Dr (Mrs) Vandana BAGRI BUCKTOWAR
Lecturer , Dept. of Obstetrics and Gynaecology
Padmashree Dr. D Y Patil Medical College
Altima Building, Ebene,Mauritius
Normal reproductive function requires precise quantitative and temporal
regulation of the hypothalamic-pituitary-ovarian axis
Hormone Biosynthesis and
Mechanism of Action - Classified as either
steroids or peptides
PEPTIDE hormones in reproduction:
LH,FSH and HCG: glycoproteins, ∞and β
Activin, Inhibin and Follistatin –polypeptides
Three polypeptide factors—inhibin, activin, and follistatin—
initially isolated from follicular fluid
Named based on their selective effects on FSH biosynthesis and
Inhibin decreases and activin stimulates gonadotrope
function.Follistatin supresses FSH gene expression
Inhibin, activin and follistatin
Inhibin consists of an ∞-subunit linked to one of two highly
homologous β -subunits to form inhibin A (alfa beta A) or inhibin B
( alfa beta B).
Activin is composed of homodimers ( A A, B B) or heterodimers ( A B)
of the same -subunits as inhibin More recently, a number of additional
-subunit isoforms have been identified.
In contrast, follistatin is structurally unrelated to either inhibin or
Steroid Hormones in Reproduction
Sex steroid hormones are synthesized in the gonads,
adrenal gland, and placenta
Cholesterol is the primary building block in
Steroid hormone production, which involves at least 17
enzymes, primarily occurs in the abundant smooth
endoplasmic reticulum found in steroidogenic cells
Steroids are metabolized mainly in the liver and to a lesser
extent in the kidney and intestinal mucosa.
Steroid Hormone Classification
Sex steroids are divided into three groups based on the
number of carbon atoms that they contain.
Each carbon in this structure is assigned a number
identifier, and each ring is assigned a letter
The 21-carbon series includes progestins as well as
glucocorticoids and mineralocorticoids.
Androgens contain 19 carbons, whereas estrogens have 18.
Derivation of Circulating Estrogens
in the Female
Circulating estrogens in the reproductive-aged female are
a mixture of both estradiol and the less potent estrone.
Although a small amount of estriol is produced through
peripheral conversion in the nonpregnant female.
Estriol production is primarily limited to production by the
placenta during pregnancy.
Derivation of Circulating Estrogens
in the Female (contd.)
Estradiol is the primary estrogen produced by the ovary
during reproductive years.
Levels are derived from both direct synthesis in
developing follicles and through conversion of estrone.
Estrone is secreted directly by the ovary and can be
converted from androstenedione in the periphery.
Androgens are converted to estrogens in many tissues,
but mainly result from aromatase activity in skin and
Contribution of the adrenal glands and ovaries to levels of androgens,
dehydroepiandrosterone (DHEA), and DHEA-sulfate (DHEAS).
The typical menstrual cycle is defined as 28 ± 7 days,
with menstrual flow lasting 4 ± 2 days, and an average
blood loss of 20 to 60 mL.
To understand the normal menstrual cycle, it is helpful
to divide the cycle into three phases: the follicular
phase, ovulation, and the luteal phase.
The Follicular Phase
The Luteal-Follicular Transition
The Normal Menstrual Cycle
Menstrual Cycle (contd.)
When viewed from a perspective of ovarian and endometrial and E/P
functions and characteristics:
Size:The adult human ovary is oval with a length of 2 to 5 cm, a
width of 1.5 to 3 cm, and a thickness of 0.5 to 1.5 cm.
During the reproductive years, ovaries weigh between 5 and 10
They are comprised of three parts: an outer cortical region, which
contains both the germinal epithelium and the follicles; a
medullary region, which consists of connective tissue, myoid-like
contractile cells, and interstitial cells; and a hilum, which contains
blood vessels, lymphatics and nerves that enter the ovary
Embryology of ovary
The ovary develops from three major cellular sources:
• (1) Primordial germ cells, which arise from the endoderm of
the yolk sac and differentiate into the primary oogonium,
• (2) Coelomic epithelial cells, which develop into granulosa
• (3) Mesenchymal cells from the gonadal ridge, which become
the ovarian stroma.
Embryology of ovary
Primordial germ cells can be identified in the yolk sac as early as
the third week of gestation.
After the primordial cells reach the gonad, they continue to multiply
through successive mitotic divisions.
Starting at 12 weeks' gestation, a subset of oogonia will enter
meiosis to become primary oocytes.
Primary oocytes are surrounded by a single layer of flattened
granulosa cells, creating a primordial follicle.
The Ovary Across the
Reproductive Life Span
All oogonia either develop into primary oocytes or become atretic. Based on
our current understanding of ovarian function, additional oocytes cannot be
The maximal number of oogonia is achieved at the 20th week of gestation, 67 million
At Birth:1-2 million oogonia
At puberty: 400,000 present at the initiation of puberty, of which less than 500
are destined to ovulate.(most lost through atresia)
There is now strong evidence that follicular atresia is not a passive, necrotic
process, but rather a precisely controlled active process, namely apoptosis,
which is under hormonal control.
Apoptosis begins in utero and continues throughout reproductive life.
Drawing illustrates the steps of meiosis and
the corresponding stages of oocyte
Ovary synthesizes and secretes the sex-steroid hormones—
estrogens, androgens, and progesterone, in a precisely controlled
pattern determined by FSH and LH.
