CH 9 The Luteal Phase of the Estrous Cycle Flashcards
When does the luteal phase occur? What phases does it consist of?
Lasts from the time of ovulation until luteolysis (regression of CL)
Consists of metestrus and diestrus
What key things have to happen in order to have the luteal phase occur?
- Corpa lutea formation
- Production of progesterone
- Luteolysis
What happens to a follicle at ovulation?
Follicle ruptures as well as the blood vessels within the follicular wall –> forms corpus hemorrhagium (bloody body)
How is the follicle observed from day 1-3 of the estrous cycle?
Appear as small, pimple-like structures on surface of ovary
What happens to CL at day 3-5 of the estrous cycle?
CL begins to increase in size as loose hemorrhagic appearance. Gains size until middle of estrous cycle.
What is happening to Cl as diestrus?
CL has reached mature suze and maximal levels of progesterone
What happens to CL at the end of the luteal phase?
At the end of the luteal phase, luteolysis occurs and CL loses functionality and decreases in size
What happens as CL degrades?
Forms corpus Albicans - scar-like tissues
What happens to the follicle after ovulation?
After ovulation, theca internal and granulosa cells undergo dramatic transformation (lutenization)
What is luteinization?
Process whereby cells of the ovulatory follicle are transformed into luteal tissue
What two cells do luteal tissue consist of?
large cells and small cells
Where do large cells originate from?
granulosa cells
What do large cells contain?
Many dense secretory granules near plasma membrane (in ruminants)
What do secretory granules in large cells contain?
Oxytocin - CL of estrous cycle
Relaxin - CL od pregnancy
Where do small cells originate from?
theca interna cells
What are characteristics of small cells?
irregular shape, numerous lipid droplets, no secretory granules
What is growth of CL mass a net effect of?
- Large luteal cells undergo hypertrophy (increase in size)
- Small luteal cells undergo hyperplasia (increase in #)
- Non-steroidogenic cells also increase in numbers (fibroblasts, capillary cells, eosinophils)
What does the vigor of CL depend on?
- number of luteal cells
- degree of which CL has become vascularized –> may be due to amount of angiogenic factors produced by follicular fluid
What are progesterone target tissues?
- surge and tonic centers of the hypothalamus
- anterior pituitary
- alveoli mammary gland
- Endometrium of the uterus –> produces maximal secretions which are essential to the developing embryo
- Myometrium of the uterus - reduces contractility and promotes “quiescence” of the tract to support embryo implantation
Why is progesterone an inhibitor?
- reduces basal GnRG amplitude and frequency
- prevents behavioral estrus
- stops the preovulatory LH surge
- reduces myometrial tone
What does progesterone synthesis require?
Requires basal (tonic) LH and cholesterol
What is luteolysis?
The disintegration of decomposition (lysis) of the CL. CL undergoes irreversible degeneration characterized by a dramatic drop in blood levels of progesterone
What are the requirements for luteolysis /and what are the two main hormones that control luteolysis?
- Presence of oxytocin receptors on endomediral glands
- Oxytocin from the CL
- PGF2a from uterine endometrium
What does PGF2a do in the luteal phase?
Causes luteolysis –> vascular counter-current exchage
What did sheep surgery experiments tell us?
- uterus is required for successful luteolysis
- uterus must be near the ovary
What is luteal oxytocin made be?
synthesized and secreted by large luteal cells
What is the difference between luteal secretory granule oxytocin and posterior pituitary oxytocin?
Analogous (same)
What is significant about PGFa metabolites?
Metabolites are easier to measure
How does PGF2a secretion occur during late luteal phase?
Secretion occurs in pulses
Pulses increase in frequency and amplitude as the end of the luteal phase approaches. A critical number of pulses in a given time period are required for luteolysis to occur.
What is required for PGF2a to be released?
Elevated levels of progesterone
What happens to first half life of PGF2a during the first half of the estrous cycle?
Progesterone prevents PGF2a secretion by blocking formation of oxytocin receptors in the uterus
What happens to progesterone after 10-12 days after the first half of the estrous cycle?
Progesterone loses its ability to black the formation of oxytocin receptors. So, oxytocin receptors can bind to oxytocin in the uterine endometrium which stimulates release of PGF2a
What does luteolysis result in?
- cessation of progesterone production
- structural regression to from a CA
- follicular development and entrance into a new follicular phase
What are the possibilities of intracellular mechanisms that cause luteolysis?
- reduced blood flow to CL
- capillary degeneration resulting in decreased blood flow to CL
- PGF2a binds to receptors on large luteal cells resulting in death of these cells and therefrom steroidgenesis
What may structural regression of CL be due to?
Immune system
Macrophages and lymphocytes present at luteolysis
What do macrophages and lymphocytes produce?
Cytokines
What are cytokines?
Non-antibody proteins that produce a variety of immune cells that act as intracellular mediators of the immune response
What do cytokines trigger?
May trigger process called apoptosis
What is apoptosis?
Programmed cell death
Cell death occurs by what 2 processes?
- necrosis
- apoptosis
What does the final destruction of CL consist of?
Performed by macrophages which phagocytize damaged luteal cells. Over time luteal cells completely disappear, leaving only connective tissue behind (CA)
