Veterinary Physiology-III

ESTROUS CYCLE 

• The estrous cycle is conveniently divided into 4 phases:

Proestrus

• One or several follicles of the ovary start to grow - number is specific for the species; lasts from one day to 3 weeks, depending on the species.
• Endometrium starts to develop under the influence of estrogen. Some animals may experience vaginal secretions that could be bloody. The female is not yet sexually receptive.

Estrus

• Estrus refers to the phase when the female is sexually receptive ("in heat," or "on heat").
• Ovarian follicles mature under the influence of estrogen which is regulated by gonadotropic hormones and estrogen secretions exert their biggest influence.
• The animal exhibits a sexually receptive behaviour that is exhibiting physiological changes visibly. A signal trait of estrus is the lordosis reflex, in which the animal spontaneously elevates her hindquarters.
• In some species, the vulvae are reddened.
• Ovulation may occur spontaneously in some species (e.g. cow), while in others it is induced by copulation (e.g. cat). If there is no copulation in an induced ovulator, estrus may continue for many days, followed by 'interestrus,' and the estrus phase starts again until copulation and ovulation occur.

Metestrus

• During this phase, the signs of estrogen stimulation subside and the corpus luteum starts to form.
• The CL begins to secrete small amounts of progesterone. This phase is brief and last 1 to 5 days. In some animals bleeding may be noted due to declining estrogen levels.

Diestrus

• Diestrus is characterised by the activity of the corpus luteum that produces progesterone.
• In the absence of pregnancy the diestrus phase (also termed pseudo-pregnancy in cats, dogs) terminates with the regression of the corpus luteum.
• The lining in the uterus is not shed, but will be reorganised for the next cycle.