The most important secretory products of ovarian steroid
biosynthesis are progesterone and estradiol.
The ovary also secretes quantities of estrone, androstenedione,
testosterone, and 17-hydroxyprogesterone.
Sex-steroid hormones play an important role in the menstrual
cycle by preparing the uterus for implantation of a fertilized ovum.
If implantation does not occur, ovarian steroidogenesis declines,
the endometrium degenerates, and menstruation ensues.
Two-Cell, Two Gonadotropin
Theory of Ovarian
First proposed by Falck in 1959,
The two-cell theory of ovarian steroidogenesis explains that estrogen
biosynthesis requires the combined action of two gonadotropins (LH and FSH)
on two cell types (theca and granulosa cells)
Theca cells express all of the genes needed to produce androstenedione
This includes high levels of CYP17 gene expression, whose enzyme product
catalyzes 17-hydroxylation—the rate-limiting step in the conversion of
progesterones to androgens
This enzyme is absent in the granulosa cells, so they are incapable of
producing the precursor needed to produce estrogens by themselves.
In response to LH stimulation, theca
cells synthesize the androgens,
androstenedione and testosterone.
These androgens are secreted into the
extracellular fluid and diffuse across
the basement membrane to the
granulosa cells to provide precursors
for estrogen production.
In contrast to theca cells, granulosa
cells express high levels of aromatase
activity in response to FSH stimulation.
Thus, these cells efficiently convert
androgens to estrogens, primarily the
potent estrogen, estradiol.
In sum, ovarian steroidogenesis is
dependent on the effects of LH and
FSH acting independently on the theca
cells and granulosa cells, respectively.
Theory of Ovarian
Steroidogenesis Across the Life
The human ovary has the capacity to produce estrogens by 8
Circulating levels of the gonadotropins, LH and FSH, vary
markedly at different ages of a woman's life.
During the second trimester of fetal development, the plasma
levels of gonadotropins rise to levels similar to those observed in
The fetal hypothalamic-pituitary axis continues to mature during
the second trimester of pregnancy, becoming more sensitive to
the high circulating levels of estrogen and progesterone secreted
by the placenta .
In response to the high levels of these steroids, fetal
gonadotropins fall to low levels prior to birth.
Steroidogenesis Across the Life
After delivery, gonadotropin levels rise abruptly in the neonate due to
separation from the placenta and subsequent freedom from inhibition by
The elevated levels of gonadotropins in the newborn persist for the first
few months of life, declining to low levels in early childhood.
The hypothalamic-pituitary axis has been found to have increased
sensitivity to negative feedback, even by the low circulating levels of
gonadal steroids at this stage.
There is growing evidence that there is an intrinsic role of the central
nervous system in maintaining low gonadotropin levels.
Steroidogenesis Across the Life
One of the first signs of puberty is a sleep-associated increase in LH
Over time, increased gonadotropin secretion is noted throughout the day.
An increased FSH:LH ratio is typical in the premenarchal girl and
During the reproductive years, LH exceeds FSH levels, reversing this ratio.
Steroidogenesis Across the Life
Span –Puberty (Contd.)
Increased gonadotropin levels stimulate ovarian estradiol production.
The rise in estrogen levels results in the growth spurt, maturation of the
female internal and external genitalia, and development of a female
habitus including breast enlargement (thelarche).
Activation of the pituitary-adrenal axis results in an increase in adrenal
androgen production and the associated development of axillary and
pubic hair (adrenache or pubarche).
Increased gonadotropin levels ultimately lead to ovulation and
subsequent menses, with the timing of the first menstrual period defining
menarche. This developmental process takes approximately 3 to 4
Diagram illustrates variations in luteinizing
hormone (LH) and follicle-stimulating hormone
(FSH) during different life stages in the female.
The Follicular phase
During the follicular phase an orderly sequence of events takes place
that ensures the proper number of follicles is ready for ovulation.
In the human ovary the end result of this follicular development is
(usually) one surviving mature follicle.
This process, which occurs over the space of 10-14 days.
It features a series of sequential actions of hormones and autocrineparacrine peptides on the follicle.
leading the follicle destined to ovulate through a period of initial growth
from a primordial follicle through the stages of the preantral, antral, and
The Primordial Follicle
The primordial follicle is nongrowing and consists of an oocyte,
arrested in the diplotene stage of meiotic prophase, surrounded
by a single layer of spindle-shaped granulosa cells.
The mechanism for determining which follicles and how
many will start growing on any given day is unknown.
The number of follicles in each growing cohort appears to
be dependent on the size of the residual pool of inactive
Reducing the size of the pool (e.g., unilateral
oophorectomy) causes the remaining follicles to
redistribute their availability over time,
The Primordial Follicle (contd.)
The follicle that is singled out to play the leading role in a particular
cycle is the beneficiary of a timely match of follicle readiness (perhaps
prepared by autocrine-paracrine actions in its microenvironment) and
appropriate tropic hormone stimulation.
The first follicle able to respond to stimulation may achieve an early
lead that it never relinquishes.
Nevertheless, each cohort of follicles that begins growth is engaged in
a serious competition that ends with only one follicle succeeding.
The size of this cohort appears to be proportional to the number of
inactive primordial follicles within the ovaries and has been estimated
at three to eleven follicles per ovary in young women.
