Lecture 23: Ovulation, Luteinization, & Luteolysis (Exam 2) Flashcards
Describe ovulation
- LH surge dep
- In most species it is a 24 H event except the mare (3 to 5 day elevation)
- The bitch ovulates 2 - 3 days after the LH surge
- Most species ovulate during estrus (except the cow)
- Secondary oocyte is ovulated except in canids (Primary)
What happens to the ovulated follicle
- Collapses
- Fills w/ blood
- Become a corpus hemorrhagicum (CH)
Why is the preovulatory LH surge critical
- It sets in motion a series of biochemical events that lead to ovulation
What else brings on ovulation
- Elevated blood flow
- Breakdown of connective tissue
- Ovarian contractions
Describe induced ovulators
- Req stimulation of the vagina, cervix, &/or uterus for ovulation to occur
- Cats, rabbits, minks, ferrets, etc.
- Camelids need stimulation but also req a substance in seminal plasma for ovulation
- Cats have 50% ovulation rate if they are mated once
What are the steps of induced ovulation
- Copulation stimulates sensory nerves in the vagina & cervix
- Impulses are then relayed to the spinal cord
- Impulses are then relayed to the surge center in the hypothalamus
- If enough stimulus is there neurons in the preovulatory center fire causing large quantities center fire causing large quantities of GnRH to be secreted that in turn stimulates the LH surge
What are the three main events of the luteal phase
- Leteinization of follicular cells to luteal cells (Granulosa cells to large luteal cells & theca cells to small luteal cells)
- Growth & dev of the CL & production of progesterone
- Luteolysis
What happens during the early luteal phase
- Begins immediately after ovulation
- Early - CL dev (luteinization) & progesterone increases
- Mid - (diestrus) CL is fully functional & progesterone plateaus
- Last 2 - 3 days - destruction of the CL occurs (luteolysis) & the luteal phase terminates & proestrus is initiate
Where does the corpus luteum originate from
Ovulatory follicle
When does the basement membrane begin to break down
As ovulation nears
When does the corpus hemorrhagicum appear
When small blood vessels rupture & the theca & granulosa cells mix
What happens after the CH forms
- The CL is not a mix of Large luteal cells (LLC) originating from granulosa cells & small luteal cells (SLC) originating from theca cells
- Insome cases there is a remnant of the follicular antrum that forms a small cavity in the center of the CL
What happens to the theca interna & granulosa cells
They are transformed to the luteal cells & produce progesterone
What produces progesterone in the CL
Small luteal cells & large luteal cells
What other hormones are produced by the CL
- Oxytocin
- Relaxin
Label the following luteal anatomy
What of the physiologic effects of progesterone on the hypothalamus
- Negative feedback
- Reduces basal GnRH amplitude & freq
- Prevents behavioral estrus
- Stops preovulatory LH surge
What physiologic effects of progesterone anterior pituitary
Negative feedback
What physiologic effects of progesterone uterus
- Positive influence on uterine glands to secrete “uterine milk” (histotroph) for potential conceptus
- Reduces myometrial tone (except mare)
What physiologic effects of progesterone Mammary glands
- Prior to parturition causes final alveolar dev
What is luteolysis
- Destruction of the CL
- Cessation of progesterone secretion
- Structural regression to form a corpus albicans
- Removal of negative feedback by progesterone upon GnRH secretion resulting in a new follicular phase
Why is luteolysis essential for the return of a new follicular phase
- Disintegration of the CL
- Dramatic drop in blood progesterone (1-3 D; releases neg feedback of progesterone on the hypothalamus & anterior pituitary)
What is the luteolytic agent in domestic animals
Prostaglandin F2 alpha (PGF2a)
What are the sources of PGF2a
- Uterine endometrium in domestic animals is the major source
- Dog uterus appears not to be involved in luteolysis
- Human PGF2a is made by the ovaries
Describe the pathway to luteolysis
- Varies by species
- A systemic pathway in the mare
- A combo of local & systemic in the sow
Describe luteolysis in ruminants
- Vascular countercurrent exchange mechanism
- Low molecular weight sub in high conc in one vessel diffuse into the adjacent vessel of lower conc
- Allows PGF2a secreted by the uterus to be transported directly to the ovary & CL w/out dilution by the systemic circulation (impt b/c around 90% of the systemic PGF2a is denatured in one circulatory pass through the pulmonary system)
Describe the mechanism of luteolysis in mares
- Systemic pathway only
- Corpus luteum is more sensitive to PGF2a
- PGF2a is metabolized less rapidly (low first pass dilution rate)
Describe the mechanism of luteolysis of a sow
- Systemic & countercurrent exchange pathways
- Medium first pass dilution rate (~40%)
- Corpora lutea are not responsive to PGF2a until @ least day 12 post ovulation
What causes luteolysis during 60% of the estrous cycle in most species
Exogenous PGF2a
Describe exogenous PGF2a w/ luteolysis
- Most potent after day 5 post ovulation therefor almost always causes luteolysis if admin @ this time
- In the first 2 to 4 days post ovulation PGF2a has negligible effect (one admin typically does not cause luteolysis & repeated admin will eventually cause luteolysis)
- In the pig the CLs are not responsive to PGF21 until day 12 - 14 post ovulation
Describe hormonal induction luteolysis
- Oxytocin receptors appear in the endcometrium in the late luteal phase (after ~ 10 - 12 D progesterone loses ability to block formation of oxytocin receptors)
- The CL contains large amounts of oxytocin
- Oxytocin release stimulates a pulse of PGF2a release
- Positive feed back system
- A critical # of PGF2a pulses w/in a given timespan are req to induce complete luteolysis
