Proestrus
Proestrus is a period of rapid follicular growth under gonadotrophic stimulation and the period in which the CL from the previous cycle in polyestrous specious regress.
The follicle growth (folliculogenesis) occurs through small, medium and large size follicles and is influenced by GnRH, FSH and LH. FSH initiates growth of follicles
LH converts the C-27 steriod cholesterol to C-19 steroid androgen in the theca interna. FSH favours the biosynthesis of C-18 steroid estrogen from androgen by the granulose cells.
In this phase, estrogen concentration increases progressively with simultaneous decline in progesterone level. It is due to the regression of the corpus luteum of the previous cycle by the luteolytic action of the PGF2α in polyestrus species.
In all domestic animals except mare Folliculogenesis is depressed during pregnancy because of high level of progesterone.
There is marked increased in the growth of the epithelial tissues, the activity of musculature of the reproductive tract, secretion of the mucus and of the vascularity of the endometrium and vaginal mucosa.
In cow and mare, the sticky, dry mucus of the preceding period changes during proestrus to a milky, less viscid discharge (serous) and finally to a clear transparent, stringy mucus late in the proestus period. Proestrus is not clinically evident in most domestic species and lasts only for 2-3 days.
In the bitch, proestrus lasts for 7-9 days and is clearly identifiable by well-defined changes in the external genitaila and behavioral signs due to increasing sexual excitement. The increased endometrial vascularity is characterized by bleeding.
The vulva becomes increasing edematous and swollen. There is a gradual relaxation of the cervix and an increased secretion of viscid, slimy mucus from the goblet cells of the cervix and anterior vagina and from the uterine glands