Rescue from Atresia
The follicle destined to ovulate is recruited in the first few days of the cycle.
The early development of follicles occurs over the time span of several menstrual
cycles, but the ovulatory follicle is one of a cohort recruited at the time of the
The total duration of time to achieve preovulatory status is approximately 85
The majority of this time (until a late stage) involves responses that are
independent of hormonal regulation.
Eventually, this cohort of follicles reaches a stage where, unless recruited
(rescued) by follicle-stimulating hormone (FSH), the next step is atresia.
Rescue from Atresia –Apoptosis
Thus, follicles are continuously available (2-5 mm in size) for a
response to FSH.
An increase in FSH is the critical feature in rescuing a cohort of follicles
from atresia, the usual fate of most follicles, eventually allowing a
dominant follicle to emerge and pursue a path to ovulation.
In addition, maintenance of this increase in FSH for a critical duration of
time is essential.
Without the appearance and persistence of an increase in the
circulating FSH level, the cohort is doomed to the process of apoptosis,
programmed physiologic cell death to eliminate superfluous cells.
Apoptosis is derived from Greek and means falling off, like leaves
from a tree.
It has been traditionally used to describe the continuing growth of
antral follicles in response to FSH.
A more useful concept is that the cohort of follicles responding to
FSH at the beginning of a cycle is rescued from apoptosis.
Remember that the very early development of follicles begins
continuously and independently from gonadotropin influence.
The fate of almost all of these follicles is apoptosis; only those
exposed to an increase in FSH stimulation because of the
juxtaposition of their readiness to respond and the increase in FSH
during the luteal-follicular transition have the good fortune to
compete for selection as a dominant follicle.
During the luteal-follicular transition, a small increase in FSH levels is
responsible for selection of the single dominant follicle that will
As previously described, theca cells produce androgens and
granulosa cells generate estrogens.
increase with increased follicular size
enhance the effects of FSH on granulosa cells
and create a feed-forward action on follicles that produce
FOLLICULAR DEVELOPMENTPrimordial follicle
The first visible signs of follicular development are an increase in the
size of the oocyte, and the granulosa cells becoming cuboidal rather
than squamous in shape.
These changes may be better viewed as a process of maturation
rather than growth.
At this same time, small gap junctions develop between the granulosa
cells and the oocyte.
Gap junctions are channels that when open permit the exchange of
nutrients, ions, and regulatory molecules.
Thus, the gap junctions serve as the pathway for nutritional,
metabolite, and signal interchange between the granulosa cells and
With multiplication of the cuboidal granulosa cells (to
approximately 15 cells), the primordial follicle becomes a primary
The granulosa layer is separated from the stromal cells by a
basement membrane called the basal lamina.
The surrounding stromal cells differentiate into concentric layers
designated the theca interna (closest to the basal lamina) and the
theca externa (the outer portion).
The theca layers appear when granulosa proliferation produces 36 layers of granulosa cells.
The Preantral Follicle
Once growth is accelerated, the follicle progresses to the
preantral stage as the oocyte enlarges and is surrounded
by a membrane, the zona pellucida.
The granulosa cells undergo a multilayer proliferation as
the theca layer continues to organize from the surrounding
This growth is dependent on gonadotropins and is
correlated with increasing production of estrogen.
The granulosa cells of the preantral follicle have the ability
to synthesize all 3 classes of steroids; however,
significantly more estrogens than either androgens or
progestins are produced.(AROMATISATION)
Although steroidogenesis in the ovarian follicle is mainly regulated by
the gonadotropins, multiple signaling pathways are involved that
respond to many factors besides the gonadotropins.
Adenylate cyclase enzyme system
Ion gate channels
Tyrosine kinase receptors,
The phospholipase system of second messengers.
These pathways are regulated by a multitude of factors, including
growth factors, nitric oxide, prostaglandins, and peptides such as
gonadotropin-releasing hormone (GnRH), angiotensin II, tissue
necrosis factor-Î±, and vasoactive intestinal peptide.
The fate of the preantral follicle is in
At low concentrations, androgens enhance their own aromatization
and contribute to estrogen production.
At higher levels, the limited capacity of aromatization is
overwhelmed, and the follicle becomes androgenic and ultimately
Follicles will progress in development only if emerging when FSH is
elevated and LH is low.
Those follicles arising at the end of the luteal phase or early in the
subsequent cycle would be favored by an environment in which
aromatization in the granulosa cell can prevail.
The success of a follicle depends on its ability to convert an
androgen-dominated microenvironment to an estrogen-dominated
Summary of Key Events in the
Initial follicular development occurs independently of
FSH stimulation rescues a cohort of follicles from
apoptosis, propelling them to the preantral stage.
FSH-induced aromatization of androgen in the granulosa
results in the production of estrogen.
Together, FSH and estrogen increase the FSH receptor
content of the follicle and stimulate the proliferation of
The Antral Follicle
Under the synergistic influence of estrogen and FSH
there is an increase in the production of follicular fluid
that accumulates in the intercellular spaces of the
This eventually coalescing to form a cavity, as the
follicle makes its gradual transition to the antral stage.
The accumulation of follicular fluid provides a means
whereby the oocyte and surrounding granulosa cells
can be nurtured in a specific endocrine environment.
The granulosa cells surrounding the oocyte are now
designated the cumulus oophorus.
In the presence of FSH, estrogen becomes the dominant
substance in the follicular fluid.
