Proestrus
Serum concentrations of estrogen rise during proestrus, leading to capillary breakage and leakage of red blood cells through uterine epithelium, as well as proliferation of the vaginal epithelium.
Examination of vaginal smears from early to late proestrus will reveal a gradual shift from intermediate and parabasal cells to superficial cells. Typically, red blood cells are present in large numbers and neutrophils are commonly observed. Large numbers of bacteria are also often present.