Overview
This lecture covers the hormonal regulation and phases of the menstrual cycle, with a focus on the uterine and ovarian cycles, their anatomy, and the coordination required for possible pregnancy.
Hormonal Regulation of the Menstrual Cycle
- Five main hormones regulate the menstrual cycle: FSH, LH, estrogen, progesterone, and HCG.
- Follicle-stimulating hormone (FSH) stimulates development of ovarian follicles.
- Luteinizing hormone (LH) triggers ovulation and is monitored for fertility.
- Estrogen and progesterone, produced by the ovaries, prepare the uterus for implantation.
- Human chorionic gonadotropin (HCG) is made post-implantation to maintain the corpus luteum.
Uterine Cycle and Anatomy
- The uterine wall has three layers: perimetrium (outer), myometrium (muscular), and endometrium (inner, mucosal).
- The endometrium has two sub-layers:
- Stratum basalis (remains after menstruation)
- Stratum functionalis (shed during menstruation, builds up for implantation)
- Three uterine phases:
- Menstrual phase (shedding of stratum functionalis)
- Proliferative phase (regrowth, thickening of endometrium)
- Secretory phase (glands and vasculature increase, prepared for implantation)
Ovarian Cycle and Oogenesis
- Three ovarian phases:
- Follicular phase (follicles grow)
- Ovulatory phase (ovulation occurs)
- Luteal phase (corpus luteum functions)
- Oogenesis starts with oogonia producing primary oocytes during fetal development.
- Primary oocytes remain arrested until puberty, then periodically resume meiosis each menstrual cycle.
- Secondary oocyte is ovulated but only completes meiosis II upon fertilization.
- Only one egg typically ovulates per cycle; others degenerate.
Follicle Development and Ovulation
- Primordial follicles protect primary oocytes until puberty.
- Under FSH, several follicles mature, but usually one becomes the dominant (tertiary or Graafian) follicle.
- Large follicles secrete increasing estrogen, signaling the uterus to prepare.
- LH surge triggers ovulation by causing the follicle to burst and release the egg (with corona radiata cells).
- Remaining follicle transforms into the corpus luteum.
Corpus Luteum and Hormone Feedback
- Corpus luteum secretes progesterone, signaling the uterus that ovulation has occurred.
- If fertilization does not occur, corpus luteum degrades into corpus albicans (scar tissue).
- After fertilization, HCG from the embryo maintains the corpus luteum and progesterone production until the placenta takes over endocrine functions.
Key Terms & Definitions
- FSH (Follicle Stimulating Hormone) — stimulates follicle growth in ovaries.
- LH (Luteinizing Hormone) — triggers ovulation.
- Estrogen — produced by growing follicles; prepares uterine lining.
- Progesterone — produced by the corpus luteum; maintains the uterine lining.
- HCG (Human Chorionic Gonadotropin) — made by embryo; maintains corpus luteum during early pregnancy.
- Corpus luteum — remnant of follicle post-ovulation; produces progesterone.
- Corpus albicans — scar tissue left after corpus luteum degenerates.
- Stratum basalis — permanent endometrial layer.
- Stratum functionalis — functional layer shed during menstruation.
- Oogenesis — process of egg cell development.
Action Items / Next Steps
- Review hormone functions and menstrual cycle phases.
- Study diagrams of ovarian and uterine cycles for visual understanding.