Oestrus cycle

Question 1

What occurs during the follicular phase?

Answer 1

1. Rise in FSH causes cohort follicles to emerge (cells have become FSH dependant)
2. Follicular cells increase their oestradiol and inhibin production
3. This has a -ve feedback effect on FSH secretion, causing it to decrease
4. Low levels of FSH are still present which promotes the conversion of androgen to oestradiol in GC
5. Rate of oestradiol secretion is dependant on LH stimulating the production of androgen in TC
6. LH pulses increase so oestradiol increases
7. As FSH declines, majority of cohort follicles become atretic leaving one dominant follicle which becomes relatively FSH dependant and survives the decline in FSH
8. At the end of the follicular phase oestradiol increases rapidly & exceeds threshold level
9. Feedback switches to +ve causing pre-ovulatory LH & FSH surges (responsible for standing heat behaviour)

Question 2

What are the effects of oestradiol on the repro. tract?

Answer 2

• proliferation & keratinisation of epithelial cells
• induces oxytocin receptors on endometrium
• glands develop for mucous secretion during oestrus
• induces prpgesterone receptors
• increases endometrial vascularity

Question 3

Describe what happens during ovulation

Answer 3

1. LH surge causes:

• increased ovarian blood flow & fluid accumulation
• release of collagenase from follicle which weakens follicle wall
• gap junctions deteriorate & cumulus oocyte complex detaches

1. Oocyte resumes meiosis - reaches met 1, expels 1st polar body & is ovulated in met 2
2. Follicle bulges oon ovary surface and bursts
3. Follicular fluid containing oocyte released
4. Oocyte adheres to fimbriae of oviduct
5. Ciliated epithelium of oviduct transport oocyte to ampulla
6. LH surge causes luteinisation of GC & TC cells to large & small luteal cells
7. Hormone production changes from oestradiol to progesterone

Question 4

Describe what happens during the luteal phase

Answer 4

1. Large & small luteal cells secrete large amounts of progesterone which represses oestradiol secretion
2. Initially a corpus rubrum forms following bleeding into ovulatory cavity & clot formation
3. Trauma heals and CL forms
4. LH pulses from anterior maintain CL activity and promote progesterone production
5. Progesterone inhibits GnRH and FSH, next follicular phase cannot begin until CL regresses

Question 5

What are the effects of progesterone on the repro. tract?

Answer 5

• uterine glands develop
• small amount of mucous secretion
• decreases myometrial activity
• decreases immuinty to stop rejection of embryo

Question 6

Describe what happens during luteolysis

Answer 6

1. If fertilisation does not occur CL undergoes spontaneous apoptosis
2. Remnants of cells phagocytosed
3. Scar forms on ovary surface where the CL was - known as carpus albicans
4. Progesterone production markedly decreases
5. GnRH not inhibited, FSH produced and next follicular phase occurs

Question 7

What is different about luteolysis in the cow?

Answer 7

• CL releases oxytocin which induces PGF-2a release from endometrium
• PGF-2a enters uterine vein then artery via counter multiplier system
• PGF-2a induces luteolysis

Question 8

What is different about luteolysis in the mare?

Answer 8

• CL releases oxytocin which causes PGF-2a release from endometrium
• PGF-2a travels systemically to CL