Vaginal cytology explained

Vaginal cytology is a microscopic examination of cells from the vaginal epithelium. In veterinary medicine, it helps differentiate the stages of the mammalian estrous cycle because the vaginal epithelium changes in response to sex hormone levels; practically, it is used to distinguish when a female canine is at a particular point in the estrous cycle.^[1] In a normal vaginal smear, lactational cells, navicular cells, endocervical cells, endometrial cells, trophoblastic cells, and leucocytes may be present.

The equipment needed for vaginal cytology includes a vaginal speculum, cotton-tipped applicators, frosted microscope slides, commercial Romanowsky stain, and light microscope.^[2]

Types of cells

See also: Vaginal epithelium.

• Basal cells - produce all the different epithelial cell types that are observed in a vaginal smear. Basal cells are comparatively small, have a round nucleus and a high rate of nucleus-to-cytoplasm. They are not commonly found during a vaginal smear and are considered rare in comparison to other cells.^[3]
• Parabasal cells - These cells are small, round or nearly round, and have a small amount of cytoplasm. They contain a vesticulated nucleus. When a swab is performed on a prepubertal animal, a large amount may exfoliate.^[3]
• Intermediate cells - the size of these cells vary heavily based on the amount of cytoplasm they contain; however, the nucleus remains of a cell remains the same size. These cells are larger than parabasal cells although their nucleus is of similar size. The nucleus of an intermediate cell is vesticulated. As an intermediate cell increases in size the cytoplasm may become irregular in appearance, causing it to resemble a superficial cell. Large intermediate cells which resemble a superficial cell are sometimes known as superficial intermediate cells or transitional intermediate cells.^[3]
• Superficial cells (dead cells) - the largest epithelial cells seen in a vaginal cytology. Superficial cells are dead cells, the nucleus of these having become pyknotic and later fading, eventually they progress to lacking a nucleus. The cytoplasm in these cells is angled, folded, and plenteous in amount.^[3]
• Metestrum cells - are described as vaginal epithelial cells that contain neutrophils in their cytoplasm. Although their name implies they are seen during metestrus, these cells are not specific for any stage of estrous.

Observable cells in each stage of estrous cycle

Proestrus

• Early- to mid-proestrus - During this stage, vaginal cytology will show a gradual shift from parabasal cells, to intermediate cells, to superficial cells. Also present are neutrophils and red blood cells in large numbers. The red blood cells are due to the proliferation of vaginal epithelium caused by increased levels of estrogen. The cells may float in basophilic mucous.
• Late-proestrus - there is a decrease in neutrophils because they are unable to cross the thickening vaginal wall. Cytology consists mostly of large intermediate and superficial cells. Red blood cells and bacteria may be present.

Estrus

Estrus is characterized by the presence of all superficial cells, many of which are anuclear (without a nucleus) or have small, pyknotic nuclei, and red blood cells may be present, along with bacteria. No neutrophils are seen unless there is inflammation. There is usually no mucous in the background.

Diestrus

In diestrus, superficial cells are replaced by parabasal and intermediate cells within one to two days of onset. Neutrophils are increased. Red blood cells may be absent or present. Bacteria is commonly observed. A smear made during late estrus to early diestrus may appear similar to one made in early- or mid-proestrus.

Anestrus

The final stage of estrous is anestrus, which is characterized by predominantly non-cornified squamous epithelial cells, such as intermediate or parabasal cells. The cells are smaller, have basophilic cytoplasm, and large round nuclei. There may be some neutrophils, but no red blood cells. The overall cellularity is low.

See also

• Canine reproduction
• Reproductive cycle
• Cytopathology

Notes and References

1. Web site: Vaginal Cytology: Introduction and Index. www.vivo.colostate.edu. 2016-02-04.
2. Web site: Determining Canine Estrus Stage via Vaginal Cytology. www.cliniciansbrief.com. en. 2019-12-08.
3. Cowell, R., & Valenciano, A. (2019). Cowell and Tyler’s Diagnostic Cytology and Hematology of the Dog and Cat. (5th ed.). St Louis, MO: Elsevier.