The relationship between mammary gland development and adipocyte formation in rat subcutaneous tissue was studied. Tissue samples from newborn rats and young rats were prepared for histological and histochemical analysis. The development of subcutaneous adipocytes in the young rat is associated temporally and spatially with mammary gland development. The growing mammary gland ducts and the blood vessels and nerves that serve the ducts may physically interact with connective tissue cells, resulting in a population of undifferentiated cells. Adipocytes and a wide variety of cell types may be derived from this pool of undifferentiated cells. Histochemical characteristics of cells of immature ends of mammary gland ducts indicated hypoxic conditions. These conditions may have directed a vascularization process that resulted in a network of adipocytes and capillaries around mammary gland ducts. Thus, under normal conditions, adipocyte formation is an integral phase of mammary gland development in the fetal and yount rat.
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