Menstruation is the cyclic, orderly sloughing of the uterine lining, in response to the interactions of hormones produced by the hypothalamus, pituitary, and ovaries. The menstrual cycle may be divided into two phases: (1) follicular or proliferative phase, and (2) the luteal or secretory phase (Fig. 1).
The length of a menstrual cycle is the number of days between the first day of menstrual bleeding of one cycle to the onset of menses of the next cycle. The median duration of a menstrual cycle is 28 days with most cycle lengths between 25 to 30 days.1-3 Menstrual cycles that occur at intervals less than 21 days are called polymenorrheic, and menstrual cycles, which are prolonged more than 35 days, are called oligomenorrheic. The menstrual cycle is typically most irregular around the extremes of reproductive life, menarche and menopause, due to anovulation and inadequate follicular development.4-6 The luteal phase is relatively constant with a duration of 14 days. The variability of cycle length is usually derived from varying lengths of the follicular phase of the cycle, ranging from 10 to 16 days.
Fig_1_Overall changes in normal menstrual cycle.jpg
Figure 1. Hormonal, Ovarian, endometrial, and basal body temperature changes and relations throughout the normal menstrual cycle. (From Carr BR, Wilson JD. Disorders of the ovary and female reproductive tract.
THE FOLLICULAR PHASE
The follicular phase begins from the first day of menses until ovulation. Lower temperatures on a basal body temperature chart and more importantly, the development of ovarian follicles characterize this phase. Folliculogenesis begins during the last few days of the preceding menstrual cycle until the release of the mature follicle at ovulation. Declining steroid production by the corpus luteum and the dramatic fall of Inhibin A and B levels allows for FSH to rise during the last few days of the menstrual cycle. This event allows for the recruitment of a cohort of ovarian follicles in each ovary, one of which is destined to ovulate during the next menstrual cycle. Once menses ensues, FSH levels decline due to the negative feedback of estrogen and the negative effects of inhibin produced by the developing follicle. FSH activates the aromatase enzyme in granulosa cells, which converts androgens to estrogen. A decline in FSH levels leads to the production of a more androgenic microenvironment within adjacent follicles to the growing dominant follicle. Also, the granulosa cells of the growing follicle secrete a variety of peptides that may play an autocrine/paracrine role in the inhibition of development of the adjacent follicles.
Development of the dominant follicle has been described in three stages: (1) Recruitment, (2) Selection, and (3) Dominance (Fig.2). During days 1 through 4 of the menstrual cycle, a cohort of follicles is recruited from a pool of nonproliferating follicles in response to FSH. Between cycle days 5 and 7 and only one follicle is selected from the cohort of recruited follicles to ovulate and the remaining follicles will undergo atresia. By cycle day 8, one follicle exerts its dominance by promoting its own growth and suppressing the maturation of the other ovarian follicles
Fig_2_Ovarian recruitment, selection, and ovulation .jpg
Figure 2. Time course for recruitment, selection, and ovulation of the dominant ovarian follicle (DF) with onset of atresia among other follicles of the cohort (N-1).
During the follicular phase, serum estrogen levels rise in parallel to the growth of follicle size as well as to the increasing number of granulosa cells. FSH receptors exist exclusively on the granulosa cell membranes. Increasing FSH levels during the late luteal phase leads to an increase in the number of FSH receptors and ultimately to an increase in estradiol secretion by granulosa cells. The increase in FSH receptor numbers is due to an increase in the population of granulosa cells and not due to an increase in the concentration of FSH receptors per granulosa cell. Each granulosa cell has approximately 1500 FSH receptors by the secondary stage of follicular development and FSH receptor numbers remains relatively constant for the remainder of development. The rise in estradiol secretion appears to increase the number of estradiol receptors. In the presence of estradiol, FSH stimulates the formation of LH receptors on granulosa cells allowing for the secretion of small quantities of progesterone and 17-hydroxyprogesterone (17-OHP) which may exert a positive feedback on the estrogen-primed pituitary to augment LH release .12 FSH also stimulates several steroidogenic enzymes including CYP19, aromatase, and 3-B-hydroxysteroid dehydrogenase.In Table 1, the production rates of sex steroids during the follicular phase, luteal phase, and at the time of ovulation are presented.
