Corpus Luteum Regression - Anatomy & Physiology

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.

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.