Estrous vaginal cytology from a bitch
Keywords: estrus, estrous, canine, bitch, vagina, red, blood, cell
Figure 1:An example of vaginal cytology during estrus in a bitch. The cytology was collected with a Pap smear brush and strained with Diff-Quick method. Size: 4080 x 3072 px
During estrus, estrogens from ovarian follicles bind to receptor sites in the vaginal mucosa and being mitogenic in nature, cause epithelial cells to divide. Cells closest to the basement membrane (parabasal cells) divide rapidly and displace cells above them, moving those cells further and further from the basement membrane. As a result, the displaced cells are deprived of oxygen and nutrients and begin to show pyknosis and karyorrhexis. Eventually, they loose their nuclei and become flat, lifeless squames. These cells are variously referred to as cornified cells or superficial cells and often, the term "superficial cell index" is used to express the ratio of superficial cells to basal cells. This author uses the term rather loosely, mainly referring to cytology with a high superficial cell index or simply omitting the term if relatively few superficial cells are present.
The thickening process of the vaginal mucosa is essential for its protection during the prolonged copulation typical of canids and is also beneficial for the long term health of the vagina.
Estrogen binding to the vaginal mucosa is therefore expressed as cytology that is typical of estrus in bitches. However, the cellular response to estrogens is neither immediate nor binary (completely on or completely off) so is only loosely correlated to the time of the LH surge and the onset of ovulation. For that reason, cytology is usually coupled with tests such as vulva turgidity, progesterone assay and/or LH assays to determine the optimal time for breeding.
Estrous cytology is also notable for its relative lack of bacteria and cell debris (a clear background), attributable in part, to vaginal flushing by cervical and vaginal secretions. In this case (but not always) numerous red blood cells are also visible. They are of endometrial origin, leaving the endometrial mucosa via diapidesis under the effect of estrogen. It is important to note that the presence of red cells is extremely variable in vaginal cytology, appearing before the onset of significant cornification and sometime lasting for a week or more after ovulation. Obviously therefore, blood is not spermicidal in dogs. Also, canine embryos do not enter the uterus until 9 days after ovulation (Reynaud et al., 2006). Therefore the presence of red cells in vaginal cytology does not necessarily mean that blood is entering the uterus in the presence of the embryos. It does mean however that the canine cervix closes very slowly after the time of ovulation!
The term "estrus" (estrous being the adjective) is used loosely because it usually refers to receptivity. However, some bitches may not be receptive even in the face of the typical cytology shown here. Reasons for this include the effect of socialization with humans vs.dogs, dominance of some females over males they have been raised with, and temerity in young bitches. Therefore one cannot be assured that a bitch will be receptive to a male merely because of the appearance of vaginal cytology.
A footnote: The term cornification is derived from the Latin term "cornu" meaning "horn", Those familiar with dermatology will know that horns are a form of keratin. In earlier years, this author used the Schorr's trichrome technique for for vaginal cytology because it provided a vivid image of orange keratinized (superficial) cells. The technique was elegant but time consuming and demanded immediate fixing in alcohol.
