MENSTRUAL CYCLE. Learning Objectives At the end of the tutorial, the student should be able to : • Define menstrual cycle. • Tell us the duration of menstrual cycle. • Describe the physiologic changes that occur in the female reproductive organs during the menstrual cycle. • Explain the regulation of menstrual cycle. • Express the applied physiology. Menstrual Cycle Definition: Periodic vaginal bleeding that occurs with the shedding of the uterine mucosa (menstruation). Menstruation • Indicate periodic shedding of the stratum functionale of the endometrium, which becomes thickened prior to menstruation under the stimulation of ovarian steroid hormones. A : Functional Layer. B: Basal layer. Duration of the cycle • Variable, • an average figure is 28 days from the start of one menstrual period to the start of the next. • First day of menstruation “day one “ of the cycle. Timing events in the menstrual cycle. 1. Onset of menstruation Day 1 Day 1 0 4 8 12 16 20 24 28 Menstruation Phases of the menstrual Cycle Changes in ovary : – THE FOLLICULAR PHASE: From first day of menstruation until day of ovulation. – LUTEAL PHASE: After ovulation is luteal phase until first day of menstruation Phases of the menstrual Cycle • Changes in the endometrium: – Menstrual , – Proliferative and – Secretory Phase. Ovarian Cycle • Follicular Phase ( Day 1 to Day 13 ): – Menstruation lasts from 1 to day 4 or 5 of the average cycle. – Secretion of Ovarian steroid hormones are at their lowest. – Ovaries contain only primary follicles. Ovarian Cycle • Follicular Phase ( Day 1 to Day 13 ): – Some primary follicles grow, and become secondary follicles. – Towards the end , one follicle in one ovary reaches maturity and reaches graffian follicle. – As follicles grow, the granulosa cells secrete estradiol ( the principal estrogen), which reaches its highest concentration in the blood at about day 12 of the cycle, 2 days before ovulation. Growth of follicles: Antral follicle Graafian follicle Primordial follicle Oocyte Ovulation Antrum (fluid filled Granulos space) a cells Thecal cells Ovarian Cycle • Follicular Phase ( Day 1 to Day 13 ): – Growth of follicles and secretion of estradiol are stimulated by FSH. – FSH in early follicular phase is slightly greater then in late follicular phase. – Towards the end , FSH and estradiol also stimulate the production of LH receptors in the graffian follicle. – Graafian follicle is prepared for the next major event. LH Surge • Begins about 24 hours before ovulation. • Reaches its peak 16 hours before ovulation. • LH surge acts to trigger ovulation. Positive feedback effect of estradiol on the pituitary , an increase in LH secretion in late follicular phase culminates in an LH s. Timing events in the menstrual cycle. 2. LH surge Ovulation • LH Surge causes graafian follicle to rupture at about day 14. • Secondary oocyte arrested at metaphase II of meiosis , is released from ovary into the uterine tube. • Ovulation occurs. Luteal phase • After Ovulation, the empty follicle is stimulated by LH to become corpus luteum. • Corpus luteum secretes both estradiol an progesterone. • Progesterone levels in the blood are negligible before ovulation but rise rapidly to a peak level during the luteal phase, approximately one week after ovulation. Luteal phase • Progesterone with estradiol during the luteal phase exert an inhibitory, or negative feedback effect on FSH n LH secretion. • Corpus luteum produces inhibin which may help to suppress FSH secretion. • This retards the development of new follicles. • Further ovulation does not occur. • Multiple ovulations and possible pregnancies on succeeding days of cycle are prevented. Luteal phase • New follicles develop towards the end of one cycle in preparation for the next. • Inhibin production is decreased at the end of luteal phase. • Estrogen and progesterone levels also fall during the late luteal phase ( starting about day 22) because corpus luteum regresses and stops functioning. • With the declining function of corpus luteum, esterogen and progesterone fall to very low levels by day 28 of the cycle. Luteal phase • WITHDRAWL OF OVARIAN STEROIDS CAUSES MENSTRUATION AND PERMETS A NEW CYCLE OF FOLLICLE DEVELOPMENT TO PROGRESS. Cyclic changes in the Endometrium • Development of the endometrium is timed by the cyclic changes in the secretion of estradiol and progesterone from the ovarian follicles. • Three phases can be identified: – The proliferative phase. – The secretory phase. – The menstrual phase. The proliferative Phase • occurs while the ovary is in follicular phase. • Increasing amounts of estradiol stimulate proliferation of stratum functionale of the endometrium. • Spiral arteries develop in the endometrium. • Estradiol stimulates the production of receptor proteins for progesterone at this time, in preparation for the next phase of the cycle. The secretory phase • Occurs when ovary is in its luteal phase. • Increased progesterone secretion stimulates the development of uterine glands. • Endometrium becomes thick, vascular and ‘ spongy’ in appearance. • Uterine glands becomes engorged with glycogen during the phase following ovulation. • Endometrium is well prepared to accept and nourish an embryo Uterine changes in the menstrual cycle. The menstrual Phase • Occurs as a result of fall in ovarian hormone secretion during the late luteal phase . • Necrosis and sloughing of the stratum functionale may be produced by constriction of spiral arteries. • It seems spiral arteries are responsible for menstrual bleeding. Cyclic changes in the female reproductive tract • High levels of estradiol secretion cause cornification of vaginal epithelium. ( the upper cells die and become filled with keratin. • During luteal phase, high levels of progesterone cause the cervical mucus to thicken and become sticky after ovulation has occured. CLINICAL ABNORMAL MENSTRUATION : Most common disorders of female reproductive tract. • Amenorrhea: Absence of Menstruation. • Dysmenorrhea : Painful menstruation which may be marked by severe cramping. • Menorhagia: Excessively profuse or prolonged bleeding. References Human Physiology. Stuart Ira Fox. Seventh Edition. Human Phsiology. Lauralee Sherwood. From Cells to Systems. Fifth Edition.