|Table 1. Production Rate of Sex Steroids in Women at Different Stages of the Menstrual Cycle|
|DAILY PRODUCTION RATE|
|Blood production and ovarian secretion rates of esuadiol-17b and estrone in women throughout the menstrual cycle.|
|*Values are expressed in milligrams or micrograms per 24 hours.|
Luteinizing hormone receptors are located on theca cells during all stages of the menstrual cycle in contrast to granulosa cells. LH principally stimulates androstenedione production and to a lesser degree testosterone production in the theca cells. In the human, androstenedione is then transported to the granulosa cells where it is aromatized to estrone and finally converted to estradiol by 17-b-hydroxysteroid dehydrogenase type I. This is known as the two-cell, two-gonadotropin hypothesis of regulation of estrogen synthesis in the human ovary (Fig 3).
Fig_3_two-gonadotropin hypothesis of regulation of estrogen
Figure 3. Two-cell, two-gonadotropin hypothesis of regulation of estrogen synthesis in the human ovary.
The primordial follicle is surrounded by a
single layer of granulosa cells and is arrested in the diplotene stage of the
first meiotic division. After puberty, the primordial follicle enlarges and
develops into a preantral follicle. It then develops a cavity and is known as an
antral follicle. Finally, it becomes a preovulatory follicle on its way towards
ovulation. Due to the presence of 5-a-reductase, preantral and early antral
follicles produce more androstenedione and testosterone in relation to
estrogens.15 5-a-reductase is the enzyme responsible for converting testosterone
to dihydrotestosterone (DHT). Once testosterone has been 5- -reduced, DHT can
not be aromatized. However, the dominant follicle is able to secrete large
quantities of estrogen, primarily estradiol, due to high levels of CYP19. This
shift form an androgenic to an estrogenic follicular microenvironment may play
an important role in selection of the dominant follicle from those follicles
that will become atretic.
Development of the follicle to the preantral stage is gonadotropin independent and any follicular growth beyond this point will require gonadotropin interaction. Gonadotropin secretion is regulated by gonadotropin releasing hormone (GnRH), steroid hormones, and various peptides released by the dominant follicle. FSH is elevated during the early follicular phase and then begins to decline until ovulation. In contrast, LH is low during the early follicular phase and begins to rise by the midfollicular phase due to the positive feedback from rising estrogen levels. For the positive feedback effect of LH release to occur, estradiol levels must be greater than 200 pg/ml for approximately 50 hours in duration.16 The gonadotropins are secreted in a pulsatile fashion and the frequency and amplitude of the pulses vary according to the phase of the menstrual cycle (Table2). During the early follicular phase, LH secretion occurs at a pulse frequency of 60 to 90 minutes with relatively constant pulse amplitude. During the late follicular phase prior to ovulation, the pulse frequency increases and the amplitude may begin to increase. In most women, the LH pulse amplitude begins to increase after ovulation.
(Dr Fora : do NOT memorize these numbers, they are presented to complete the issue)
|Table 2. Mean (SEM) Luteinizing Hormone Secretory Burst Characteristics During Phases of the Menstrual Cycle*|
|PERODICITY (min)||AMPLITUDE** (mlU/ml/min)||HALF-DURATIONS (min)||LH HALF-LIFE (min)||TOTAL DAILY SECRETION (mlU/ml/24 hr)|
|Early follicular 175±1.4a||80 ± 3a||0.43 ± 0.02a||6.5 ± 1.0a||131 ± 13a||49 ± 6a|
|Late folicular 26.9±1.6b||53 ± 1b||0.70 ± 0.03b||3.5 ± 0.9b||128 ± 12a||56 ± 8a|
|Midluteal 10.1±1.0c||177 ± 15#||0.26 ± 0.02c#||11.0 ± 1.1e||103 ± 7a||52 ± 4a|
|395 ± 37d#||0.95 ± 0.05d#|
*Entries in each column identified by a, b, c, d differ significantly
(Duncan's multiple-range test, P <.05). Periodicity is itersecretory burst
interval. LH, Luteinizing hormone.
