Menstrual Cycle (Class 12 Biology)

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1. Introduction to Menstrual Cycle Definition: The cyclical changes occurring in the female reproductive system (primarily the uterus and ovary) at monthly intervals. Duration: Approximately 28-29 days, but can vary from 20 to 45 days. Onset: Begins at puberty (menarche, around 10-14 years). Cessation: Ends around 45-50 years (menopause). Purpose: Prepares the uterus for a potential pregnancy. If fertilization does not occur, the uterine lining sheds. 2. Hormonal Regulation The menstrual cycle is regulated by hormones from the hypothalamus, pituitary gland, and ovaries. Hypothalamus: Secretes Gonadotropin-Releasing Hormone (GnRH). Anterior Pituitary: Stimulated by GnRH to secrete: Follicle-Stimulating Hormone (FSH): Stimulates follicular development in the ovary. Luteinizing Hormone (LH): Triggers ovulation and formation of corpus luteum. Ovaries: Secrete: Estrogens: Primarily estradiol, promotes growth of ovarian follicles and uterine lining (endometrium). Progesterone: Maintains the uterine lining for pregnancy. 3. Phases of the Menstrual Cycle The cycle is typically divided into four main phases: 3.1. Menstrual Phase (Day 1-5) Events: Breakdown of the endometrial lining of the uterus and its blood vessels. Blood, mucus, and tissue are discharged through the vagina (menstruation/menstrual flow). Caused by a sharp decline in progesterone and estrogen levels due to the degeneration of the corpus luteum. Duration: Typically 3-7 days. Indicator: Absence of menstruation indicates pregnancy, but can also be due to stress, poor health, etc. 3.2. Follicular/Proliferative Phase (Day 6-13) Hormonal Changes: FSH secretion from anterior pituitary increases. FSH stimulates the growth and development of primary follicles into Graafian follicles in the ovary. Growing follicles secrete increasing amounts of estrogens. Uterine Changes: Estrogens stimulate the repair and proliferation of the uterine endometrium. Endometrium becomes thick and vascularized. Ovarian Changes: Primary follicle $\to$ Secondary follicle $\to$ Tertiary follicle $\to$ Graafian follicle. Feedback: High estrogen levels exert positive feedback on the pituitary, stimulating LH surge. 3.3. Ovulatory Phase (Day 14) Key Event: Ovulation – release of the secondary oocyte from the Graafian follicle. Trigger: Rapid secretion of LH (LH surge) reaches its peak around mid-cycle (day 14). LH surge induces rupture of the Graafian follicle. Timing: Occurs typically on day 14 of a 28-day cycle. 3.4. Luteal/Secretory Phase (Day 15-28) Ovarian Changes: The remaining parts of the Graafian follicle transform into the corpus luteum under the influence of LH. Corpus luteum secretes large amounts of progesterone and some estrogen. Uterine Changes: Progesterone is crucial for maintaining the uterine endometrium. Endometrium thickens further, becomes highly vascularized, and glandular, preparing for implantation. Fate of Corpus Luteum: If fertilization occurs: Corpus luteum persists and continues to secrete progesterone, supported by hCG (human Chorionic Gonadotropin) from the developing embryo. If fertilization does NOT occur: Corpus luteum degenerates around day 28 (corpus albicans). This leads to a sharp fall in progesterone and estrogen levels, triggering the breakdown of the endometrium and the onset of the next menstrual phase. 4. Diagrammatic Representation The following diagram illustrates the cyclical changes in pituitary hormones, ovarian hormones, ovarian events, and uterine events during the menstrual cycle. Days of Menstrual Cycle 1 5 14 21 28 Menstrual Phase Follicular Phase Ovulation Luteal Phase Pituitary Hormones FSH LH Ovarian Hormones Estrogen Progesterone Ovarian Events Growing Follicles Ovulation Corpus Luteum Uterine Events Menstruation Proliferative Secretory 5. Clinical Significance Pregnancy: If fertilization occurs, the menstrual cycle stops, and the uterine lining is maintained. Contraception: Many hormonal contraceptives work by mimicking pregnancy (high estrogen/progesterone) to inhibit FSH/LH and prevent ovulation. Menstrual Disorders: Irregular periods (oligomenorrhea), heavy bleeding (menorrhagia), painful periods (dysmenorrhea) are common issues.