior to ovulation, activin suppresses granulosa progesterone production to prevent premature luteinization. Activin A directly stimulates the synthesis of GnRH receptors in pituitary cells.Follistatin is a glycopeptide secreted by a variety of pituitary cells. The peptide has also been called FSH—suppressing protein because of its main action. It causes inhibition of FSH synthesis and secretion and the FSH response to GnRH by binding to activin and causing its decrease.
Summary of inhibin and activin activity- Pituitary secretion of FSH is regulated by balance between activin and inhibin
- Follistatin causes inhibition of activin and enhances inhibin action.
- In ovarian follicle, activin and inhibin influence growth and development by modulating theca and granulosa responses to gonadotropes.
- Growth factors:
- Insulin like growth factors: IGF, IGF I and IGF II are single chain polypeptide containing three disulfide bonds.10
- IGF I is mainly for growth promoting action and depends on growth hormone.
- IGF II is not dependent on growth and is hormone important in fetal growth and development.
- IGF II is an important factor in dominant follicle than IGF I Insulin like growth factor binding proteins (IGFBP I to 6) nonglycosylated peptides. Function as IGF binding proteins. It carrys IGF in serum, prolongs half-life and regulates tissue effect of IGFS. IGFBP-3 is main BP. These do not bind to insulin.
- IGF receptor (IGFI receptors, IGF II receptor): IGF I binds to insulin receptors with low affinity. Insulin binds to IGF I receptor with moderate affinity. IGF II receptor does not bind insulin.
- IGF I causes DNA synthesis, steroidogenesis, aromatase activity, LH receptor synthesis and inhibin secretion.
- IGF II stimulates granulosa mitosis
- IGF I synergy with FSH, stimulates aromatase activity and causes production of estrogen. Similarly, IGF I enhances LH-induced progesterone from granulosa—luteal cells.
- IGF II is highly expressed in both theca and granulosa cells. The IGF II is capable of stimulating steroidogenesis and proliferation of theca and granulosa cells.
- IGF BP-1 inhibits IGF I mediated steroidogenesis and proliferation of luteinized granulosa cells. BP counteracts synergistic action of gonadotropins and growth factor. IGFBP1-6-IGFBPI is present in granulosa cells, IGFBP2 in the theca cells. IGFBP 2, 4, 5 in both theca and granulosa of antral and atretic follicle IGFBP6 are not present in ovary.
- IGFBP expression in polycystic ovary is similar to that seen in atretic follicle; decrease in IGFBP-3 occurs in dominant follicle allowing increase in IGF levels and actively circulating levels of IGFBP1 decrease in response to insulin and thus circulating levels are decreased in women with anovulation and polycystic ovaries who have elevated levels of insulin. These patients also have increased circulating levels of IGFI because of LH stimulated synthesis and secretion in theca cells. The level of IGF PB-2 and 4 in follicular flood in anovulatory patients are increased.
- Estrogen-dominant follicular fluid contains very low levels of IGFBP-4. In contrast they hare high levels of androgen in follicular flood. This occurs as there is presence of IGFBP-4 protease causing decrease IGFBP activity and enhanced IGF activity another mechanism for ensuring success of dominant follicle.
Summary of insulin like growth factors action on ovary- The most abundant IGF in human follicle is IGF II
- IGF II stimulates granulosa cells proliferation, aromatase and progesterone synthesis.
- FSH and LH stimulate IGF production
- FSH inhibits binding protein synthesis and thus maximizes growth factor availability.
- Epidermal growth factor (EGF) is produced in luteinized granulosa cells in response to LH and induce progesterone synthesis.
- Transforming growth factor (TGF): TGF-α get attached to EGF receptors. TGF-β inhibits androgen production. GDF-9 originates in oocyte and is required for growth and development of ovarian follicle (GDF-9 member of TGF-β family).
- Fibroblast growth factor (FGF): Causes decrease in estrogen production and so has the opposite action of TGF-β.
- Platelet: Derived growth factor (PDGF) PDGF and EGF modify prostaglandin production within follicle.
- Angiogenic growth factor: Vasculariztion of follicle is influenced by peptides in follicular flood mainly on vascular endothelial growth factor (VEGF). This factor is involved in the co-ordinated growth and regression of follicles and corpus luteum.
- The interleukin system: Prominent component of ovarian follicle and main source of interleukin. It is an immunomediator.
- Tumor necrosis factor α (TNF-α): Main role in apoptosis, a feature of follicular atresia and luteolysis of corpus luteum.
- Other peptides:
- ACTH, β-lipotropins, prorenin, corticotrophin releasing hormone (CRH), AMH, pregnancy-associated plasma protein A (PAPP-A), endothelin I and oxytocin are other peptides present in follicular flood.
- Prorenin causes stimulation of steroidogenesis to provide androgen for estrogen production also stimulates angiogensesis.
- CRH inhibits LH stimulated androgen production in theca cells.
- AMH function (already discussed prior in chapter) causes oocyte maturation and follicle development. It activates oocyte maturation inhibitor (OMI).
