Reproductive cycle





Introduction


Every healthy human female experiences reproductive cycles that organize their reproductive system for pregnancy. The reproductive cycles are under the dynamic influence of integrated action of hormones from hypothalamus, anterior pituitary, and gonadal ovarian steroids. Ovarian cycle is directly regulated and synchronized by the anterior pituitary follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Uterine cycle is controlled by a direct reaction to ovarian steroid hormones (estrogen and progesterone). The definitive rheostat of the pituitary gonadotropins (FSH and LH) is situated centrally in the hypothalamus, which is highly responsive to plasma concentration of ovarian steroid hormones. The gonadotropin synthesis and secretion from anterior pituitary are augmented and regulated by the release of gonadotropin-releasing hormone (GnRH) from hypothalamus into hypothalamic–hypophyseal–portal circulation.


Puberty in females


An overview of menarche


In girls, puberty is considered as the development of the genital organs, secondary sexual characteristics, and the appearance of menarche. Thus, the main physiological mechanisms that determine puberty in girls include ovarian growth and maturation with increased synthesis and secretion of sex steroid hormones with folliculogenesis and ovulation. These changes are responsible for the occurrence of menarche in girls. Menarche is the memorable beginning of a woman’s reproductive life. It is commonly accepted that the cyclical hormonal variabilities that control the reproductive cycle have an essential natural impact on a woman’s physical and mental health. This cycle duration is from 25 to 35 days, usually of 28 and 26 days of length in women’s reproductive life. The usual age group at menarche is 9–14 years and the mean is 12–13 years. Body mass index and lifestyle may influence the age at menarche. Menarche is associated with an anovulatory cycle and ideally appears 2–2.5 years after the initial stages of mammary gland development. In the 1st year postmenarche, the menstrual cycles are typically irregular and anovulatory ranges from 21 to 45 days. Menstrual cycles can remain continuously irregular until the 3–5th year postmenarche. The predominance of primordial and preantral follicles occurs before puberty; tiny follicles can grow through this period of development. These small follicles are gonadotropin-independent. The volume of ovary increases by the commencement of adolescence, attains increase in bulk or size rapidly after menarche especially between 13 and 16 years, and remains constant or decreased marginally subsequently. Polycystic ovarian morphological changes are noticed in healthy young girls; this morphology is not associated with reduced ovulatory rate, hyperandrogenism, or metabolic disorders. In the initial phase of postmenarche, ovarian morphological changes on transabdominal ultrasound scan show multicystic ovaries and significantly raised ovarian volume that differs from ovarian morphological features observed in elder women.


The menstrual cycle


In the normal ovulatory menstrual cycle, four hormones FSH, LH, estrogen, and progesterone, with the central pulsatile release of GnRH, regulate the normal reproductive activities in women like continuous cycles of follicular growth and development, ovulation, and endometrial preparation for the implantation of the blastocyst ( Fig. 3.1 ). Active high secretion of FSH through the luteal–follicular switch leads to the staffing of a troop of follicles and the appearance and progression of a dominant follicle called mature Graafian follicle. From the ovary, estradiol and inhibin release are the main inhibitors of ongoing excitatory release of FSH, while high estradiol with other prospective elements is significant for the LH surge, centrally regulated at anterior pituitary in women. Corpus luteum releases progesterone and estradiol to prepare the uterine endometrium for implantation, and its decease permits FSH to become high with the start of a next cycle.




Fig. 3.1


Reproductive cycle.

From Koyama A, Hagopian L, Linden J. Emerging options for emergency contraception. Clin Med Insights Reprod Health. 2013:7;CMRH-S8145.


The phases of menstrual cycle with endocrine control


Follicular phase


Early follicular period is best described as “an early rise in FSH with mobilization of fresh follicular squad toward the growing bank of follicles with high secretions of inhibin B and an initial rise in estradiol levels.” The initial part of the follicular phase of the well-maintained cycle is significantly associated with obvious declining of GnRH pulsatile release during sleep. Sleep may cause a fall in pulsatile GnRH release from hypothalamus ( Fig. 3.2 ) and may assist the role of maintaining FSH synthesis during this sensitive phase of follicular trooping.