**Duration of the deconvolution-resolved LH secretory burst at half-maximal amplitude.
#Maximal rate of LH secretion attained with the deconvolution-resolved LH secretory burst. The midluteal phase has been divided into small (less than 0.65 mIU/ml/min) and large (greater than 0.65 mIU/ml/min) secretory burst amplitudes.
There are numerous substances found in follicular fluid that regulate the microenvironment of the ovary and that regulate steroidogenesis in granulosa cells, such as steroids, pituitary hormones, plasma proteins, proteoglycans and non-steroidal ovarian factors. Growth factors such as insulin-like growth factor 1 and 2 (IGF1, IGF2) and epidermal growth factor (EGF) are recognized as playing important roles in oocyte development and maturation. The concentration of ovarian steroids is much higher in follicular fluid in comparison to plasma concentrations. There are 2 populations of antral follicles: (1) large follicles, which are greater than 8mm in diameter, and (2) small follicles, which are less than 8mm. In the large follicles, the concentrations of FSH, estrogen, and progesterone are high while prolactin concentration is low. In the small follicles, prolactin and androgen levels are higher than in large antral follicles.
Ovulation occurs approximately 10-12 hours after the LH peak.21 The LH surge is initiated by a dramatic rise of estradiol produced by the preovulatory follicle (Figure 4). To produce the critical concentration of estradiol needed to initiate the positive feedback, the dominant follicle is almost always >15mm in diameter on ultrasound.22 The LH surge occurs 34 to 36 hours prior to ovulation and is a relatively precise predictor for timing ovulation. The LH surge stimulates luteinization of the granulosa cells and stimulates the synthesis of progesterone responsible for the midcycle FSH surge. Also, the LH surge stimulates resumption of meiosis and the completion of reduction division in the oocyte with the release of the first polar body. It has been demonstrated in cultured granulosa cells that spontaneous luteinization can occur in the absence of LH. It is hypothesized that the inhibitory effects of factors such as oocyte maturation inhibitor or luteinization inhibitor are overcome at ovulation.
Fig_4_Changes in gonadotropins and ovarian steroids during mensis.jpg
Figure 4. Changes in gonadotropins and ovarian steroids at midcycle, just prior to ovulation. The initiation of LH surge is at time 0. Abbreviations: E2, estrogen; P, progesterone
Prostaglandins and proteolytic enzymes such as
collagenase and plasmin, are increased in response to LH and progesterone.
Although the precise mechanism is not known, proteolytic enzymes and
prostaglandins are activated and digest collagen in the follicular wall, leading
to an explosive release of the oocyte-cumulus complex. Prostaglandins may also
stimulate ovum release by stimulation of smooth muscle within the ovary. The
point of the dominant follicle closest to the ovarian surface where this
digestion occurs is called the stigma. There is no evidence to support the
theory that follicular rupture occurs as a result of increased follicular
pressure, although precise measurements precisely at rupture have not been
performed. In humans, ovulation probably occurs randomly from either ovary
during any given cycle, not preferentially in the contralateral ovary during the
next menstrual cycle as in the primate model.
The concentrations of prostaglandins E and F series and hydroxyeicosatetraenoic acid (HETE) reach a peak level in follicular fluid just prior to ovulation.28,29 Prostaglandins may stimulate proteolytic enzymes while HETEs may stimulate angiogenesis and hyperemia.30 Patients treated with high dose prostaglandin synthetase inhibitors such as indocin, can block prostaglandin production and effectively block follicular rupture. This gives rise to what is known as the luteinized unruptured follicle syndrome and it presents in fertile and infertile patients equally.31 Therefore, infertility patients are advised to avoid taking prostaglandin synthetase inhibitors, as well as cyclo-oxygenase (COX)-2 inhibitors, especially around the time of ovulation. A schematic diagram illustrating the proposed mechanisms involved in follicular rupture is presented in figure 5.
Fig_5_mechanisms involved in follicular rupture.jpg
Figure 5. Proposed mechanisms involved in follicular rupture.
Estradiol levels fall dramatically immediately prior to the LH peak. This may be due to LH downregulation of its own receptor or because of direct inhibition of estradiol synthesis by progesterone. Progesterone is also responsible for stimulating the midcycle rise in FSH. Elevated FSH levels at this time are thought to free the oocyte from follicular attachments, stimulate plasminogen activator, and increase granulosa cell LH receptors. The mechanism causing the postovulatory fall in LH is unknown. The decline in LH may be due to the loss of the positive feedback effect of estrogen, due to the increasing inhibitory feedback effect of progesterone, or due to a depletion of LH content of the pituitary from downregulation of GnRH receptors.
