Autoradiographic analysis of lymphocyte migration into the mammary epithelium and milk of lactating female rats

Adoptively transferred radiolabeled lymphoid cells migrating to the lactating mammary gland were shown to enter the alveolar epithelium and ultimately the milk. Lactating female rats were injected intravenously with [³H]uridine-labeled syngeneic mesenteric lymph node cells, and the distribution of the cells 24 and 48 h later within intestinal and mammary tissues and their presence in milk were assessed autoradiographically. At both time periods, numerous labeled cells were found in the Peyer's patches and mesenteric lymph nodes. The mammary tissue and intestinal mucosa (lamina propria and epithelium) contained fewer labeled cells per unit area than those sites. Labeled cells within the mammary tissue were distributed equally between the alveolar epithelium and the intralobular connective tissue, where they were seen in both blood vessels and in the loose connective tissue. Occasional labeled cells were observed among the mononuclear cells seen in the alveolar lumina, and an average of 4.8% of the cells harvested from milk samples were labeled. Labeled cells within the secretory epithelium and milk always had the morphologic characteristics of mononuclear leukocytes. Thus, at least a portion of the lymphoid cells which have been shown to migrate to the mammary tissue in increased numbers during lactation actually enter and traverse the epithelium and contribute to the lymphoid component of mammary secretions.