Vaginal Cytology

Changes in normal vaginal cytology during the estrous cycle of the bitch have been well outlined by several authors. Essentially, estrus has four normal stages (anoestrus, proestrus, estrus, and metestrus) that may be recognized cytologically.

Anoestrus is the quiescent phase of the heat cycle. During normal anoestrus the vaginal vault is dry and secretions absent. Smears taken from the vaginal vault (during normal anoestrus contain low numbers of squamous epithelial cells from all of the several layers of stratified epithelium lining the vagina. The most superficial epithelial cells are undergoing cornification (keratinization) and have basophilic cytoplasm with angular cell margins and pyknotic to absent. Mature but non‑cornified epithelial cells are round to oval with centrally located vesicular nuclei and less cytoplasmic basophilia. The smaller more immature parabasal cells also are round to oval with large vesicular nuclei but they exhibit more cytoplasmic basophilia than mature cells due to a higher content of cytoplasmic RNA. An occasional neutroplil may also be seen.

Proestrus is the first active phase of the estrous cycle and is of variable length (4‑20 days). During proestrus the vulva is swollen and eventually a bloody discharge is detected. Bitches attract males but do not allow copulation.

Cytologically, red blood cells are seen in vaginal smears about 2‑7 days before discharge is seen grossly. As proestrus progresses there is a steady increase in the number of cornified epithelial cells. As cells become cornified the nuclei become smaller and more condensed. The cell increases in overall size and the cytoplasm becomes more basophilic. When cornification is complete the nucleus has often disappeared and cytoplasmic wrinkles and folds are commonly seen. The occasional leukocytes seen in anoestrus are absent in proestrus.

Estrus begins when the bitch accepts the male. Like proestrus, estrus is variable in length ranging from about 4 to 13 days. Ovulation occurs during estrus but the exact time of ovulation can only be estimated. In general, ovulation occurs 24‑36 hours before the appearance of leukocytes in the vaginal smear.

Cytologically, early estrus is characterized by a marked decrease in red blood cells and a continuing increase in the number of cornified epithelial cells. At the height of estrus, only cornified epithelial cells are seen. In late estrus, leukocytes begin to appear.