Luteolysis is the degradation of the corpus luteum (as opposed to luteinisation – the formation of the corpus luteum). Luteolysis occurs in the absence of pregnancy, at the end of the luteal phase. The process of luteolysis is initiated by oxytocin (secreted by the corpus luteum) and prostaglandin F2a in domestic animals.

Mechanism

Progesterone is produced by the corpus luteum, which inhibits the hypothalamus (the hypothalamus secretes GnRH, therefore progesterone inhibits GnRH secretion). The corpus luteum also secretes oxytocin.

Initially the oxytocin appears to have no effect, however after a short period of time (e.g. 12-15 days in the cow) oxytocin receptors begin to form. When these oxytocin receptors are stimulated by the oxytocin secreted by the corpus luteum, prostaglandin F2a synthesis and secretion by the endometrium is stimulated.

Prostaglandin inhibits the production of progesterone (which is inhibiting the GnRH secretion and thus preventing the emergence of new dominant follicles). If progesterone production is inhibited then the oestrous cycle is able to begin again.

Prostaglandin also stimulates further oxytocin release, stimulating more oxytocin receptors that cause further prostaglandin F2a release. This is known as a positive cascade system and is used to quickly progress a biological situation, here the situation would be the prevention of inhibition of progesterone (which is inhibiting GnRH secretion).

The reduction of plasma progesterone concentration means follicular growth can now continue and dominant follicles can now emerge. In pregnancy, there is no corpus luteum formation (luteinisation) so there is no luteolysis – therefore progesterone levels remain high.

Summary

1. Corpus luteum produces progesterone and oxytocin – progesterone is inhibiting GnRH secretion
2. Oxytocin receptors form
3. Stimulated receptors cause prostaglandin F2a by endometrium
4. Prostaglandin inhibits the secretion of progesterone and stimulates further oxytocin release
5. Positive cascade system rapidly increases plasma prostaglandin concentration
6. Progesterone levels are low again and GnRH secretion resumes
7. Follicular development begins again, ready to repeat the oestrous cycle