Ovarian function and control Flashcards
Describe the stages of folliculogenesis
- initial recruitment phase: primordial follicles begin to grow, leaving the resting pool. The granulosa cells chnage from flattened to cuboidal. (primary follicle forms).
- futjer granulosa cell growth occurs, form multiple layers, basement membrane forms, stromal cells are recruited- theca cells. (Secondary follicle)
- follicular fluid is generated, collects in the spaces between the cells, forming a fluid filled structure (antral follicle)
Antral follicle growth: - recruitment: a cohort of secondary follicles are recruited, initiated by FSH wave, some undergo atresia. Small amounts of E2 secreted by follicles
- selection: follicles are larger in size, smaller in number (atresia), selected fro further growth, follicles are FSH dependent, begin to produce Inhibin and E2, inhibin suppresses FSH: become LH dependent
Dominance: transition from FSH to LH dependence, LH receptors appear on granulosa cells, increased E2 and I production. the growth of these follicles suppresses subordinant follicle growth. They will now either ovulate or undergo atresia.
Explain the concept of a follicular wave
3 follicular waves occur in the animal, across the ovaries, NOT 3 WAVES PER OVARY (only 1 ovary will ovulate, they seem to take turns but no known pattern found as to whether the left or right will ovulate). Once one ovary has ovulated, the dominant follicle on the other ovary will undergo atresia
The oestrus cycle is split into the luteal phase and the follicular phase, the oestrus cycle is predominantly the luteal phase (the CL is the dominant structure), highlighted by the dominance of P4.
After ovulation, the corpus luteum forms when progesterone levels are high. It then undergoes destruction as the progesterone levels drop- this is the beginning of the follicular phase. The diameter of follicle increases over time as other follicles become atresed.
1st wave: dominant follicle undergoes atresia because P4 high
2nd wave: same again
3rd wave: progesterone falls, feedback is released, ovulation can occur due to FSH and LH levels, high oestrogen levels, dominant follicle can be ovulated.
Describe the function of the ovarian follicle
Responsible for oestradiol production via cooridinated activity of theca cells and granulosa cells
stimulated by LH and FSH, invovles the enzyme aromatase
Explain the 2-cell 2-gonadotrophin theory
Theca cells have LH receptors, granulosa cells have FSH receptors.
Theca celsl bind LH, triggers a 2nd messenger system, results in the formation of testosterone, released
Testosterone taken up by granulosa cells, bind FSH which activates the conversion of T to E2.
E2 then released into the blood
Outline the process of oogenesis
- primordial germs cells from the hindgut migrate to the genital ridge, proliferate by mitosis (completed at birth)
- the primordial cells enter meisosis I, arrest at the GV phase before cytokinesis occurs- primary oocytes. They can be arrested at this stage for years
At fertilisation, a massive mitotic event occurs, lots of primary oocytes formed, production then declines over gestation, no more creation after birth - very small proportion of oocytes survive to be ovulated: quality control
Describe the process of oocyte maturation
- Cytoplasmic maturation: getting ready for early stages of embryo development (structural capabilities- organelles and RNA), embryo is reliant on what is in the oocyte up until around the 8th division. Lose DNA via polar body but maintain cytoplasm volume: maintain space for organelles
- Nuclear maturation occurs
- the LH surge occurs: Completion of Meiosis I but then Meiosis II arrested
- Ovulation
- Fertilisation: Influx of Ca2+- Meiosis II activation, 2nd polar body formed and extruded.
Describe the process of ovulation
= the release of the oocyte from the ovary
this occurs over the entire surface of the ovary in most species
Initiated by the LH surge, which is stimulated by E2 in the absence of P4
Describe spontaneous ovulation
- Progesterone drops, leading to luteolysis
- GnRH increases, increased secretion of LH and FSH
- results in increased follicular development. inhibins cause drop in FSH, E2 increases over threshold, LH surge occurs
- Increased PGF2A secretion, increases blood flow to ovary and dominant follicle
- odema formation occurs within the follicle, increasing folicular pressure
- PGF2A also causes increased ovarian SM contraction
- follicular wall bursts, oocyte released
Describe induced ovulation
- Ovulation can be induced by coitus
- A single act of intercourse causes an LH surge around 50-60% of the time
- Sensory stimulation from the act of coitus stimulates the surge centre
- The more intercourse that the cat has, the more stimulation of the surge centre that occurs
- Once sufficient stimulation has occurred, all the dominant follicles will be ovulated
Describe how the corpus luteum develops
- the basement membrane of the follicle breaks down, lutinisation occurs. breadown allows bloodflow into structure, aids with proliferation
- theca cells become small luteal cells, grnaulosa cells become large luteal cells
- cells become progesterone producers, proliferate and increase in size
Give examples of luteotrophic hormones
= hormones that support the growth/function of the CL LH (ruminants, sows mares) prolactin (rodents) Others: - IGF-1 - GH -PGE2
Detail the funciton of the corpus luteum
Produces progesterone
large luteal cells produce the most progesterone
small luteal cells are stimulated by LH
Explain the regression of the corpus luteum
= luteolysis, occurs in the absence of an embryo, initiated by PGF2A
- Luteal oxytocin acts on the endometrium in a positive feedback loop, stimulated PGF2A production
- Utero-ovarian countercurrent system important in many species
structural and function
destruction:
- structural: reduced blood flow, increased macrophages, apoptosis and tissue resorption
- functional: down regulation of P4