Length of the Menstrual Cycle
Ovulation occurs on only one day in the cycle and is followed about 2 weeks later by menstruation, in the absence of pregnancy. Normally the time between ovulation and the next menstruation does not vary to any great extent; the length of the menstrual cycle is dependent upon variations in the time from the beginning of the cycle up to ovulation, as illustrated below.
The location of ovulation determines the length of the cycle:
The length of time from the beginning of menstruation up to ovulation can vary. Ovulation is often delayed at times of stress, during lactation and at pre-menopause.
On this one day of ovulation in the cycle, one or more ova may become available for fertilization. The ovum lives for no more than 24 hours, and the sperm cells for a variable time. In the absence of satisfactory mucus the sperm cells are unlikely to survive beyond an hour or so, but with the support of good cervical mucus they may survive for up to 2 or 3 days, even rarely 4 or 5 days.
After ovulation, the granulosa cells continue to enlarge, become vacuolated in appearance, and begin to accumulate a yellow pigment called lutein. The luteinized granulosa cells combine with the newly formed theca-lutein cells and surrounding stroma to become what is known as the corpus luteum. The corpus luteum is a transient endocrine organ that predominately secretes progesterone and its primary function is to prepare the estrogen primed endometrium for implantation of the fertilized ovum. The basal lamina dissolves and capillaries invade into the granulosa layer of cells in response to secretion of angiogenic factors by the granulosa and thecal cells.34 Eight or nine days after ovulation approximately around the time of expected implantation, peak vascularization is achieved. This time also corresponds to peak serum levels of progesterone and estradiol. The central cavity may accumulate with blood and become a hemorrhagic corpus luteum. The life span of the corpus luteum depends upon continued LH support. Corpus luteum function declines by the end of the luteal phase unless human chorionic gonadotropin is produced by a pregnancy. If pregnancy does not occur, the corpus luteum undergoes luteolysis under the influence of estradiol and prostaglandins, and forms a scar tissue- the corpus albicans.
Estrogen levels rise and fall twice during the menstrual cycle. Estrogen levels rise during the midfollicular phase and then drop precipitously after ovulation. This is followed by a secondary rise in estrogen levels during the midluteal phase with a decrease at the end of the menstrual cycle. The secondary rise in estradiol parallels the rise of serum progesterone and 17-hydroxyprogesterone levels. Ovarian vein studies confirm that the corpus luteum is the site of steroid production during the luteal phase.35
The mechanism by which the corpus luteum regulates steroid secretion is not completely understood. Regulation may be determined in part by LH secretory pattern and LH receptors or variations in the levels of the enzymes regulating steroid hormone production, such as 3-ك-HSD, CYP17, CYP19, or side chain cleavage enzyme. The number of granulosa cells formed during the follicular phase and the amount of readily available LDL-cholesterol may also play a role in steroid regulation by the corpus luteum. The luteal cell population consists of at least two cell types, the large and small cells.36 Small cells are thought to have been derived from thecal cells while the large cells from granulosa cells. The large cells are more active in steroidogenesis and are influenced by various autocrine/paracrine factors such as inhibin, relaxin, and oxytocin.
In studies looking into the mechanisms regulating the menstrual cycle, LH was established as the primary luteotropic agent in a cohort of hypophysectomized women.39 After induction of ovulation, the amount of progesterone secreted and the length of the luteal phase is dependant on repeated LH injections. Administration of LH or HCG during the luteal phase can extend corpus luteum function for an additional two weeks.
The secretion of progesterone and estradiol during the luteal phase is episodic, and correlates closely with pulses of LH secretion (figure 6). The frequency and amplitude of LH secretion during the follicular phase regulates subsequent luteal phase function and is consistent with the regulatory role of LH during the luteal phase. Reduced levels of FSH during the follicular phase can lead to a shortened luteal phase and the development of a smaller corpus lutea. Also, the life span of the corpus luteum can be reduced by continuous LH administration during the follicular or luteal phase, reduced LH concentration, decreased LH pulse frequency, or decreased LH pulse amplitude The role of other luteotropic factors such as prolactin, oxytocin, inhibin and relaxin is still unclear.