LH is not normally present in follicular fluid until the midcycle. If
LH is prematurely elevated in the circulation and antral fluid,
mitotic activity in the granulosa decreases, degenerative changes
ensue, and intrafollicular androgen levels rise.
Therefore, the dominance of estrogen and FSH is essential for
sustained accumulation of granulosa cells and continued follicular
Antral follicles with the greatest rates of granulosa proliferation
contain the highest estrogen concentrations and the lowest
androgen/estrogen ratios, and are the most likely to house a
healthy oocyte.and become a PREOVULATORY FOLLICLE.
An androgenic milieu antagonizes estrogen-induced granulosa
proliferation and, if sustained, promotes degenerative changes in
Summary of Key Events in the
Follicular phase estrogen production is explained by the
two-cell, two-gonadotropin mechanism, allowing the critical
creation of an estrogen-dominated microenvironment.
Selection of the dominant follicle is established during days
5-7, and consequently, peripheral levels of estradiol begin to
rise significantly by cycle day 7.
Estradiol levels, derived from the dominant follicle, increase
steadily and, through negative feedback effects, exert a
progressively greater suppressive influence on FSH release.
While directing a decline in FSH levels, the midfollicular rise
in estradiol exerts a positive feedback influence on LH
LH levels rise steadily during the late follicular phase, stimulating
androgen production in the theca and optimizing the final maturation
and function of the dominant follicle.
A unique responsiveness to FSH allows the dominant follicle to utilize
the androgen as substrate and further accelerate estrogen production.
FSH induces the appearance of LH receptors on granulosa cells by
growth factors and autocrine and paracrine peptides
Inhibin-B, secreted by the granulosa cells in response to FSH, directly
suppresses pituitary FSH secretion.
Activin, originating in both pituitary and granulosa, augments FSH
secretion and action.IGF enhances all actions of FSH and LH.
Transition from supression to induction
of LH release IN midfollicular phase
With increasing concentrations of estrogen within the
follicle, FSH changes its focus of action, from upregulating its own receptor to generation of the LH
The combination of a capacity for continued response
despite declining levels of FSH and a high local estrogen
environment in the dominant follicle provides optimal
conditions for LH receptor development.
LH can induce the formation of its own receptor in FSHprimed granulosa cells, but the primary mechanism
utilizes FSH stimulation and estrogen enhancement.
There are two critical features in this mechanism:
The concentration of estradiol, and the length of time during
which the estradiol elevation is sustained.
The estradiol concentration necessary to achieve a positive
feedback is more than 200 pg/mL, and this concentration
must be sustained for approximately 50 hours.
This level of estrogen essentially never occurs until the
dominant follicle has reached a diameter of 15 mm.
The estrogen stimulus must be sustained beyond the
initiation of the LH surge until after the surge actually begins.
Otherwise, the LH surge is abbreviated or fails to occur at all.
Inside a preovulatory follicle
Estrogen production becomes sufficient to achieve and maintain
peripheral threshold concentrations of estradiol that are required
in order to induce the LH surge.
Acting through its receptors, LH initiates luteinization and
progesterone production in the granulosa layer.
The preovulatory 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,
derived from the thecal tissue of lesser, unsuccessful follicles.
The preovulatory follicle, through the elaboration of
estradiol, provides its own ovulatory stimulus. Ovulation
approximately 10-12 hours after the LH peak and 24-36
hours after peak estradiol levels are attained.
The onset of the LH surge is the
most reliable indicator of
impending ovulation, occurring
34-36 hours prior to follicle
A threshold of LH concentration
must be maintained for at least
14-27 hours in order for full
maturation of the oocyte to
occur.Usually the LH surge lasts
The gonadotropin surge stimulates a large collection of
events that ultimately leads to ovulation, the physical
release of the oocyte and its cumulus mass of granulosa
A complex series of changes must occur which cause the
final maturation of the oocyte and the decomposition of the
collagenous layer of the follicular wall.
The LH surge initiates the continuation of meiosis in the
oocyte (meiosis is not completed until after the sperm has
entered and the second polar body is released),
luteinization of granulosa cells, expansion of the cumulus,
and the synthesis of prostaglandins and other eicosanoids
essential for follicle rupture.
Premature oocyte maturation and luteinization are
prevented by local factors.
LH-induced cyclic AMP activity overcomes the local
inhibitory action of oocyte maturation inhibitor (OMI)
and luteinization inhibitor (LI).
LI may be endothelin-1, a product of vascular
OMI originates from the granulosa cells, and its activity
depends on an intact cumulus oophorous.
Activin also suppresses progesterone production by
luteal cells, providing yet another means of preventing
With the LH surge, levels of progesterone in the follicle
continue to rise up to the time of ovulation.
Progressive rise in progesterone terminate the LH
surge as a negative feedback effect. Progesterone
increases the distensibility of the follicle wall.
A change in the elastic properties of the follicular wall
occurs just prior to ovulation, unaccompanied by any
significant change in intrafollicular pressure.
FSH, LH, and progesterone stimulate the activity of
proteolytic enzymes, resulting in digestion of collagen
in the follicular wall and increasing its distensibility.
The gonadotropin surge also releases histamine.
The proteolytic enzymes are activated in an orderly sequence
The granulosa and theca cells produce plasminogen activator
in response to the gonadotropin surge.
Plasminogen activators produced by granulosa cells activate
plasminogen in the follicular fluid to produce plasmin.
