Reproduction Index Glossary

Classification of Vaginal Epithelial Cells


A majority of cells observed in a normal vaginal smear are, not surprisingly, vaginal epithelial cells. In addition, varying numbers of leukocytes, erythrocytes and bacteria are usually evident, and small numbers of other contaminating cells and microorganisms are sometimes observed.

Analyzing a vaginal smear is largely an exercise in classifying the epithelial cells into one of three fundamental types: parabasal, intermediate or superficial cells. Keep in mind, however, that the epithelial cells reflect a developmental continuum; some of the cells you observe will not fit perfectly into these rigidly-defined categories.

All of the figures on this page are presented at the same magnification and are from a single dog.

Parabasal Cells

Parabasal cells are the smallest epithelial cells seen on a typical vaginal smear. They are round or nearly round and have a high nuclear to cytoplasmic ratio.

Parabasal cells are prevalent on smears taken during diestrus and anestrus, and not uncommon during early proestrus. Parabasal cells are conspicuously absent during estrus.

Intermediate Cells

Intermediate cells vary in size and shape, but typically have a diameter two to three times that of parabasal cells. Many cytologists subclassify these cells into:

  • small intermediates: nearly round or oval shape with large, prominent nuclei
  • large intermediates: polygonal shape with a small nuclear/cytoplasmic ratio

In the figures below, all of the cells are typical intermediates except for the one cell in the middle panel (arrow), which might be classified as a superficial cell (small, dark nucleus). Note that the intermediate cells vary in size and that some have rounded outlines (small intermediates), while others have a polygonal shape (large intermediates).

Intermediate cells are prevalent during all stages of the cycle except estrus.

Superficial Cells

Superficial cells are the largest cells seen on a vaginal smear. The are polygonal in shape and distinctly flat, sometimes having the appearance of being rolled up. Their nuclei are either absent or pyknotic (very small and dark). Superficial cells without nuclei are often referred to as being "fully cornified".

Superfical cells are often seen in large sheets or strings, as seen below with fully cornified cells.

Superficial cells are not normally seen during anestrus and increase in prevalence during proestrus. The presence of large numbers of superficial cells or only superficial cells is the defining characteristic of cytologic estrus, and their abrupt and precipitous decline marks the onset of diestrus.

Other Cells

Aside from the epithelial cells described above, a number of other cells are seen on vaginal smears.

  • Erythrocytes are usually observed in large numbers during proestrus. In some bitches, they are seen through estrus and even into early diestrus.
  • Neutrophils are often abundant in smears taken during early diestrus, and are not uncommon at other stages, though rare during estrus. Moderate numbers of neutrophils are a common, though not consistent feature of normal canine vaginal smears and not by themselves indicative of vaginitis.
  • "Foam cells" is a term given to non-descript epithelial cells containing numerous vacuoles that are typically seen on smears prepared during anestrus.

The following figure, of a proestrus smear, shows a group of intermediate cells associated with neutrophils and red blood cells.

Finally, bacteria are often seen on vaginal smears in huge numbers, covering cells and spilling onto the background. The minute dark specks covering the superfical cells in the image below are bacteria.


Index of: Vaginal Cytology
Techniques for Preparing a Canine Vaginal Smear Cytologic Changes Through the Canine Estrous Cycle

Last updated on April 11, 1998
Author: R. Bowen
Send comments via form or email to rbowen@.colostate.edu