AP2 Reproduction Menstrual Cycle Steele Buie

Menstrual Cycle

Although the uterus is the site where a young embryo implants, it is receptive to implantation for only a brief interval each month Menstrual cycle = cycle of endometrium changes 28-day cycle Three phases: Menstrual phase (Days 1-5) Proliferative phase (Days 6-14) Secretory phase (Days 15-28)

Degeneration of Corpus Luteum

Corpus luteum → Corpus albicans ↓ estrogen + progesterone Endometrium is no longer maintained => menstruation Inhibition of GnRH, FSH, LH removed (follicle development resumes)

Summary: Menstrual and Ovarian Cycles

Days 1-14 Follicular Phase (Ovary) Menstrual Phase (Uterine, Days 1-5) + Proliferative Phase (Uterine, Days 6-14) Day 14 = Ovulation Days 15-28 Luteal Phase (Ovary) Secretory Phase (Uterine)

Corpus Luteum

Endocrine organ: releases estrogen + progesterone - Causes endometrium proliferation + maintenance - Inhibits FSH + LH (no need at this time to stimulate development of more follicles)

Fertilization and Implantation

Fertilization occurs in the fallopian tube within 24 hours of ovulation Sperm can live ~ 3 days Implantation into the uterine wall occurs ~1 week after fertilization If fertilization and implantation occur: - Placenta is formed from cells of endometrium and developing embryo - Placenta secretes hCG (human chorionic gonadotrophin) which keeps the corpus luteum alive and secreting estrogen + progesterone (prevents menstruation) - Estrogen + progesterone from the corpus luteum keep the endometrium intact until the placenta starts secreting these hormones (which then keeps endometrium intact throughout pregnancy)

Menstural Phase

(Days 1-5) Menstruation due to shedding of stratum functionalis - occurs because estrogen + progesterone levels are low (because corpus luteum has regressed to corpus albicans and stopped secreting hormones)

Secretory Phase

(Days 15-28) Endometrium development continues/is maintained - under the influence of estrogen and progesterone from corpus luteum - In preparation for implantation of embryo: glands in endometrium begin to secrete nutrients to nourish embryo Higher progesterone levels causes cervical mucus to thicken (blocks sperm entry) If egg not fertilized, corpus luteum regresses to corpus albicans - estrogen and progesterone levels fall - no endometrium maintenance => leads to menstruation

Proliferative Phase

(Days 6-14) Endometrium proliferates (increase in glands and blood vessels) in response to increasing levels of estrogen (from developing follicles) Higher estrogen causes cervical mucus to become thinner for easier passage of sperm into uterus

Hormonal Changes: Ovarian & Menstrual Cycles

(a) Fluctuation of gonadotropin levels: Fluctuating levels of pituitary gonadotropins (follicle-stimulating hormone and luteinizing hormone) in the blood regulate the events of the ovarian cycle. (b) Ovarian cycle: Structural changes in the ovarian follicles during the ovarian cycle are correlated with (d) changes in the endometrium of the uterus during the uterine cycle. (c) Fluctuation of ovarian hormone levels: Fluctuating levels of ovarian hormones (estrogens and progesterone) cause the endometrial changes of the uterine cycle. The high estrogen levels are also responsible for the LH/FSH surge in (a). (d) The three phases of the uterine cycle: • Menstrual: Shedding of the functional layer of the endometrium. • Proliferative: Rebuilding of the functional layer of the endometrium. • Secretory: Begins immediately after ovulation. Enrichment of the blood supply and glandular secretion of nutrients prepare the endometrium to receive an embryo. Both the menstrual and proliferative phases occur before ovulation, and together they correspond to the follicular phase of the ovarian cycle. The secretory phase corresponds in time to the luteal phase of the ovarian cycle.