- PAPP—a inhibits proteolytic activity within follicle before ovulation.
- Endothelin-1 causes inhibition of luteinization.
Summary of Events in Antral Follicle
- Size of follicle becomes 500 µm with oocyte remaining 80 µm with formation of follicular fluid and surrounding granulosa cells designated to from cumulus oophorus.
- Two cell, two gonadotropins mechanism causes follicular phase estrogen production.
- Increasing level of estrogen causes negative feedback leading to suppressive influence on FSH release.
- Midfollicular rise in estradiol exerts positive feedback influence on LH secretion.
- Positive action of estrogen causes increase in quantity and quality (bioactivity) of gonadotropins.
- LH levels rise in late follicular phase, stimulating androgen production in theca cells.
- FSH induces appearance of LH receptors on granulosa cells.
- Autocrine–paracrine peptides have main role on follicular response to gonadotropins main are inhibin, activin, follistatin, insulin like growth factor.
- Inhibin-β from granulosa cells in response to FSH, causes direct suppresses pituitary FSH secretion.
- Activin originating from pituitary and granulosa augments FSH secretion and its action.
PREOVULATORY FOLLICLE
- Granulosa cells in the preovulatory follicle enlarge and acquire lipid inclusions while theca cells becomes vaculotaed and richly vascular, giving the preovulating follicle a hyperemic appearance.
- There is a increasing amount of estrogen production in preovulatory follicle. The rapid increase causes peak of estrogen level approximately 24–36 hours prior to ovulation.
- LH promotes luteinization of the granulosa in the dominant follicle, resulting in production of progesterone. This production of progesterone has immense physiological importance. Progesterone receptors begin to appear in granulosa cells of dominant follicle in preovulatory period.
- Progesterone introduced after adequate estrogen priming causes positive feedback and enhances LH surge.
- Appropriate low level of progesterone derived from the maturing follicle contribute to precise synchronization to midcycle surge.
- The preovulatory period is associated with rise in plasma levels of 17α-hydroxyl progesterone. This rise signal the LH stimulation of P450 SCC and P450C17 causing production of theca androgens, the substrate for granulosa estrogen. Some theca cells loss ability to express P450C17 and the rest remain with expression actively to P450C17 causing aromatization of androgen to estrogen.
- The midcycle increase in androgen by rise in inhibin is caused by LH stimulation of theca cells.
- Androgen production at this stage in the cycle serves two purposes: (1) Local role within the ovary to enhance the process of atresia and (2) A systemic effect to stimulate libido.
Summary of Events
- Estrogen production becomes sufficient to achieve and maintain peripheral threshold concentration of estradiol in order to induce the LH surge.
- Acting through its receptors, LH initiates luteinization and progesterone production in granulosa layer.
- Preovulatry rise in progesterone facilitates the positive feedback action of estrogen and may be required to induce the midcycle FSH peak.
- A midcycle increase in local and peripheral androgens occurs, which is derived from the theca tissue of lesser, unsuccessful follicles.
OVULATION
- Ovulation takes place approximately 10–12 hours after LH peak and 24–36 hours of estrogen peak level.
- LH concentration must be maintained for 14–74 hours in order to cause full maturation of oocyte.
- Ovulation occurs more frequently in the right ovary compared with the left and oocytes from the right ovary have a higher potential for pregnancy.
- Ovulation alternates between two ovaries in younger women. Contralateral ovulation favors pregnancy more than ipsilateral ovulation. The gonadotropins surge stimulates a large collection of events that ultimately leads to ovulation, the physical release of the oocyte and its cumulus mass of granulosa cells presence of luteinization of granulosa cells and progesterone production, expansion of the cumulus and the synthesis of prostaglandins and other eicosanoids essential for follicle rupture. To cause this, LH surge is required.
- Increase in cyclic AMP occurs due to LH induction. Cyclic AMP causes decrease in oocyte maturation. Locally produced activin suppresses progesterone production by luteal cells, providing yet another means of preventing premature luteinization.
- Oocyte exerts control over granulosa functions, affecting both metabolism and proliferation through the secretion of proteins in transforming growth factor β family. The differentiation and maintenance of cumulus cells from the preantral granulosa cells is under the direction of the oocyte.
- The cumulus oophorus differs from other granulosa cells in being lacking in LH receptors and progesterone production. The oocyte enables cumulus cells to respond to the gonadotropins induced physical and biochemical changes just before ovulation.
- The local factors prevent premature oocyte maturation and luteinization under control of oocyte. Nitric oxide is another mediator which maintain gap junction system of communication.
- Increased level of LH causes continuous rise in progesterone level in follicle at ovulation. Increased level of progesterone causes negative feedback on LH surge. Progesterone causes distending distensibility of follicle wall and increase in follicular flood also occurs. There is also degenerative changes in collagen of follicle wall so, it becomes thin and stretched.14
- FSH, LH and progesterone stimulate the activity of proteolytic enzymes, resulting in digestion of collagen in the follicular wall and increasing its distensibility. The gonadotropins surge also release histamine, which can induce ovulation.
