Photomicrograph of exfoliative cytologic slides from dogs in different stages of the estrous cycle.

A. Anestrus. There is low cellularity, and the vaginal epithelial cells are mostly parabasal. Original magnification, 200×; Romanowsky variant stain.

B. Late proestrus. The vaginal epithelial cells are mainly large intermediate cells. Few superficial cells, RBCs, and bacteria are also noted. Original magnification, 200×; Romanowsky variant stain.

C. Estrus. There is a predominance of superficial vaginal epithelial cells (full cornification) and clear background. Few bacteria are noted. Original magnification, 100×; Romanowsky variant stain.

D. Early diestrus. There are small intermediate and parabasal vaginal epithelial cells, bacteria, numerous neutrophils, and moderate debris. Two metestrum cells (asterisk) are evident. Original magnification, 200×; Romanowsky variant stain.