Fig_6_Episodic secretion of LH and progesterone during mensis.jpg
Figure 6. Episodic secretion of LH (top) and progesterone (bottom) during the luteal phase of a woman. Abbreviations: LH, luteinizing hormone: P, progsterone E2, estradiol; LH + 8, LH surge plus 8 days.
The corpus luteum function begins to decline
9-11 days after ovulation. The exact mechanism of how the corpus luteum
undergoes its demise is unknown. Estrogen is believed to play a role in the
luteolysis of the corpus luteum. Estradiol injected into the ovary bearing the
corpus luteum induces luteolysis while no effect is noted after estradiol
injection of the contralateral ovary. However, the absence of estrogen receptors
in human luteal cells does not support the role of endogenous estrogen in corpus
luteum regression. Prostaglandin F2a appears to be luteolyic in nonhuman
primates and in studies of women. Prostaglandin F2a exerts its effects via the
synthesis of endothelin-1 which inhibits steroidogenesis and stimulates the
release of a growth factor, tumor necrosis factor alpha (TNFa), which induces
cell apoptosis. Oxytocin and vasopressin exert their luteotropic effects via an
autocrine/paracrine mechanism.55 Finally, luteinizing hormone's ability to
downregulate its own receptor may play a role in termination of the luteal
Not all hormones undergo marked fluctuations during the normal menstrual cycle. Androgens, glucocorticoids, and pituitary hormones, excluding LH and FSH, undergo only minimal fluctuations. Due to extra-adrenal 21-hyroxylation of progesterone, plasma levels of deoxycorticosterone are increased during the luteal phase.
HORMONAL EFFECTS ON THE REPRODUCTIVE TRACT
The effects of varying concentrations of estrogen and progesterone through the course of the menstrual cycle have characteristic effects on the endometrium (Fig 7). This allows for histologic dating of the endometrium and is most accurately accomplished by performing an endometrial biopsy 2-3 days prior to menstruation. The proliferative phase is more difficult to date accurately in comparison to the luteal phase. The glands during the proliferative phase are narrow, tubular, and some mitosis and pseudostratification is present. The endometrium thickness is usually between 0.5 and 5mm. In a classical 28 day menstrual cycle, ovulation occurs on day 14. On cycle day 16, the glands take on a more pseudostratified appearance with glycogen accumulating at the basal portion of the glandular epithelium and some nuclei are displaced to the midportion of the cells. In a formalin fixed specimen, glycogen is solubulized resulting in the characteristic basal vacuolization at the base of the endometrial cells. This finding confirms the formation of a functional progesterone producing corpus luteum. In the luteal phase, progesterone decreases the biologic activity of estradiol on the endometrium by: decreasing the concentration of estradiol receptors, increasing the enzymatic activity of 17-ك-hydroxysteroid dehydrogenase type II, the enzyme responsible for the conversion of estradiol to estrone, and by increasing the activity of estrone sulfotransferase.
Fig_7_Dating of the Endometrium.jpg
Figure 7. Dating of the Endometrium.
On cycle day 17, the endometrial glands become more tortuous and dilated. On cycle day 18, the vacuoles in the epithelium decrease in size and are frequently located next to the nuclei. Also, glycogen is now found at the apex of the endometrial cells. By cycle day 19, the pseudostratification and vacuolation almost completely disappear and intraluminal secretions become present. On cycle day 21 or 22, the endometrial stroma begins to become edematous. On cycle day 23, stromal cells surrounding the spiral arterioles begin of enlarge and stromal mitoses become apparent. On cycle day 24, predecidual cells appear around the spiral arterioles and stromal mitoses become more apparent. On cycle day 25, the predecidua begins to differentiate under the surface epithelium. On cycle day 27, there is a marked lymphocytic infiltration and the upper endometrial stroma appears as a solid sheet of well-developed decidua-like cells. On cycle day 28, menstruation begins.
