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Writer's pictureCatherine

The Menstrual Cycle

Updated: Apr 26, 2020


Menstrual Cycle


1. Primordial follicle (primary oocyte)

  • formed prenatally

  • diploid, prophase I

  • resumes meiosis on individual basis when ovulated

  • granulosa cells undergo hypertrophy and proliferate under influence of local growth factors

2. Unilaminar Primary Follicle

  • characterized by cuboidal granulosa


3. Multilaminar Primary Follicle

  • zona pellicuda: glycoproteins, barrier to autoimmunity

  • FSH-R-deficient females show a block in folliculogenesis before antral follicle formation


4. Secondary/Antral Follicle

  • develops under FSH control (10-20 formed per cycle)

  • granulosa cells secrete follicular liquor to antrum (similar to plasma - polysaccharides, steroid binding proteins, inhibin, follistatin (inhibits FSH))

  • cumuls oophorus starts to form - see next

  • characterized by antrum


5. Mature Graffian Follicle

  • granulosa = avascular

  • theca = vascular

  • cumulusoophorus = ova + zona pellucida

  • corona radiata: layer of granulosa cells immediately around ova

  • theca: highly vascular, secrete androgens, which are converted to estrogens by granulosa cells (more and more as grows)

  • the mature graffian follicle is the largest follicle

  • increased estrogen --> decreased FSH --> increased LH --> ovulation

  • LH induces completion of first meiotic division - forms first polar body and secondary oocyte

  • then second meiotic division --> stops at metaphase (diploid)


Follicular Cycle

6. Corpus Hemorrhagicus


  • ovulation occurs

  • walls of follicle collapse, ruptures capillaries in theca interna


7. Corpus Luteum

  • granulosa cells secrete follistatin and inhibin --> decrease FSH

  • LH stimulates transformation into luteal cells

  • granulosa luteal cells - secrete progesterone, convert androgens to estrogen

  • theca luteal cells - secrete progesterone and androgens

  • corpus luteum gets larger until 9th day after ovulation --> more progesterone secreted --> decreased LH by negative feedback

  • if no pregnancy, decreased LH --> corpus luteum degenerates

  • if pregnancy, corpus luteum continues to enlarge (via HCG)


8. Corpus Albicans


  • corpus luteum invaded by fibroblasts, type I collagen fibers from scar

  • scar slowly contracts









9. Atretic Follicle

  • degenerating follicle, forms small connective tissue scar

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