Plasmin, in turn, generates active collagenase to disrupt the
The inhibitor system, which is very active in the thecal and
interstitial cells, prevents inappropriate activation of
plasminogen and disruption of growing follicles.
*Premature luteinisation prevented by OMI and luteinisation inhibitor(left)
and events at ovulation(right)
A large number of leukocytes enter the follicle prior to ovulation
Neutrophils are a prominent feature in the theca compartment of
both healthy and atretic antral follicles.
These immune cells probably contribute to the cellular changes
associated with ovulation, corpus luteum function, and apoptosis.
Estradiol levels plunge as LH reaches its peak. This may be a
consequence of LH down-regulation of its own receptors on the
The granulosa cells that are attached to the basement
membrane and enclose the follicle become luteal cells.
The cumulus granulosa cells attach to the oocyte. The
mechanism that shuts off the LH surge is unknown.
Within hours after the rise in LH, there is a precipitous
drop in the circulating estrogens.
The decrease in LH can be due to a loss of the
positive stimulating action of estradiol or to an
increasing negative feedback of progesterone.
The abrupt fall in LH levels may also reflect a depletion
in pituitary LH content due to down-regulation of GnRH
Summary of the Key Ovulatory
The LH surge initiates the continuation of meiosis in
the oocyte, luteinization of the granulosa, and
synthesis of progesterone and prostaglandins within
Progesterone enhances the activity of proteolytic
enzymes responsible, together with prostaglandins, for
digestion and rupture of the follicular wall.
The progesterone-influenced midcycle rise in FSH
serves to free the oocyte from follicular attachments, to
convert plasminogen to the proteolytic enzyme,
plasmin, and to ensure that sufficient LH receptors are
present to allow an adequate normal luteal phase.
Before rupture of the follicle and release of the ovum, the
granulosa cells begin to increase in size and assume a
characteristic vacuolated appearance associated with the
accumulation of a yellow pigment, lutein, which lends its
name to the process of luteinization and the anatomic
subunit, the corpus luteum.
During the first 3 days after ovulation, the granulosa cells
continue to enlarge.
In addition, theca lutein cells may differentiate from the
surrounding theca and stroma to become part of the
Dissolution of the basal lamina and rapid vascularization
and luteinization make it difficult to distinguish the origin of
Capillaries begin to penetrate into the granulosa layer after the
cessation of the LH surge, reach the central cavity, and often fill it with
Angiogenesis is an important feature of the luteinization process,
By day 8 or 9 after ovulation, a peak of vascularization is reached,
associated with peak levels of progesterone and estradiol in the blood.
The corpus luteum has one of the highest blood flows per unit mass in
On occasion, this ingrowth of vessels and bleeding results in unchecked
hemorrhage.and an acute surgical emergency that can present at any
time during the luteal phase. Indeed, this is a significant clinical risk in
women who are anticoagulated; such women should receive
medication to prevent ovulation.
The lifespan and steroidogenic capacity of the corpus luteum are
dependent on continued tonic LH secretion.
The corpus luteum is not homogeneous.
Contents of Corpus luteum:luteal cells,endothelial cells, leukocytes, and
The nonsteroidogenic cells form the bulk (70-85%) of the total cell
The leukocyte immune cells produce several cytokines, including
interleukin-1 and tumor necrosis factor, cytolytic enzymes,
prostaglandins, and growth factors involved in angiogenesis,
steroidogenesis, and luteolysis.
Endothelial cells constitute about 50% of the cells in
a mature corpus luteum
The increased perfusion provides these luteal cells with access
to circulating low-density lipoprotein (LDL), which is used to
provide precursor cholesterol for steroid biosynthesis.
These endothelial cells participate in immune reactions and
endocrine functions. They produce endothelin-1, a mediator of
Luteal cell population are of 2 types:large and small cells.
The small cells that contain LH and hCG receptors.and the large
luteal cells produce peptides (oxytocin, relaxin, inhibin, and other
Large luteal cells produce peptides are more active in
steroidogenesis, with greater aromatase activity and more
progesterone synthesis than small cells.
Human granulosa cells (already luteinizing when recovered from in
vitro fertilization patients) contain minimal amounts of P450c17
This is consistent with the two-cell explanation, which assigns
androgen production (and P450c17) to the cells derived from thecal
With luteinization, expression of P450 and 3-hydroxysteroid
dehydrogenase markedly increases, to account for the increasing
production of progesterone, and the continued expression of
mRNAs for these enzymes requires LH.
The aromatase system (P450arom) continues to be active in
luteinized granulosa cells.
Progesterone levels normally rise sharply after ovulation, reaching a peak
approximately 8 days after the LH surge.
Initiation of new follicular growth during the luteal phase is inhibited by
the low levels of gonadotropins due to the negative feedback actions of
estrogen, progesterone, and inhibin-A.
With the appearance of LH receptors on the granulosa cells of the
dominant follicle and the subsequent development of the follicle into a
corpus luteum, inhibin expression comes under the control of LH, and
expression changes from inhibin-B to inhibin-A.
The circulating levels of inhibin-A rise in the late follicular phase to reach
a peak level at the midluteal phase.
Inhibin-A, therefore, contributes to the suppression of FSH to nadir levels
during the luteal phase, and to the changes at the luteal-follicular
The secretion of progesterone and estradiol during the luteal phase is
episodic, and the changes correlate closely with LH pulse.