The mucous secreting glands of the endocervix are affected by the changes in steroid hormone concentration. Immediately after menstruation, the cervical mucous is scant and viscous. During the late follicular phase, under the influence of rising estradiol levels, the cervical mucous becomes clear, copious and elastic. The quantity of cervical mucous increases 30 fold compared to the early follicular phase. The stretchability or elasticity of the cervical mucous can be evaluated between two glass slides and recorded as the spinnbarkeit. Under the microscope, the cervical mucous displays a characteristic ferning or palm-leaf arborization appearance. After ovulation, as progesterone levels rise, the cervical mucous once again becomes thick, viscous and opaque and the quantity produced by the endocervical cells decreases.
The changes in hormonal levels of estrogen and progesterone also have characteristic affects on vaginal epithelium. During the early follicular phase, exfoliated vaginal epithelial cells have vesicular nuclei and are basophilic. During the late follicular phase, the vaginal epithelial cells display pyknotic nuclei and are acidophilic due to the influence of rising estrogen levels. As progesterone rises during the luteal phase, the acidophilic cells decrease in number and are replaced by an increasing number of leukocytes.
In the absence of a pregnancy, steroid hormone
levels begin to fall due to declining corpus luteum function. Progesterone
withdrawal results in increased coiling and constriction of the spiral
arterioles. This results in tissue ischemia due to decreased blood flow to the
superficial endometrial layers, the spongiosa and compacta. The endometrium
releases prostaglandins that cause contractions of the uterine smooth muscle and
sloughing of the degraded endometrial tissue. The release of prostaglandins may
be due to decreased stability of lysosomal membranes in the endometrial cells.67
Infusions of prostaglandin F2-a in women during the luteal phase has been shown
to induce endometrial necrosis and bleeding.68 The use of prostaglandin
synthetase inhibitors decreases the amount of menstrual bleeding, and can be
used as therapy in women with menorrhagia. Menstrual fluid is composed of
desquamated endometrial tissue, red blood cells, inflammatory exudates, and
proteolytic enzymes. Within two days after the start of menstruation, estrogen
stimulates the regeneration of the surface endometrial epithelium, while
concomitant endometrial shedding is occurring. The estrogen secreted by the
growing ovarian follicles, causes prolonged vasoconstriction enabling the
formation of a clot over the denuded endometrial vessels. Also, the regeneration
and remodeling of the uterine connective tissue is regulated in part by the
matrix metalloproteinase (MMP) system.70
The average duration of menstrual flow is between four to six days, but the normal range in woman can be from as little as two days up to eight days. The average amount of menstrual blood loss is 30 ml and greater than 80 ml is considered abnormal.
Primary amenorrhea is the given diagnosis for any female over the age of 16 who has never experienced a menstrual period. There are multiple causes for this uncommon disorder that usually stem from biochemical, genetic or developmental abnormalities or, in rare cases, inadequate nutrition.
Diagnostic test may include
•Progesterone challenge test
•Urine chemistry, 17-ketosteroids
•Head CT (computed tomography)
•Head MRI scan
•Ultrasound, pelvic region
Treatment Treatment varies and is dictated by the cause of the amenorrhea. Pituitary tumors, which lead to excessive prolactin secretion, may be controlled with drugs such as bromocriptine; otherwise, surgical removal may be necessary. Radiation therapy is usually reserved until all other options have been exhausted. Developmental abnormalities may be treated with hormonal therapy, surgery or both. If the amenorrhea is the result of a systemic disease, treatment of the disease should resolve the condition.
Secondary amenorrhea is defined as the absence of menstrual periods for six months in a woman who had been experiencing regular cycles, or for 12 months in a woman who had irregular periods previously. Causes of secondary amenorrhea are generally due to conditions such as pregnancy, hyperprolactinemia, premature ovarian failure, drug side effects, excessive weight gain or loss, stress, extensive exercise, Cushing's syndrome, hypothyroidism, Sheehan's syndrome or Asherman'ssyndrome.
Diagnostic test may include
•Progesterone challenge test
•TSH, T3, Free T4
•HCG or progesterone
•Testosterone and SHBG (for obtaining Free Androgen Index [FAI] or calculated free testosterone levels)
•Head CT (computed tomography)
Treatment Treatment is designed to address the underlying causes of the disorder.
Dr Mahmoud Ahmad Fora
Last Updated Mar 25, 2006