James Crawley
Animal Science (Behaviour & Welfare)
The role of hormones in the female reproductive system
The oestrus cycle is the continuing process of fertility and non-fertility controlled by several hormones and the endocrine system. This consists of the hypothalamus, the anterior pituitary gland, the ovaries and the uterus, known as the hypothalamic-pituitary-ovarian-uterine axis. Oestrus normally lasts for 18 to 24 days but this figure can vary in specific circumstances such as early puberty when the cycle lasts for a lot less time (3 to 12 days) and also when the cycle commences after pregnancy and parturition (Fuquay 2002). The oestrus cycle halts during pregnancy due to the large amounts of progesterone in the blood supplied by the Corpus Luteium (CL). Progesterone is vital for maintaining pregnancy and also for stimulating the growth of mammary alveoli for the purpose of lactation.
Gonadotrophin releasing hormone (GnRH) begins a cascade of hormones through the animal that are responsible for the oestrus cycle taking place. It is secreted in a pulsating manner with rises and falls in concentrations from the hypothalamus where it is transported to the anterior pituitary gland. It is here that the GnRH controls the release of two protein hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) which are known collectively as gonadotrophins. Because the release of GnRH is in waves, consequently FSH and LH are secreted in waves too. These gonadotrophins are released into the general blood system and are transported to the gonads where they have their major effect. FSH lives up to its name and is the principal hormone for stimulating follicle growth in the ovaries. LH surges through the ovaries towards the end of maturation and just before ovulation, it causes the maturing follicle to ‘rupture’ and separate leaving the developing luteal cells behind. LH is responsible for the development of the CL and this in turn is responsible for producing progesterone. At the same time as the LH surge, the ovarian steroid hormone Oestradiol is released and this works alongside progesterone in a feedback mechanism to control gonadotrophin release. High concentrations of progesterone and oestradiol in the blood act to lower the amount of gonadotrophins released.