Because of this episodic secretion, relatively low midluteal progesterone
levels, which some believe are indicative of an inadequate luteal phase,
can be found in the course of totally normal luteal phases.
In the normal cycle the time period from the LH midcycle surge to
menses is consistently close to 14 days.
For practical purposes, luteal phases lasting between 11 and 17 days
can be considered normal.
The incidence of short luteal phases is about 5-6%.
It is well known that significant variability in cycle length among
women is due to the varying number of days required for follicular
growth and maturation in the follicular phase.
The luteal phase cannot be extended indefinitely even with
progressively increasing LH exposure, indicating that the demise of
the corpus luteum is due to an active luteolytic mechanism.
The corpus luteum rapidly declines 9-11 days after ovulation, and the
mechanism of the degeneration remains unknown.
The morphologic regression of luteal cells may be induced by the estradiol
produced by the corpus luteum.
This action of estrogen may be mediated by nitric oxide. Nitric oxide
stimulates luteal prostaglandin synthesis and decreases progesterone
Nitric oxide and hCG have opposing actions in the human corpus luteum;
nitric oxide is associated with apoptosis of luteal cells.
The final signal for luteolysis, however, is prostaglandin F, produced within
the ovary in response to the locally synthesized luteal estrogen.
In view of the known estrogen requirement for the synthesis of progesterone
receptors in endometrium, luteal phase estrogen may be necessary to allow
the progesterone-induced changes in the endometrium after ovulation.
Inadequate progesterone receptor content due to inadequate estrogen
priming of the endometrium is an additional possible mechanism for
infertility or early miscarriage, another form of luteal phase deficiency.
Experimental evidence indicates that the luteolytic effect of
prostaglandin F2 is partially mediated by endothelin-1
Prostaglandin F2Î± stimulates the synthesis of endothelin; endothelin-1
inhibits luteal steroidogenesis, and in turn, endothelin-1 stimulates
prostaglandin production in luteal cells.
In addition, endothelin-1 stimulates the release of tumor necrosis
factor-Î±, a growth factor known to induce apoptosis.
Gap junctions are a prominent feature of luteal cells, just as they are in
the follicle before ovulation.
The process of luteolysis involves proteolytic enzymes, especially the
matrix metalloproteinases (MMPs).
Luteolysis is believed to involve a direct increase in MMP expression.
The human ovary contains the complete interleukin-1 system, providing
another resource for cytolytic enzymes.
The survival of the corpus luteum is prolonged by the emergence of a
new stimulus of rapidly increasing intensity, hCG.
This new stimulus first appears at the peak of corpus luteum
development (9-13 days after ovulation), just in time to prevent luteal
hCG serves to maintain the vital steroidogenesis of the corpus luteum
until approximately the ninth or tenth week of gestation, by which time
placental steroidogenesis is well established.
In some pregnancies placental steroidogenesis will be sufficiently
established by the seventh week of gestation.
In addition, the rescue of the corpus luteum by an early pregnancy with
hCG is associated with maintenance of the vascular system (not new
vessel growth), a process dependent on the angiogenic factors VEGF
Summary of Key Events in the Luteal
Normal luteal function requires optimal preovulatory follicular development
(especially adequate FSH stimulation) and continued tonic LH support.
The early luteal phase is marked by active angiogenesis mediated by VEGF.
New vessel growth is held in check by angiopoietin-1 working through its
receptor Tie-2 on endothelial cells. Regression of the corpus luteum is
associated with a decrease in VEGF and angiopoietin-1 expression and an
increase in angiopoietin-2 activity.
Progesterone, estradiol, and inhibin-A act centrally to suppress
gonadotropins and new follicular growth
Regression of the corpus luteum may involve the luteolytic action of its own
estrogen production, mediated by an alteration in local prostaglandin
concentrations and involving nitric oxide, endothelin, and other factors.
In early pregnancy, hCG rescues the corpus luteum, maintaining luteal
function until placental steroidogenesis is well established.
The interval extending from the late luteal decline of estradiol and
progesterone production to the selection of the dominant follicle.
It is a critical and decisive time, marked by the appearance of menses,
but less apparent and very important are the hormone changes that
initiate the next cycle.
The critical factors include GnRH, FSH, LH, estradiol, progesterone,
Through the FSH-mediated actions on the granulosa cells, the
recruitment of a new ovulating follicle is directed by a selective
increase in FSH that begins approximately 2 days before the onset of
There are at least two influential changes that result in this important
increase in FSH:
a decrease in luteal steroids and inhibin and
a change in GnRH pulsatile secretion.
ROLE OF INHIBIN , ACTIVIN ,FOLLISTATIN AND OTHER
FACTORS AFFECTING FSH
Inhibin-B, originating in the granulosa cells of the corpus luteum and
now under the regulation of LH, reaches a nadir in the circulation at the
Inhibin-A reaches a peak in the luteal phase
These may help to suppress FSH secretion by the pituitary to the lowest
levels reached during a menstrual cycle.
The process of luteolysis, with the resulting demise of the corpus
luteum, affects inhibin-A secretion as well as steroidogenesis.
Thus, an important suppressing influence on FSH secretion is removed
from the anterior pituitary during the last days of the luteal phase.
The selective action of inhibin on FSH (and not LH) is partly responsible
for the greater rise in FSH seen during the luteal-follicular transition,
compared to the change in LH.
