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Updated: Mar 28 2018

Menstrual Cycle

4.9

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Images
https://upload.medbullets.com/topic/116013/images/menstruation1.jpg
https://upload.medbullets.com/topic/116013/images/menstruation2.jpg
https://upload.medbullets.com/topic/116013/images/ms cycle.jpg
https://upload.medbullets.com/topic/116013/images/menstrual cycle.jpg
  • 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
        • a 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
        • a 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
        • a 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
        • a 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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