Uterine gland development begins postnatally and is accompanied by estrogen and progesterone receptor expression in the dog.

During neonatal and juvenile life, mammalian uteri undergo extensive structural and functional changes, including uterine gland differentiation and development. In sheep and mice, inhibition of neonatal uterine gland development induced by progestin treatment led to a permanent aglandular uterine phenotype and adult infertility, suggesting that this strategy might be useful for sterilizing dogs and other companion animals. The goal of this study was to define temporal patterns of adenogenesis (gland development), cell proliferation, and progesterone and estrogen receptor expression in uteri of neonatal and juvenile dogs as a first step toward determining whether neonatal progestin treatments might be a feasible contraceptive approach in this species. Uteri obtained from puppies at postnatal wk 1, 2, 4, 6, or 8 were evaluated histologically and immunostained for MKI67, a marker of cell proliferation, estrogen receptor-1, and progesterone receptor. Adenogenesis was under way at 1 wk of age, as indicated by the presence of nascent glands beginning to bud from the luminal epithelium, and rapid proliferation of both luminal epithelial and stromal cells. By Week 2, glands were clearly identifiable and proliferation of luminal, glandular, and stromal cells was pronounced. At Week 4, increased numbers of endometrial glands were evident penetrating uterine stroma, even as proliferative activity decreased in all cell compartments as compared with Week 2. Whereas gland development was most advanced at Weeks 6 to 8, luminal, glandular, and stromal proliferation was minimal, indicating that the uterus was nearly mitotically quiescent at this age. Both estrogen receptor-1 and progesterone receptor were expressed consistently in uterine stromal and epithelial cells at all ages examined. In summary, canine uterine adenogenesis was underway by 1 wk of age and prepubertal glandular proliferation was essentially complete by Week 6. These results provided information necessary to facilitate development of canine sterilization strategies based on neonatal progestin treatments designed to permanently inhibit uterine gland development and adult fertility.