Bitches with uteri devoid of endometrial glands should be sterile, and consequently could contribute to the population control of dogs. Considering that an inadequate exposure of the female reproductive system to steroids can lead to the formation of the uterine gland knock-out (UGKO) phenotype in some species, the aim of this study was to evaluate the effect of serial applications of medroxyprogesterone acetate (MPA) from birth until the age of 6 months on the development of endometrial glands in bitches. For this purpose, 16 female mongrel dogs from different litters were distributed into either an MPA group (n = 8), animals treated with medroxyprogesterone acetate (MPA,10 mg/kg/sc), from day 1 after birth until the age of 6 months, at 3-week intervals, or a control group (n = 8), composed of animals that only received a 0.9% NaCl solution in place of MPA. At 6 months of age, ovariohysterectomy was performed and uterine horn samples were collected for histological and immunohistochemical examinations. The variables evaluated included the total thickness of the uterine wall (μm), endometrium (μm), and myometrium (μm); diameter of the endometrial glands (μm); number of endometrial glands/mm2 ; and the epithelial height (μm). Five fields per histological sample were evaluated for the calculation of the total thickness of the uterine lining, endometrium, and myometrium, and the epithelial height. The glandular diameter was calculated by measuring 10 glands per histological sample. The measurements were performed using the ImageJ software version 1.43u (Wayne Rasband, National Institutes of Health, USA). The expression of the ER- α, ER- β and PR on the surface epithelium and endometrial stroma was evaluated. A total of 200 cells with stained nuclei were counted in each slide. The histological data were evaluated by descriptive analysis, and the morphometric data were analysed by the Tukey’s test using the System for Statistical Analyses (SAS) software version 9.1 (2007). The immunohistochemical data were analysed using the GraphPad Prism 5.0 software (Prism Software, Irvine, CA, USA). Data normality was demonstrated using the Kolmogorov-Smirnoff test. Statistical differences were evaluated using the Student’s t-test, and the analyses were considered significant if the P-value was ≤ 0.05.The bitches from the MPA-treated group presented a 35% decrease in the number of endometrial glands, a larger diameter of the endometrial glands, a greater epithelial height, as well as a greater thickness of the uterine wall, endometrium, and myometrium. However, no significant differences were observed between the two groups in the expression of ER-α, ER-β, and PR on the surface epithelium and endometrial stroma. Therefore, serial injections of MPA from birth until the age of 6 months caused a decrease in the development of endometrial glands by approximately 35% in bitches. However, the ER α and β and PR were expressed at normal levels on the uterine surface epithelium and endometrial stroma.
