Lecture 11: The Follicular Phase of the Estrous Cycle

Reading

• Chapter 8

Lecture 11: The Follicular Phase

General Regulation of Hormonal Pattern in Female

• Hypothalamus secrets GnRH
• Anterior Pituitary secrets LH and FSH
  • stimulates follicle and/or CL growth and maintenance
• CL produces progesterone
  • progesterone has negative feedback
• Follicle produces Estradiol and Inhibin
• estradiol has negative feedback when progesterone present
• inhibin inhibits FSH release

Preovulatory Hormonal Regulation

• progesterone decreases due to effects of PGF
  • removal of negative feedback by progesterone
  • in response, GnRH and then LH increase
  • follicle still produces inhibin so FSH does not change much
• preovulatory follicle produces inhibin and estradiol
  • inhibin keeps FSH under check
  • estradiol negative feedback effects decrease so GnRH and LH increase
  • estradiol increases to high levels, estradiol stimulates the surge center in the hypothalamus

Follicle Progression

• primordial, primary, secondary and tertiary follicles
• atresia can occur at any step

Follicular Growth

• gonadotropin independent phase
• FSH dependent
• recruitment = emergence
• selection
• dominance

Follicluar Waves

• presence of 2 and 3 wave cycles
  • FSH causes recruitment, follicles then secrete estradiol that at this point have negative feedback
  • Selection then occurs to trigger some of the follicles to grow in size, they also produce both estradiol and inhibin. The inhibin and estradiol have negative feedback controlling both LH and FSH.
  • Dominance is when one or more follicles enlarges more. Tjhey produce large amounts of estradiol and inhibin that both decrease FSH and LH or at least prevent an LH surge.
  • If a CL is present and producing progesterone, then the dominant follicle becomes atretic, the estradiol and inhibin decrease. There is then an icnrease in FSH (mainly due to decrease in inhibin).
  • a second follicular wave then occurs. The same events occur with the second wave if the CL is present and a third wave is recruited and then ovulates.
  • 2 wave cycles simply result in ovulation of the second wave.
• recruitment = emergence
  • FSH always controls emergence
  • Increase in FSH precedes emergence
• species variation
  • cattle 2 - 3 waves/cycle; sheep 4 - 5 waves/cycle likely CL produces progesterone only
  • Pigs, horses and humans have only 1 wave/cycle and maybe progesterone and inhibin produced by CL.

Production of estradiol by antral follicle

• theca interna has LH receptor and produces androgen (testosterone)
• granulosa cell has FSH receptor which when activated causes cell to take testosterone from theca and convert it to estradiol
  • FSH causes increased activity of aromatase enzyme (converts testosterone to estradiol)
• after recruitment and selection, dominant follicle has LH receptors on granulosa and granulosa can then respond to principally LH to drive estradiol production.Ovulatory Follicle

• layers oocyte must cross to ovulate
• events in follicle which if blocked will prevent ovulation
  • protein synthesis
  • steroid synthesis
  • prostaglandin synthesis (both PGF and PGE)
  • plasminogen activator synthesis

Ovulation (in response to LH)

• cumulus cells produce hyaluronic acid and proteins to cause cumulus expansion
  • the cumulus and oocyte separate from the granulosa cells
• triggers changes in protein synthesis and cellular differentiation within follicle
  • steroid biosynthesis shifts to production of progesterone
    • triggers production of inactive collagenase from theca interna
  • PGE2 and PGF2 are produced
    • triggers production of plasminogen activator from granulosa
• increased blood flow to follicle
  • local response mediated by increases in PGE2 and histamine
  • follicle also produces angiogenic factors
  • increased vascular permeability produces edema in tissue surrounding follicle and increased plasminogen
  • follicular pressure increases
• plasminogen is converted to plasmin by plasminogen activator
  • plasmin converts inactive collagenase to the active form which then weakens the collagen of the tunica albuginea and thecal layer
  • granulosa cells also increase production of follicular fluid
  • stigma begins to push out and weaken
• follicle ruptures
• PGF stimulates smooth muscle to contract and force the ruptured follicle to empty

Types of Ovulators

• spontaneous
  • ovulate with regular frequency and do not require copulation
• induced
  • requires vaginal and/or cervical stimulation for ovulation
• even spontaneous ovulators retain some neural control over ovulation
  • heifers with clitoral message can shift time of LH surge
  • rape leads to more higher pregnancy that expected

Oogenesis

• mitotic divisions of primordial germ cells
• nuclear arrest at dictyotene
• cytoplasmic growth
• nuclear maturation
• cytoplasmic maturation
• resumption of meiosis

Corona radiata regulate resumption of meiosis via gap junctions

• LH surge causes gap junction loss
• cAMP, PKA increase and pre-MPF converted to MPF
• GV goes through GVBD

Ovulation and fertilization of secondary oocyte in most species

Dogs, Fox - ovulation and seprm penetration occurs at primary oocyte stage

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