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Menstrual Cycle

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Topic updated on 07/16/16 2:43pm

Overview
 

Introduction
  • Stages of menstrual cycle
    • phase 1: follicular (proliferative) phase
    • phase 2: mid-cycle or ovulation
    • phase 3: luteal (secretory) phase
    • phase 4: menses/pregnancy
  • Events of menstrual cycle
    • ovarian follicle and oocyte develop
    • reproductive tract prepares to receive fertilized ovum
    • endometrial lining sheds if fertilization fails
  • Menstrual cycle length
    • menstrual cycle averages 28 days, but varies from 28 days to 35 days
    • variability in cycle length is attributed to follicular (proliferative) phase
      • luteal (secretory) phase is constant (14 days)
    • ovulation day + 14 days = menstruation
Phase 1: Follicular/Proliferative Phase (Day 0 - Day 13)
  • Folliculogenesis overview
    • folliculogenesis is the process by which primordial follicles develop into a single mature, dominant Graafian follicle
    • it is controled by the hypothalamic-pituitary axis
      • GnRH → ↑ FSH → (+) folliculogenesis
    • growth of follicles is fastest during week 2 of follicular (proliferative) phase
    • follicular (proliferative) phase varies in length

  • Stage of folliculogenesis
    • stage 1: primordial follicle → primary follicle
      • GnRH → ↑ FSH → (+) primordial follicle → primary follicle
      • primordial follicle consists of a primary oocyte surrounded by a single layer of squamous pre-granulosa cells
      • 15-20 primordial follicles develop into 15-20 primary follicles
      • primary follicle consists of a primary oocyte surrounded by a single layer of cuboidal granulosa cells
        • granulosa cells acquire FSH receptors
        • granulosa cells secrete layer of glycoproteins on surface of primary oocyte
          • zona pellucida
    • stage 2: primary follicle → secondary follicle
      • GnRH → ↑ FSH → (+) primary follicle → secondary follicle
      • primary follicles develop into secondary follicles
      • secondary follicle consists of a primary oocyte surrounded by several layers of cuboidal granulosa cells
      • as the secondary follicle develops, stromal cells differentiate to form theca cells
        • theca cells acquire LH receptors
        • basement membrane separates theca cells and granulosa cells
    • stage 3: secondary follicle → tertiary follicle → graafian follicle
      • GnRH → ↑ FSH and ↑ LH → ↑ 17β-estradiol (major) and ↑ progesterone (minor)
        • 17β-estradiol prepares uterus for embryo implantation
          • causes proliferation of endometrium
      • negative feedback mechanism
        • ↑ 17β-estradiol → (-) anterior pituitary → ↓ FSH and ↓ LH
      • secondary follicles develop into tertiary ("antral") follicles
      • granulosa cells increase in number and in size
        • granulosa cells secrete fluid into center of follicles, creating a fluid-filled center
      • tertiary ("antral") follicle contains a fluid-filled space or antrum
      • antrum enlarges and surrounds primary oocyte
        • a mature, pre-ovulatory follicle is called a Graafian follicle
    • stage 4: Graafian follicle development
      • a single, mature Graafian follicle will rupture and release oocyte during ovulation
      • other follicles in initial pool of 15-20 primordial follicles will undergo atresia
      • immediately prior to ovulation, primary oocyte completes meiosis I yielding secondary oocyte and the first polar body
Phase 2: Mid-Cycle or Ovulation (Day 14)
  • Hypothalamic-pituitary axis
    • positive feeback mechanism
      • ↑↑ 17β-estradiol → (+) anterior pituitary → ↑↑ FSH and ↑↑ LH
    • LH surge
      • rising levels of estradiol triggers LH surge, triggering ovulation
    • ovulation
      • Graafian follicle ruptures and releases secondary oocyte into peritoneal cavity
      • secondary oocyte is picked up by fimbriae of Fallopian tube and is transported through infundibulum into ampulla by means of ciliary movement of tubal epithelium and muscular contractions of tube
Phase 3: Luteal or Secretory Phase (Day 15 - Day 28)
  • Hypothalamic-pituitary axis
    • LH Surge
      • ↑↑ LH → (+) theca cells → ↑ progesterone (major) and ↑ 17β-estradiol (minor)
      • post-ovulation "surge" drives development of corpus luteum
        • corpus luteum is composed of residual elements of ruptured Graafian follicle
          • includes granulosa cells and theca cells
        • corpus luteum synthesizes and secretes progesterone and 17β-estradiol
          • progesterone maintains endometrium to support embryo implantation
          • progesterone increases hypothalamic-set point
            • raises basal body temperature
Phase 4: Menses/Pregnancy
  • Menses
    • corpus luteum regresses and is replaced by a fibrotic scar (corpus albicans)
      • regression causes abrupt loss of progesterone and 17β-estradiol
    • endometrial lining sloughs, causing menstrual bleeding
  • Pregnancy
    • if fertilization occurs, syncytiotrophoblasts secrete hCG that "rescues" corpus luteum
      • corpus luteum continues to synthesize/secrete progesterone and 17β-estradiol
Pathology of Menstrual Cycle
  • Oligomenorrhea
    • menstrual cycles occurring at intervals > 35 days
  • Polymenorrhea
    • menstrual cycles occurring at intervals < 21 days
  • Metrorrhagia
    • menstrual cycles occurring at irregular intervals
    • inter-menstrual bleeding
  • Menometrorrhagia
    • menstrual cycles occurring at irregular, more frequent intervals
    • heavy menstrual bleeding


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