Insemination

 

Hormones in Inter-oestrus

From around day 6 of the cycle, the yellow body (corpus luteum) has developed enough for the pregnancy hormone (progesterone) it produces to cause effects in the animal via the bloodstream. The secretion of gonadotropin-releasing hormone (GnRH) in the hypothalamus is inhibited. This is called a negative feedback mechanism.

Follicles not maturing fully during the yellow body phase produce the oestrus hormone (oestrogen), which can cause oestrus behaviour during mid-cycle. However, ovulation does not take place because luteinising hormone is not secreted by the pituitary (an effect of the negative feedback mechanism).
If a successful insemination (and hence fertilisation) has taken place during oestrus, the embryo produces the hormone interferon-tau from around day 16.
This embryonic hormone acts as a signal of “pregnancy” to the uterus, preventing it from producing prostaglandin F2 alpha.
The yellow body persists and produces the pregnancy hormone progesterone, which is vital for foetal survival, for most of the gestation period.
Without fertilisation and hence without a signal from the embryo’s interferon-tau, the uterus produces the hormone prostaglandin F2 alpha from around day 17 of the cycle, triggering resorption of the yellow body. As a result, the level of progesterone in the blood falls rapidly and GnRH is secreted by the hypothalamus (end of negative feedback). A new cycle gets underway.

See right to view the effects of individual hormones or the hormone curves.