- Tissue type plasminogen activator and urokinase type plasminogen activator causes production of plasminogen in granulosa and theca cells. Plasminogen activator in granulosa cells is expressed only at the right preovulatory stage in response to LH. The inhibitor system which is very active in theca and interstitial cells prevents inappropriate activation of plasminogen and disruption of growing follicles. Movement of the follicle destined to ovulate to the surface of the ovary is important in that the exposed surface of follicle is now prone to rupture because it is separated from cells rich in plasminogen inhibitor system.
- Ovulation is the result of proteolytic digestion of follicular apex, site called the stigma.
- Prostaglandins E2 and F2α, but mainly prostaglandin E2 and other eicosanoids increase markedly in preovulatory follicular phase in response to LH surge and reaching peak concentration at ovulation. Inhibition of cyclooxygenase-2 (COX-2) synthesis blocks follicle rupture without effecting the other LH-induced process of leuteinization and oocyte maturation.
- Prostaglandins act to free proteolytic enzymes within the follicular wall and the HETEs may promote angiogenesis and hyperemia.
- LH and PGE2 both activate the epidermal growth like factor. Signaling pathway that leads to cumulus expansion and resumption of oocyte meisosis. Prostaglandins may also contract smooth mucule cells that have been identified in ovary, thereby aiding the extrusion of the oocyte—cumulus cell mass.
- A large number of leukocytes enter the follicle prior to ovulation. Neutrophils are prominent feature in the theca compartment of both healthy and atertic antral follicles.
- Ovarian follicular cells themselves in response to LH express the genes involved with immune responses resulting in the release of the product that affect the cellular reactions associated with ovulation and the remodeling process that leads to corpus lutem (Fig. 1.5).
- Extradiol level plunges as LH reaches its peak. Theca tissue derived from healthy antral follicle exhibits marked suppression of steroidogenesis when exposed to high levels of LH. The low midcycle levels of progesterone exert an inhibitory action on further granulosa cell multiplication and drop in estrogen may reflect this local follicular role of progesterone.
- Proliferation of the cumulus cells is suppressed by FSH, while FSH stimulates mural granulosa cell proliferation, supported by oocyte factor. Within hours after the rise in LH, there is a drop in plasma estrogen. Decrease in LH may be due to loss of positive stimulation action of estradiol. LH may further be controlled by short negative feedback of LH upon hypothalamus.15
- Another possibility has been suggested as called gonadotropin surge inhibiting factor (GnSIF) originated in ovary. GnSIF major role in prevention of premature luteinization. These all influences cause rapid decline in LH secretion.
- The follicle must be at appropriate stage of maturation in order for it to respond to ovulating stimulus. In normal cycle, gonadotropin release and final maturation coincide with gonadotropin surge is cotrolled by level of estradiol which in turn a function of follicular growth and maturation.
- In majority of human cycles, the requisite feedback relationship in system allow only one follicle to reach point of ovulation.
- Nonidentical (Dizygote) multiple births may be, in part, reflect the random statistical chance of more than one follicle fulfilling all requirement for ovulation.
Summary of Ovulatory Events
- Progesterone enhances activity of proteolytic enzymes responsible, together with prostaglandins for digestion and rupture of follicular wall.
- The progesterone-influenced midcycle rise in FSH serves to free the oocyte from follicular attachment to convert plasminogen to proteolytic enzyme plasmin and to ensure that sufficient receptors are present to allow an adequate normal luteal phase.So physiology of ovulation is starting with primordial follicle to formation of preovulatory follicle which is required at antral follicle phase to become dominant follicle.
Preovulatory follicle undergoes ovulation. Follicular phase is not fixed phase in time interal. Next to ovulation ovum fully mature with completed Ist meiotic division is free for fertilization in presence of sperm or autolysis in absence of sperm (Fig. 1.6).
To treat cases of infertility due to ovarian factor in concern with anovulatory can be appropriate, if physiology of ovulation is clear.
Luteal phase starts from ovulation in which continued tonic LH support is required. Progesterone, estradiol and inhibin A act centrally to suppress gonadotropins and new follicular growth.
Regression of corpus leteum may involve luteolytic action of its own. Estrogen production mediated by an alteration in local prostaglandin and involving nitric oxide, endothelin and other factor.
In early pregnancy, hCG rescues the corpus luteum, maintains luteal function until placental steroidogenesis initiates.
Demise of corpus luteum results in nadir in circulating levels of estrodiol, progesterone and inhibin. Decrease in inhibin A level removes suppressing influence on FSH secretion. Decrease in estradiol and progesterone removes negative feedback suppression and causes increase in frequency of GnRH pulsatile secretion. There is greater secretion of FSH compared with LH. The increase in FSH is instrumental in rescuing approximately a 70-day old group of ready follicle from atresia allowing dominant follicle to begin its emergence. Then entire process of follicular growth takes place.