03. Ovarian Function

• primary oocytes arrest in prophase I until ovulation
• secondary oocytes arrest in metaphase II until fertilisation
• polar bodies are produced due to asymmetrical meiotic division. These are by-products of meiosis with much smaller cytoplasms
• second polar body is formed after the second meiotic division, signifies sperm entry
• granulosa and theca cells are present in follicles
• granulosa cells are epithelial and produce oestrogen and progestogens
• theca cells are interstitial and produce androgens and progestogens
• development of the follicles up until the secondary follicle is fsh and lh independent, this takes about a year
• development of the follicles from secondary follicle to graafian follicle is fsh and lh receptive and occurs in the mesntrual cycle.
• primordial follicle = primary oocyte surrounded by a single layer of granulosa cells
• atresia = selective apoptotic process of oocytes
• primary follicles:

  • granulosa cells become cuboidal in a single layer around the oocyte
  • fibroblast matrix surrounds granulosa cells
  • zona pellucida become visible
• zona pellucida is a glycoprotein layer that surrounds oocytes.

  • porous, allows for contact between granulosa cells and oocyte
• secondary follicles:

  • granulosa cells proliferate and become 3-6 layers
  • forms theca interna and externa
  • granulosa cells secrete follicular fluid
• fsh stimulates follicle development and oestradiol secretion
• lh stimulates follicle maturation, development of corpus luteum, oestradiol secretion and ovulation
• progesterone is important for completing the endometrium and stimulating mammary gland development
• during menses, oestrogen and progesterone levels are low. This means there is no negative feedback so GnRH is released and FSH and LH levels increase
• LH stimulates theca cells to produce androgens. FSH stimulates granulosa cells to turn those androgens into oestrogen

  • two cell hypothesis
• when FSH levels peak, the follicle with the highest number of FSH receptors outcompete the others
• only one oocyte is normally ovulated
• oestrogen normally has negative feedback onto the hypothalamus and anterior pituitary.

  • mid cycle, there is high levels of oestrogen that positively feedback leading to high LH and FSH
  • high LH leads to ovulaiton
• during ovulation there is an increase in granulosa cells and accumulation of follicular fluid

  • the cumulus oophorus loosens and the follicular wall weakens
  • proteases break down side of ovary
  • stigma forms
• during the luteal phase the follicle forms the corpus luteum, and granulosa and theca interna cells form lutein cells which produce oestrogen and progesterone

  • if no HCG present, the corpus luteum degrades and forms the corpus albicans
  • cycle recommences as low oestrogen and progesterone