Posts Tagged ‘ oestrous ’

Luteolysis

Introduction

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
Advertisements

The Oestrous Cycle

Remember that only the introduction to this article is given here, to view the full article head over to www.jameswatts.co.uk where it can also be downloaded

Introduction

The oestrous cycle is the reproductive cycle found in most mammalian placental females whereby there are recurring periods when the female is fertile and sexually receptive (oestrus) interrupted by periods in which the female is not fertile and sexually receptive (anoestrus). Animals that have oestrous cycles reabsorb the endometrium (inner membrane of the mammalian uterus) if conception does not occur during that cycle. The oestrous cycle is contrasted with the menstrual cycle in which the endometrium is shed through menstruation when pregnancy does not occur and in which the female may be sexually receptive at any time during the cycle.

There are many variations among animals in terms of their oestrous cycles. Some may undergo oestrous only one time a year during a particular season (white-tailed deer, foxes), while others may undergo a succession of cycles during a certain time of the year if they do not become pregnant (horses, sheep), and others may undergo cycles throughout the whole year (mice, cows, pigs).

Below is an overview of the hormonal changes during the oestrous cycle: (Download article to see this)