Ovarian Function and its Control

Question 1

What is the corpus luteum?

Answer 1

Highly vascularized, transient endocrine gland, solid, produces progesterone

Question 2

What happens during oogenesis in the fetal ovary?

Answer 2

Primary oocytes produced, enter prophase of meiosis I, arrest at GV (germinal vesicle) stage

Question 3

How is a follicle formed?

Answer 3

By recruitment of follicular cells, granulosa cells form the primordial follicle, local ovarian growth factor stimulates early growth

Question 4

What happens in the secondary follicle?

Answer 4

Becomes surrounded by granulosa cells, zona pellucida (glycoproteins) forms, recruits theca cells

Question 5

What happens in the antral follicle?

Answer 5

Escapes the influence of granulosa cells, resumes meiosis

Question 6

What three stages occur to antral follicles?

Answer 6

Recruitment – cohort of small antral follicles start to develop, initiated by FSH wave, secrete small amounts of oestradiol, occurs in luteal as well as follicular phase, some undergo atresia
Selection – cohort of growing follicles, dependent on FSH for support, produce inhibin and oestradiol, become dependent on LH, some will start to undergo atresia
Dominance – transition from FSH to LH dependence, LH receptor appear in granulosa cells, increased oestradiol and inhibin production inhibits growth of other follicles, remain for approximately 6 days then either ovulate or undergo atresia depending on progesterone levels

Question 7

How is oestradiol produced by the follicles?

Answer 7

Co-ordinated action of theca and granulosa layers, key enzyme involved is aromatase, LH stimulates theca cells to produce testosterone which is converted into oestradiol by granulosa cells

Question 8

What causes release of the oocyte?

Answer 8

Oestradiol stimulates the LH surge

Question 9

What is the normal LH surge to ovulation interval?

Answer 9

Cows – 24-30 hours, dog – 24-96 hours, mare – gradual increase in LH during oestrus

Question 10

What events are initiated by the preovulatory LH surge?

Answer 10

Increased blood flow to ovary and dominant follicle, increased fluid accumulation, increased synthesis of collagenase, increased PGF2α – increases contraction of ovarian smooth muscle & causes the release of lysosomal enzymes, shift from oestradiol to progesterone by dominant follicle

Question 11

Describe the ovulation event

Answer 11

Follicular wall bursts, oocyte & granulosa cell released from GC anchor point, oocyte carried with follicular fluid into the peritoneal cavity, captured by fimbria infundibulum

Question 12

What causes activation of meiosis II in the oocyte?

Answer 12

Fertilisation

Question 13

Describe the transition to form a CL

Answer 13

Granulosa -> large luteal cells, theca -> small luteal cells, progesterone now produced, intense angiogenesis, basement membrane breakdown

Question 14

How do events differ in the bitch?

Answer 14

Pre-ovulatory luteinisation results in an early progesterone rise

Question 15

List some luteotrophic hormones

Answer 15

LH (ruminants, sows, mares) & prolactin (rodents)

Question 16

What is the key regulatory step of progesterone production by CL?

Answer 16

Transfer of cholesterol to the mitochondria

Question 17

What does luteolysis enable?

Answer 17

Pre-ovulatory follicular development, ovulation & a return to oestrus

Question 18

What is the key luteolytic hormone?

Answer 18

PGF2α

Question 19

What factors initiate luteolysis?

Answer 19

Endometrial PGF2α stimulated by luteal oxytocin

Question 20

What structural and functional mechanisms change during luteolysis?

Answer 20

Functional – downregulation of steroidogenic enzymes, rapid fall in progesterone
Structural – a reduction in blood flow, increase in macrophages and lymphocytes, apoptosis and tissue resorption – corpus albicans

Question 21

When will the CL respond to prostaglandins?

Answer 21

Once it has developed and fully expresses PGF receptors