Fig. 3.2


Hypothalamic–pituitary–ovarian axis.


The middle part of follicular phase is mainly characterized by the dominant follicle appearance and lower secretion of FSH in response to inhibin B and high estradiol concentration with a later high level of inhibin A. High estradiol causes endometrial growth and development. In the mid-follicular period, GnRH pulsatile release becomes high and interpulse break minimizes to almost 1 h. LH pulse amplitude is significantly reduced, demonstrating the downregulation (negative feedback) of estradiol release from growing follicles in response to the GnRH pulse amplitude and due to the straight anterior pituitary suppression of gonadotrope release. The latter part of the follicular period is best described by rapid and increased secretion of estradiol and inhibin A in association with inhibin B and FSH decreased concentrations. The cyclical discharge of GnRH that started at the mid-follicular period is sustained to the latter part of the follicular phase. However, LH secretion increases as a result of the excitatory signaling from high estradiol and possibly inhibin A on gonadotropes, with increased responsiveness to GnRH. This significant increase in estradiol levels is responsible for active endometrial growth and proliferation.


Mid-cycle surge


Due to exponential high estradiol and possibly inhibin A secretion in the late follicular phase, LH concentration increases 10-fold in 48–72 h, while FSH concentration raises 4-fold. The LH surge in the middle cycle is mainly needed for final oocyte maturation and start of follicular rupture, commonly ensuing almost 36 h post-LH surge. LH surge is widely accepted as a crucial factor for normal regular reproductive cycles. The levels for estrogen secretion are sensitive to positive feedback. Recommendations of synthetic estradiol to healthy ladies at the time of initial follicular phase or to postmenopausal women prompt rise in basal and GnRH induced LH release, which is more reliant on the dosage and extent of estrogen secretion. The high estradiol always favors the LH surge.


Luteal phase


After ovulation, the development of corpus luteum results in the discharge of progesterone, estradiol, and inhibin A and generates suppressive effects on gonadotropins (FSH, LH) release. Reduced pulsatile GnRH secretion starts at the end of the mid-cycle surge and lasts throughout the luteal phase; this effect occurs due to progesterone. In the luteal phase, LH pulse rates are highly expressive as compared to the follicular phase due to reduced pulsatile GnRH secretion induced by progesterone and the inverse association among LH sensitivity to GnRH and GnRH pulse frequency and also due to the direct effect of progesterone on the anterior pituitary to enhance LH sensitivity to GnRH. The corpus luteum has a limited life, and during the nonexistence of gestation, reduced blood concentration of progesterone and estradiol leads to endometrial shedding.


Luteal–follicular transition


The weakening of corpus luteum and reduced secretion of ovarian steroid hormones and inhibin A facilitates increased FSH secretion that starts before menstruation and participates in the accumulation of fresh troop of follicles in the emerging bank. Maintenance of the mid-luteal-period concentration of estradiol controls rise in FSH. While proof for the effect of estradiol in the negative feedback control of FSH secretion in women is well established , the role of inhibins A and B relative to estradiol and relative to each other remains disputed. Control of the FSH rise across the luteal–follicular transition is regulated in part by release of negative feedback associated with deteriorating levels of estradiol from the decease of the corpus luteum. Therefore, scientists investigated the effects of tamoxifen to suppress the estrogen receptor in normal cycles explored that inhibin A participates in preventing the FSH release through normal luteal phase. LH pulse frequency raises just prior to the start of menstrual discharge. LH pulse frequency is negatively associated with progesterone levels, and administering the mid-luteal-period quantities of progesterone in combination with estradiol controls normal luteal–follicular rise in GnRH pulse frequency in healthy women.


