Periovulatory time in the bitch: What's new to know?: Comparison between ovarian histology and clinical features

• The paired ovaries of 40 cycling bitches were histologically analyzed.
• Number and dimension of 414 follicles, corpora hemorrhagica and corpora lutea were evaluated.
• Comparison was made with vaginal cytology and progesteronemia in the reproductive cycle.
• Special attention was paid to the periovulatory period which is the reproductive core in the bitch.
• This study might deepen the knowledge on some critical steps in canine reproduction.

The ability to recognize specific events happening in the ovaries during periovulatory time allows optimal management of canine reproduction. The objective of this study was to assess the efficacy of vaginal cytology and blood progesterone (P4) assay to identify accurately the changes occurring at the ovarian structures, mainly during the fertile period. Tertiary follicles, corpora hemorrhagica (CHs) and corpora lutea (CLs) from forty healthy bitches undergoing ovariohysterectomy were evaluated by histo-morphometry based on their aspect, number and size. The tertiary follicles distribution (small, medium and large) was statistically different (P < 0.002) among all the stages of the reproductive cycle, except for small follicles (<2 mm), which were always observed from proestrus to anestrus. Very large follicles (>4 mm) were predominant (P = 0.008) around ovulation when P4 mean level was 6.1 ± 1.7 ng/mL. The early postovulatory estrous period was characterized by CHs (P < 0.002) and P4 level of 16.7 ± 5.9 ng/mL. The end of the fertile period – start of diestrus – coincided with the development of CLs (P = 0.001) associated with a P4 mean level of 73.9 ± 9.9 ng/mL. The small (P < 0.001) and medium (P < 0.05) follicle diameters were positively correlated with the bitch size. The number of follicles larger than 4 mm was significantly lower in bitches younger than 4 years (P < 0.02). This study provides insight into some critical steps in the canine reproductive processes in the periovulatory phase and the end of the fertile period, essential to plan breeding programs.