Lecture 72: Ovulation, Lutenization, and Luteolysis

Question 1

Describe the events in which the preovulatory LH surge causes ovulation

Answer 1

• Increase in PGE2, increases the blood flow to ovary and dominant follicle, causes edema, which increases follicular presure
• Increase in PGF2a causes an increase in contraction of ovarian smooth muscle which also increases follicular pressure
• increase in PGF2a causes release of lysosomal enzymes, which weakens the follicle wall
• Shift from E2 to P4 by the dominant follicle, causes an increase in P4, an increase in collagenase, which also weakens the follicle wall

Question 2

The luteal phase is made up of which two smaller phases of the estrous cycle?

Answer 2

Metestrus and diestrus

Question 3

What are the 3 main events of the luteal phase?

Answer 3

1) Lutenization of follicular cells to luteal cells (granulosa cells to large luteal cells and theca cells to small luteal cells)
2) Growth and development of the corpus luteum and production of progesterone
3) Luteolysis

Question 4

Describe the luteal phase from beginning to end if there is no maternal recognition of pregnancy

Answer 4

The luteal phase begins right after ovulation. During the early luteal phase, the CL develops (luteinization) and progesterone increases. During mid luteal phase (diestrus) the CL is fully functional and progesterone plateaus. During the lase 2-3 days of the luteal phase, destruction of the CL occurs and the luteal phase terminates and proestrus is initiated (follicular phase is resumed).

Question 5

What are the physiologic effects of progesterone on the hypothalamus?

Answer 5

Negative feedback
- Reduces basal GnRH amplitude and frequency
- prevents behavioral estrus
- stops the preovulatory LH surge

Question 6

What are the physiologic effects of progesterone on the anterior pituitary?

Answer 6

Negative feedback

Question 7

What are the physiologic effects of progesterone on the uterus?

Answer 7

• Positive influence on uterine glands to secrete “uterine milk” (histotroph) for potential conceptus
• Reduces myometrial tone (except mare)

Question 8

What are the physiologic effects of progesterone on the mammary glands?

Answer 8

Prior to parturition causes final alveolar development in the mammary gland

Question 9

What is luteolysis?

Answer 9

The destruction of the corpus luteum
Essential in order to return to a new follicular phase

Question 10

What is the major luteolytic source in domestic animals?

Answer 10

PGF2a

Question 11

what is the source of PGF2a

Answer 11

Uterine endometrium is the MAJOR source
- Dog uterus appears to not be involved in luteolysis
- Human PGF2a is made by the ovaries

Question 12

What is the mechanism of luteolysis in ruminants?

Answer 12

Vascular countercurrent exchange mechanism
- low molecular weight substances in high concentrations in one vessel diffuse into the adjacent vessel of lower concentration (from utero-ovarian vein to the ovarian artery)
- this allows PGF2a secreted by the uterus to be transported directly into the ovary and the CL without dilution from systemic circulation
- this is very important in ruminants because 90% of PGF2a is denatured with just one pass through the pulmonary system

Question 13

What is the mechanism of luteolysis in the mare?

Answer 13

Systemic pathway only
- CL is more sensitive to PGF2a
- PGF2a is metabolized less rapidly (low first-pass dilution rate)

Question 14

What is the mechanism of luteolysis in the sow?

Answer 14

Systemic and countercurrent exchange pathways
- Medium first-pass dilution (~40%)
- Corpa lutea are not responsive to PGF2a until at least Day 12 post ovulation

Question 15

Explain the hormonal induction of luteolysis

Answer 15

• Oxytocin receptors appear in the endometrium in the late luteal phase
• The CL contains large amounts of oxytocin
• Oxytocin release stimulates a pulse of PGF2a release
• PGF2a stimulates more oxytocin
• Positive feedback system