Histological changes during menstrual cycle


Early proliferative phase


In the early proliferative period, the endometrial width is normally < 2 mm. In lower basal zones, the multiplication of cells occur and epithelial layers enduring in the basal uterine divisions lead to renovation and luminal epithelium repair through day 5 of the normal cycle. During this, mitosis is apparent in the glandular and stromal epithelial lining. This frequent practice of wound healing generally avoided the scarring. Endometrial stem cells are highly proficient in producing precursors of stromal and epithelial origins. Adult progenitor cells have the capacity to remarkably contribute to the regenerative process. In the early proliferative phase, the endometrial glands are narrow and straight, with the lining of low columnar cells. The ultrastructure showed epithelial cell cytoplasm with several ribosomes; however, endoplasmic reticulum with Golgi apparatus is not well developed in these cells.


Late proliferative phase


As an outcome of endometrial glandular hyperplasia and rise in stromal extracellular matrix, the thickness of endometrium improves in the late proliferative phase. The glands are extensively parted proximate to the surface of the endometrium and more congested and convoluted deeply into the endometrium. The glandular epithelial cells become tall and pseudostratified near ovulation. The impact of steroid hormones on endometrium growth, development, and secretion is mainly reliant on basalis and functionalis zones, and endometrial proliferation is confined to the functionalis layer.


Early secretory phase


Ovulation indicates the commencement of secretory period of endometrial cycle, while it is distinguished, the endometrial luminal cells and glandular cells also demonstrate secretory action after the proliferative period. Mitotic events in epithelial and stromal cells are limited to initial 3 days postovulation and are hardly witnessed in the subsequent part of the cycle. Ultrastructural level of endometrial epithelia discloses plentiful endoplasmic reticulum, un-remarkably big-sized mitochondria with noticeable and dominant cristae. A network of collagen type 1 and collagen type 3 is recognized in stroma through early secretory period. Edematous changes in stroma contribute to the endometrial thickening during this time.


Mid-secretory phase


The distinguishing point of the mid-secretory period is the expansion of spiral arteries. These blood vessels become largely spiraled. The glands are tortuous with secretory function, reaches at a highest level 6 days postovulation, demonstrated through the absence of vacuoles from the epithelial cell cytoplasm, the nucleolar channel system, visible briefly in nucleoli of 10% of the secretory phase epithelial cells within 16–24 days. The nuclear channel system comprises of amorphous matrix, dense granules, and a series of tubular channels. The nuclear channel system is the inward folding of the inner nuclear membrane, for the transport of mRNA to the cytoplasm. The nucleolar channel system is an ultrastructure assurance of the secretory phase during implantation. The expansion of stromal cells around the vessel attains an eosinophilic cytoplasm with pericellular extracellular matrix in the mid-to-late secretory period. These modifications are called predecidualization to differentiate them from additional stromal alteration that arises in a fertile cycle, emphasizing the demarcation among the subepithelial compact and spongy zone.


Decidual stromal cells contribute primarily in maternal–fetal interfaces and their precursors confine to the endometrial and decidual perivascular region where these precursors share the alpha-smooth muscle actin expression and contraction under the cytokine influence. Ultrastructurally, the predecidual stromal cells show well established Golgi apparatus and endoplasmic reticulum. The laminin, fibronectin, heparan sulfate, and type IV collagen are parts of nearby matrix. Stromal cells of the mid-secretory and late secretory phases express the protein stock that stimulates hemostasis by a tissue factor, a membrane-bound protein that starts blood coagulation once it comes in contact with ruptured blood vessel, blood, and plasminogen activator inhibitor type 1, which prevent fibrinolysis.


Premenstrual and menstrual phase


Principal histologic characteristics of premenstrual duration are the extinction of stromal reticular nexus, assemble, or catalyzed by matrix metalloproteinase (MMPs) infiltration of stromal layer by polymorphonuclear and mononuclear leukocytes. The endometrial glands become fatigued and tired due to massive secretion, and epithelial cells show nuclei at the lowest regions. The nuclei of granular lymphocytes, undergo pyknosis and karyorrhexis indicating apoptosis, suggested as few initial events portend menses. These modifications occur before the degradation of the extracellular matrix and leukocytic penetration. In the glandular epithelium, the nucleolar channel system, oversized mitochondria, and features of the early and mid-secretory periods are disappeared. The uterine endometrial lining reduces and paves the way for endometrial shedding in menstruation, partially due to the reduced secretory process and breakdown of the extracellular matrix.


