Estrus - Estrous cycle
This is the period of heat, and copulation is permitted only at this time. This condition lasts from 9 to 15 hours and is characterised by a high rate of running activity. Under the influence of EPH, a dozen or more ovarian follicles grow rapidly: estrus is thus a period of heightened estrogen secretion. Behavioural changes include quivering of the ears, and lordosis (arching the back in response to handling or to approaches by the male). The uteri undergo progressive enlargement and become distended owing to the accumulation of luminal fluid. Many mitosis occur in the vaginal epithelium and as new cells accumulate, the superficial layers become squamous and cornified.
The latter cells ah exfoliated into the vaginal lumen, and their presence in vaginal smears is indicative of estrus. During late estrus, there are cheesy masses of cornified cells with degenerate nuclei present in the vaginal lumen, but few if any leukocytes are found during estrus. Ovulation occurs during estrus and is preceded by histologic changes in the follicle suggestive of early luteinisation. Much of the luminal fluid in the uteri is lost before ovulation.