Inhibin-B levels begin to rise shortly after the increase in FSH (a
consequence of FSH stimulation of granulosa cell secretion of inhibin)
and reach peak levels about 4 days after the maximal increase in FSH.
Thus, suppression of FSH secretion during the follicular phase is an
action exerted by inhibin-B, whereas escape of FSH inhibition during
the luteal-follicular transition is partly a response to decreasing
inhibin-A secretion by the corpus luteum.
Circulating levels of activin increase in the late luteal phase to peak at
It contribute to the rise in FSH during the luteal-follicular transition.
Activin enhances and follistatin suppresses GnRH activity.
Evidence in vivo and in vitro indicates that gonadotropin response to
GnRH requires activin activity
The selective rise in FSH is also significantly influenced by a change
in GnRH pulsatile secretion, previously strongly suppressed by the
high estradiol and progesterone levels of the luteal phase exerting a
negative feedback effect at the hypothalamus.
A progressive and rapid increase in GnRH pulses occurs during the
From the midluteal peak to menses, there is a 4.5-fold increase in LH
pulse frequency (and presumably GnRH) from approximately 3 pulses
per 24 hours to 14 pulses per 24 hours.
During this time period, the mean level of LH increases approximately
2-fold, from approximately a mean of 4.8 IU/L to 8 IU/L.
The increase in FSH is greater than that of LH.
FSH pulse frequency increases 3.5-fold from the midluteal period to
the time of menses, and FSH levels increase from a mean of
approximately 4 IU/L to 15 IU/L.
An increase in GnRH pulse frequency from a low level of secretion has
been associated with an initial selective increase in FSH in several
experimental models, including the ovariectomized monkey with
destruction of the hypothalamus.
Treatment of hypogonadal women with pulsatile GnRH results first in
predominance of FSH secretion (over LH).
This experimental response and the changes during the lutealfollicular transition are similar to that observed during puberty, a
predominance of FSH secretion as GnRH pulsatile secretion begins to
The pituitary response to GnRH is also a factor.
Estradiol suppresses FSH secretion by virtue of its classic negative
feedback relationship at the pituitary level.
The decrease in estradiol in the late luteal phase restores the
capability of the pituitary to respond with an increase in FSH
Summary of Key Events in the
The demise of the corpus luteum results in a nadir in the circulating levels
of estradiol, progesterone, and inhibin.
The decrease in inhibin-A removes a suppressing influence on FSH
secretion in the pituitary.
The decrease in estradiol and progesterone allows a progressive and rapid
increase in the frequency of GnRH pulsatile secretion and a removal of the
pituitary from negative feedback suppression.
The removal of inhibin-A and estradiol and increasing GnRH pulses
combine to allow greater secretion of FSH compared with LH, with an
increase in the frequency of the episodic secretion.
The increase in FSH is instrumental in rescuing an approximately 70-dayold group of ready follicles from atresia, allowing a dominant follicle to
begin its emergence.
ENDOMETRIUM- Histologic Menstrual
The endometrium consists of two layers: the basalis layer, which lies
against the myometrium, and the functionalis layer, which is apposed to
the uterine lumen.
The basalis layer, which does not change significantly across the
menstrual cycle, is critical for regeneration of the endometrium following
The functionalis layer of the endometrium can be further divided into the
superficial, thin, stratum compactum, which consists of gland necks and
dense stroma, and the underlying stratum spongiosum, which contains
glands and large amounts of loosely organized stroma and interstitial
After menstruation, the endometrium is only one to two millimeters thick.
Under the influence of estrogen, the glandular and stromal cells of the
functionalis layer proliferate rapidly following menses.
This period of rapid growth,
termed the proliferative phase,
corresponds to the ovary's
As this phase progresses,
glands become more tortuous
and cells lining the glandular
The stroma remains compact.
Endometrial thickness is about
12 millimeters at the time of the
LH surge and does not increase
Following ovulation, the endometrium transforms into a secretory tissue. The
period defined as the secretory phase of the endometrium and correlates to
the ovary's luteal phase.
Glycogen-rich subnuclear vacuoles appear in cells lining the glands.
Under further stimulation by
progesterone, these vacuoles
move from the glandular base
toward the lumen and expel their
This secretory process peaks on
approximatelypostovulatory day 6,
coinciding with the day of
Glands become increasingly
tortuous, and the stroma becomes
In addition, spiral arteries that
feed the endometrium increase
their number and coiling.
If a blastocyst does not implant, then the corpus luteum is not maintained
by placental hCG, progesterone levels drop, and endometrial glands begin
Polymorphonuclear leukocytes and monocytes from the nearby
vasculature infiltrate the endometrium.
The spiral arteries constrict leading to local ischemia, and lysosomes
release proteolytic enzymes that accelerate tissue destruction.
Prostaglandins (PGs), particularly prostaglandin F2 , are present in the
endometrium and likely contribute to arteriolar vasospasm. Prostaglandin
F2 also induces myometrial contractions, which may aid in expelling the
The entire endometrial functionalis layer is thought to exfoliate with
menstruation, leaving only the basalis layer to provide cells for
Following menstruation, re-epithelialization of the desquamated
endometrium is believed to be initiated within 2 to 3 days after the onset of
menses and to be completed within 48 hours.
Menstrual phase: fragmented endometrium with condensed stroma and
glands with secretory vacuoles are seen in a background of blood.
