By examining exfoliated vagina cells one can have a simple technique to monitor the progression through the estrous cycle in the bitch. Cells are easily obtained by the use of a cotton swab or by means of imprint smears of the vestibular mucosa. There are several types of vaginal epithelial cells. The classification of these different cell types is based upon the location of the cell in the layers of squamous epithelium. In order of development from basement membrane towards vaginal lumen, they are called basal cells, parabasal cells, intermediate cells and superficial cells.

 Basal cells. The basal cells are usually not seen in vaginal smears. They are small cells, with a minimal amount of cytoplasm present. The basal cells are attached to the basement membrane and give rise to all other epithelial cells in the vaginal smear.

 Parabasal cells. The parabasal cells are the smallest epithelial cells seen in vaginal smears. These cells are usually round-oval cells with a round nucleus and only a very slim amount of dark blue or dark grey cytoplasm present. The parabasal cells are very uniform in appearance. In very young animals sheets of parabasal cells can be encountered when the vagina is swabbed.

 Intermediate cells. The size of intermediate cells may vary, although in general they are larger than the parabasal cells. The low and small, intermediate cell has a darker cytoplasm color and is more rounded. This in contrast to the upper, and larger, intermediate cell, which has a lighter cytoplasm color. The large intermediate cell is usually also round, but becomes more irregular and a bit folded.

 Superficial cells. This is the largest epithelial cell type seen in vaginal smears. As in the upper intermediate cells the cytoplasm is abundant, folded and angular. This cell type, however, starts to keratohyalinize and small droplets, vacuoles, can be found in the cytoplasm. At the same time the nucleus starts to fade away or become pycnotic. When they are completely cornified, no nucleus is seen any longer.

In addition to the cell types described above there are two other epithelial cell types that can be found in a vaginal smear:

 "Metestrum" cells. The metestrum cell is thought to be a parabasal cell that has one or more neutrophils in its cytoplasm. Although its name suggest that they can only be found during metestrus, their presence is not as much related to the estrus cycle as such, but more to the presence of leucocytes in the vagina.

 "Foam" cells. The foam cells are also believed to be parabasal cells. They contain cytoplasmatic vacuoles that may vary in size. Their significance is not known.

The change of cell pattern in vaginal smears reflects the change of the vaginal epithelium due to change of ovarian hormonal activity. During the follicular phase the estrogens promote the proliferation and maturation of the epithelial cells towards keratinized squamous epithelium.

Anestrus

During the anestrus some parabasal and intermediate cells can be found in a vaginal smear. Sometimes a few neutrophils can be seen. Typical is the absence of superficial cells. At the end of the anestrus and at the start of the follicular phase some erythrocytes may be present. Bacteria can be absent or present in low numbers.

Onset of Follicular Phase

In the onset of the follicular phase there is a proliferation of epithelial cells and a leakage of erythrocytes by the smaller capillaries. As a result many erythrocytes are present in vaginal cytologic smears. The main epithelial cell type is the intermediate cell, but some parabasal cells and superficial cells are usually also present. Neutrophils are also observed. Neutrophils become less prominent and may even be absent.

Progressing Follicular Phase, Ovulation, Start Luteal Phase

In addition to the erythrocyte, the superficial epithelial cell is the most common exfoliated cell in this stage, some bitches reaching 60%, others 90%. Although these cells have usually a pycnotic nucleus, anuclear cells can also be found. Large intermediate cells may also be present, but parabasal cells and small intermediate cells are absent. Bacteria may be seen. Characteristic no neutrophils are present.

Progressing Luteal Phase or Diestrus/Metestrus

A rapid shift in numbers of superficial cells is observed in this stage. The percentage of superficial cells decreases and at the same time parabasal cells and intermediate cells reappear again. Neutrophils are usually also coming back. As a result, an occasional 'metestrum' cell can be found. Erythrocytes may or may not be present.

One should realize that, although vaginal cytology gives an indication of the stage of the cycle, it is not a trusted indicator of the preovulatory LH surge or of ovulation.

References

1.  Olson PN, Thrall MA, Wykes PM, Nett TM. Vaginal cytology: Part 1. A useful tool for staging the canine estrous cycle. Compend. Contin. Ed. Pract. Vet. 6; 288-298, 1984.

2.  Roszel JF. Genital cytology of the bitch. Scope 19, no 1, 2-15, 1975.

3.  Allison RW, Thrall MA, Olson PN. Vaginal cytology. In: Diagnostic cytology of the dog and cat. Eds RL Cowell, RD Tyler JH Meinkoth, DB DeNicola. Mosby-Elsevier, Missouri, 378-389, 2008.

4.  Hiemstra M, Schaefers-Okkens AC, Teske E, Kooistra HS. The reliability of vaginal cytology in determining the optimal mating time in the bitch. Tijdschr Diergeneeskd, 126, 685-689, 2001.