Menstruation


This phase mainly ensues by progesterone decline, sign of failure to attain conception, and necessity to remove the specialized uterine endometrial layer developed during decidualization. The uniqueness of the phase is determined by progesterone and estrogen decline with corpus luteum deterioration. In infertile cycles, the menses appear nearly and exclusively in humans and some old primates. The molecular mechanisms activated as a result of progesterone decline involve the excitation of the nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κβ) transcriptional pathway, a principal target of cytokines and subsequent genetic expression like endometrial bleeding-associated factor (EBAF), an anti-TGF-β cytokine that restricts the mechanism of action of other constitute of TGF-β family that endorses the endometrial rectitude. The coordinated blockage of the events of TGF-β seems to commence major succeeding measures of menses, comprising expansion of MMPs.


Menstrual disturbances associated with the pathological conditions


Disturbance in the menstrual cycle may influence the reproductive life of young girls as well as adult women. The developing high prevalence of dysmenorrhea (painful menses), irregularity in monthly cycles, prolonged heavy menstrual bleeding in young girls and adult women is associated with a significant amount of physical and mental stress. Dysmenorrhea restricts their routine physical activities and affects the quality of life. Dysmenorrhea may be of primary/unknown origin or secondary when there is an identified underlying cause. Endometriosis is considered the commonest reason for painful menstruation. Regular exercise, early childbirth, and proper management can help to control dysmenorrhea and chances of related complications in these girls.


The lengthy interval between the cycles and heavy menstruation are alarming features when associated with anovulation, hirsutism, and acne. Cushing syndrome, thyroid disorders, adrenal abnormalities, ovarian tumors, and premature decline in ovaries are the common pathological conditions associated with oligomenorrhea, which refer to the occurrence of menses at intervals of more than 35 days due to a prolonged follicular phase. Oligomenorrhea needs logical evaluation and treatment, regardless of age. Oligomenorrhea in young girls is significantly associated with hyperandrogenism and insulin resistance, the hallmark of the polycystic ovarian syndrome (PCOS). Amenorrhea (primary and secondary) is also a common problem among girls and adult women. Primary amenorrhea means menarche fails to arise and secondary amenorrhea means the termination of menses after the commencement. These problems in young females are mainly because of the instabilities of the hypothalamic–pituitary–ovarian axis (H–P–O axis). The variation in prognosis from females with secondary amenorrhea, compared with females having a history of oligomenorrhea indicates diverse mechanisms responsible for the malfunctioning of menstrual cycle. Hypothalamic suppression of the ovarian axis leads to secondary amenorrhea, while hyperandrogenism and PCOS are commonly observed in girls with oligomenorrhea. The majority of cases (61%) of PCOS present with oligomenorrhea at their first visit. Hence, it is suggested that menstrual irregularities with the reduced menstrual flow in young girls are an initial alarming signal for PCOS.


Menorrhagia refers to the heavy bleeding of more than 80 mL with passage of clots, occurring at normal intervals (from 21 to 35 days) and the duration of flow for more than a week. Such a problem is mostly seen in young girls and adult women with frequent bruising due to inherited coagulopathy; in these situations, consultation with a hematologist should be considered an essential step in the absence of suitable screening tests for underlying disorders. Polymenorrhea refers to the luteal-phase defects, which results in shortened cycles (< 21 days). Menorrhagia and polymenorrhea are the characteristic features of dysfunctional uterine bleeding (DUB), which is not associated with gestation or any systemic disorder and needs emergency care. The actual underlying mechanism is unidentified, but it may be due to abnormal ovarian steroid hormone levels, disruption of the H–P–O axis that results in menstrual irregularities after menarche, or particularly during perimenopausal years when deteriorating secretions of estrogen remain unsuccessful to achieve LH surge and ovulation. Irregular bleeding and spotting between periods are also associated with DUB.



References

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Jan 4, 2021 | Posted by in GYNECOLOGY | Comments Off on Reproductive cycle
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