Basic research into the histology, endocrine control, and local regulation of prepuberal ruminant mammogenesis was conducted to provide a better understanding of this important developmental period. Histologic features of prepuberal ruminant mammary parenchymal morphogenesis were examined in tissue samples taken from ewe lambs at 2 (n = 5), 3 (n = 15), 6 (n = 26), 9 (n = 7), 12 (n = 5), and 13 wk (n = 20), and from Holstein heifers at 4 (n = 1) and 6 mo (n = 2). Examination of approximately 8000 histologic sections revealed that mammary parenchymal morphogenesis in sheep and cattle occurs through the proliferation of highly arborescent ductal structures embedded in a dense stroma. These observations contrast strongly with models of mammogenesis based on murine mammary development. The formation of luminal spaces and the expansion of ducts also differed from murine mammogenesis models. Luminal spaces were shown to develop through a progressive separation of opposing sides in initially solid ductal structures. Likewise, our investigation of prepubertal ovine mammogenesis revealed that parenchymal weight, 3H-thymidine labeling, stromal weight, and parenchymal DNA were all unaffected by ovariectomy (P > 0.05), in marked contrast to the dramatic reduction in mammary development following ovariectomy in rats, mice, and heifers. Responsiveness to exogenous estrogen (0.1 mg/kg) was demonstrated by increased 3H-thymidine labeling (P < 0.05) in both intact and ovariectomized lambs. Three dimensional collagen gel cultures of bovine mammary organoids from the peripheral (OUTER) and medial (INNER) parenchymal zones were used to characterize the proliferative and morphogenetic responses to local-acting growth factors. The proliferation of OUTER cells was 2 to 3 times greater than INNER cells (P < 0.0001) in response to IGF-I stimulation. Dramatic differences in the morphology of INNER and OUTER organoids were also observed. INNER cells grew into smooth-edged colonies when treated with heifer serum but stellate colonies when treated with other mitogens. OUTER cells grew into stellate colonies regardless of mitogen treatment. These investigations highlight the fact that a great deal more research into the basic physiology of prepuberal ruminant mammogenesis is required and that dogma developed in murine model systems may not be applicable to ruminant mammary physiology. / Ph. D.