The Canine Estrous Cycle

Stages of the canine estrous cycle can be defined by sexual behavior, vulvar swelling, vaginal bleeding or by vaginal cytology. The period of receptivity to a male is different for all the bitches. Some of them are receptive before and after the period of potential fertility. Changes in the cells indicate the events of the cycle. They are a better way to predict the fertile time and gestation length than are behavioral or physical signs. Examination of a smear can provide useful information. For example, it is often difficult to differentiate proestrus and diestrus from an isolated smear. The reason why multiple smears are recommended is because sometimes they can be misleading at evaluation.

The first one is called Anestrus, which consists of mainly Intermediate, and Parabasal cells dominate. Superficial cells are found in very small numbers. Neutrophils may also be present or absent. Proestrus consists mainly of Serum concentrations of estrogen, leading to capillary breakage and leakage of red blood cells through uterine epithelium. When examining vaginal smears from early to late proestrus it will show a small shift from intermediate and parabasal cells to superficial cells. Red blood cells are present in large numbers as well as neutrophils and large amount of bacteria are also often present.

Proestrus may last for two to three weeks in bitches. Estrus would be the next stage, which is characterized by the predominance of superficial cells.

Most bitches will undergo full cornification, and the smear will reveal a monotonous pattern composed almost exclusively of anucleate superficial cells. If the bitch has been bred within a day of preparing a vaginal smear, it is quite likely that sperm will be observed among the epithelial cells.

Diestrus is noticeable by a quick decrease in the number of superficial cells and reappearance of intermediate and parabasal cells. Most commonly, the cellular profile changes within a single day from essentially 100% superficial cells to less than 20% superficial cells.

It is best to confirm the onset of diestrus by examining a smear prepared on diestrus day 2. The importance of identifying the beginning of diestrus is that it is a more accurate predictor of the time of ovulation.

Dogs ovulate 5-7 days prior to the onset of diestrus and gestation length is usually 57 + 1 day from the onset of diestrus day 1. The period of behavioral estrus is variable, and often extends up to several days before or after cytologic estrus.

Gestation lengths calculated from the onset or cessation of receptivity are correspondingly inaccurate. The beginning of diestrus also correlates well with loss of fertility. The majority of cells observed in a normal vaginal smears are vaginal epithelial cells. 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 basically classifying the epithelial cells into one of three important types: parabasal, intermediate or superficial cells. Parabasal cells are the smallest epithelial cells seen on a typical vaginal smear. They are round and have a high nuclear to cytoplasmic ratio. Parabasal cells are predominant on smears taken during diestrus and anestrus, and not uncommon during early proestrus. Parabasal cells are absent during estrus. Intermediate cells vary in size and shape, but typically have a diameter two to three times that of parabasal cells.

Intermediate cells are prevalent during all stages of the cycle except estrus. Superficial cells are the largest cells seen on a vaginal smear. They are polygonal in shape and flat, sometimes having the appearance of being rolled up. Their nuclei are either absent or very small and dark. Superficial cells without nuclei are often referred to as being fully cornified. Aside from the epithelial cells described, 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, but rare during estrus. Moderate numbers of neutrophils are a common, but 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 that are prepared during anestrus.