In the human, the embryo enters the uterine cavity 2 to 3 days after
fertilization with implantation beginning approximately 4 days later
It can be defined as the temporal window of endometrial maturation
during which trophectoderm attaches to the endometrial epithelial
cells with subsequent invasion of the endometrial stroma.
Based on a number of studies, the window of implantation in the
human is relatively broad, extending from day 20 through day 24 of the
Several investigators have attempted to correlate biochemical markers
and ultrastructural features of the endometrium with the presence of
Endometrial maturation is associated with loss of both surface microvilli
and ciliated cells, as well as the development of cellular protrusions,
called pinopods, on the apical surface of the endometrium
The presence of pinopods is considered to be an important morphologic
marker of peri-implantation endometrium.
Pinopod formation is known to be highly progesterone dependent.
Biochemical markers of uterine receptivity have included studies of
mucin 1 (MUC1) and keratin sulfate.
These transmembrane glycoproteins are significantly increased on the
glandular cell surface during the peri-implantation period
Integrins are also marker for identification of the implantation window
The corpus luteum is the major source of steroid production in early
By the seventh week of gestation, approximately 50 percent of
estrogen in the maternal circulation is produced in the placenta
Removal of the corpus luteum will result in miscarriage before the
switch to placental steroid production.
Using a conservative approach, progesterone supplementation should
be considered in any woman who has surgical removal of the corpus
luteum before 10 weeks of gestation, with supplementation continued
at least to this time.
Placental progesterone is synthesized from cholesterol primarily
derived from maternal circulation.
Progesterone is secreted continuously into the maternal circulation
rather than stored, as occurs for peptide products.
Maternal progesterone serum levels increase from approximately 25
ng/mL during the midluteal phase to 150 ng/mL at term.
Progesterone has been postulated to be the critical mediator of uterine
quiescence during pregnancy, possibly via inhibition of prostaglandin
Progesterone is also a potent immunomodulatory agent that may
block immune rejection of the developing fetus
Estriol-The pregnancy estrogen
The placenta lacks the steroidogenic enzyme CYP17, which is required for
converting C21 steroids, including progesterone, into C19 androgens.
Therefore, placental estrogen production is dependent on precursors
provided by other systems.
While progesterone production is dependent on maternal precursors,
estrogen production is dependent on precursors from the fetal adrenal
The fetal adrenal gland is nearly as large as the fetal kidney, and therefore
is proportionally enlarged relative to the adult adrenal gland.
The fetal adrenal cortex produces DHEAS, which is subsequently
hydroxylated to form 16 -hydroxy-DHEAS in the fetal liver.
The high levels of aromatase activity in the placenta convert 16 -hydroxyDHEAS to estriol, explaining the high circulating levels of this steroid
Steroid Production Related to Fetal
It has been appreciated for many decades that measurement of urinary
estriol can be used as a marker for fetal health.
Estrogen levels are markedly blunted in patients making low levels of
androgen precursors, such as occurs in anencephaly, adrenal
hypoplasia, or fetal demise.
Serum estriol is now used in the second trimester as part of the triple
screen and quadruple screen to look for Down syndrome and neuraltube defects, among other fetal anomalies.
The Normal Menstrual Cycle
Menstrual cycle length is determined by the rate and quality of follicular
growth and development, and it is normal for the cycle to vary in individual
Cycle lengths are the shortest (with the least variability) in the late 30s, a time
when subtle but real increases in FSH and decreases in inhibin are occurring.
This can be pictured as accelerated follicular growth.
At the same time, fewer follicles grow per cycle as a woman ages.
Approximately 2-4 years prior to menopause, the cycles lengthen again. In
the last 10-15 years before menopause, there is an acceleration of follicular
This accelerated loss begins when the total number of follicles reaches
approximately 25,000, a number reached in normal women at age 37-38
Eventually menopause occurs because the supply of follicles is depleted.
Variations in menstrual flow and cycle length are common at the
extremes of reproductive age, during the early teenage years and the
years preceding the menopause.
The prevalence of anovulatory cycles is highest in women under age
20 and over age 40.
Menarche is typically followed by approximately 5-7 years of
relatively long cycles that gradually decrease in length and become
Although menstrual cycle characteristics generally do not change
appreciably during the reproductive years, overall cycle length and
variability slowly decrease.
On average, mean cycle length and variability reach their lows at
about age 40-42.
Over the subsequent 8-10 years before the menopause, the trend is
reversed; both average cycle length and variability steadily increase
as ovulations become less regular and frequent.
Mean cycle length is greater in women at the extremes of body mass
and composition; both high and low body mass index are associated
with an increased mean cycle length.
In general, variations in cycle length reflect differences in the length
of the follicular phase of the ovarian cycle.
Women who have a 25-day cycle ovulate on or about cycle day 1012, and those with a 35-day cycle ovulate approximately 10 days
Within a few years after menarche, the luteal phase becomes
extremely consistent (13-15 days) and remains so until the
At age 25, over 40% of cycles are between 25 and 28 days in
length; from age 25 to 35, over 60% are.
Although it is the most often reported intermenstrual interval, only
approximately 15% of cycles in reproductive aged women are
actually 28 days in length.
Less than 1% of women have a regular cycle lasting less than 21
days or more than 35 days.300 Most women have cycles that last
from 24 to 35 days, but at least 20% of women experience irregular