Canine Reproductive Anatomy and Estrous Cycle

The Estrous Cycle Phases

The canine is typically monestrous, meaning she undergoes one to two cycles per year, though some breeds may have more. The cycle is divided into four distinct stages:

1. Proestrus: The initiation of overt activity, often characterized by physical changes like vulvar swelling and hemorrhagic discharge.
2. Estrus: The period of sexual receptivity, or "breeding time".
3. Diestrus: The luteal phase, which occurs during pregnancy or pseudopregnancy.
4. Anestrus: A period of quiescence and repair between cycles.

The duration of these phases can vary significantly between individuals. While the average duration of proestrus and estrus is 7–9 days each, the actual range can span from 2 to 21 days. The inter-estrus period (the time from the end of one estrus to the start of the next) averages about 7 months, but can range from 4 months in Rottweilers to 12 months in Basenjis.

Clinical Note on Puberty: The onset of puberty is influenced by age and body size. Smaller breeds (e.g., Corgis) may reach puberty at 6–12 months, while giant breeds (e.g., Great Danes) may not reach it until nearly 2 years of age.

Hormonal Profiles and Behavior

Hormonal fluctuations drive both the physical and behavioral changes seen in the bitch. During Proestrus, estrogen is the dominant hormone. Behaviorally, the bitch may attract males but will typically show negative responses, such as growling or snapping, if they attempt to mount.

As the bitch transitions into Estrus, estrogen levels begin to fall while progesterone starts to rise. This rise in progesterone is linked to the LH (luteinizing hormone) peak and the luteinization of follicles. During estrus, the bitch exhibits "standing behavior," characterized by tail deviation and muscle contractions to facilitate mating.

In Diestrus, progesterone dominates the profile. This phase is the luteal phase, and its duration is generally about 60 days. If the bitch is not pregnant, she enters pseudopregnancy, driven by the continued presence of progesterone. Finally, during Anestrus, progesterone returns to baseline levels (<1 ng/mL).

Vaginal Cytology

Vaginal cytology is a vital diagnostic tool used to characterize the stage of the estrous cycle by observing the response of the stratified squamous epithelium to estrogen.

Cell Types

The vaginal epithelium consists of several cell types that change in proportion throughout the cycle:

• Parabasal cells: Small, round/oval with a large nucleus.
• Intermediate cells (Small and Large): Larger than parabasal cells with irregular borders.
• Superficial cells: The largest cells; they may be nucleated or become "cornified" (anucleated squames) as they mature.

Interpreting Cytology Results

Clinicians use the ratio of these cells to estimate the cycle stage:

Pro-Tip for Cytology: A common rule of thumb to confirm estrus is to look for a slide that is 100% cornified with at least 50% anuclear squames.

To obtain a sample, a dry sterile cotton-tipped swab is introduced into the dorsal vestibule or caudal vagina. The swab should be rolled gently on a slide, air-dried, and stained (e.g., with Diff-Quik) before examination under high power (400x–1000x). Because the canine vagina is quite long, the swab must be inserted significantly deep to reach the cranial vagina for an accurate reading.