Corpus Luteum Regression - Anatomy & Physiology
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The Luteolytic Mechanism
- Prostaglandin F2α (PGF2α)
- PGF2α from the uterus is transported to the ipsilateral ovary through a vascular counter-current exchange mechanism.
- This involves two closely associated blood vessels in which blood from one vessel flows in the opposite direction to that of the adjacent vessel.
- Low molecular weight substances in high concentrations in one vessel cross over into the adjacent vessel, where they are low in concentration.
- The ovarian artery lies in closs association with the utero-ovarian vein.
- By counter-current exchange, PGF2α is transferred across the wall of the uterine vein into the ovarian artery by passive diffusion.
- This ensures a high proportion of PGF2α produced by the uterine glands will be transported directly to the ovary and corpus luteum without dilution in the systemic circulation.
- This is an important mechanism, because much PGF2α is denatured during one pass through the circulatory system. This is about a 90% loss in the ewe and cow.
Luteolysis in Ruminants
- The ruminant corpus luteum secretes large amounts of oxytocin as well as progesterone.
- In the late luteal phase, oxytocin receptors develop on the uterine endometrium.
- Binding of oxytocin to the receptors stimulates the pulsatile release of PGF2α.
- PGF2α secretion by the endometrium is almost non-existant during the first half of the luteal phase.
- During the late luteal phase, pulses of PGF2α secretion increase in frequency and amplitude as the end of the luteal phase approaches.
- A critical number of PGF2α pulses are required within a given time span to induce complete luteolysis.
- In the ewe, this is ~5 pulses in 24 hours.
- During the first half of the cycle, progesterone prevents the secretion of PGF2α by blocking the formation of oxytocin receptors on the uterus.
- Oxytocin and PGF2α stimulate each other in a positive feedback manner.
- The greater the number of endometrial oxytocin receptors, the greater the ability of oxytocin to stimulate PGF2α synthesis.