Developmental response of adult mammary epithelial cells to various fetal and neonatal mesenchymes.

Adult mouse mammary epithelial cells were isolated and grown in combination with mesenchyme from the following sources: embryonic preputial gland, foot skin, tail skin, genital tubercle skin, mammary gland, and neonatal uterus, vagina and urinary bladder. Following 1 month of in vivo cultivation of the tissue recombinants as grafts underneath the renal capsule of normal female or hyperprolactinaemic (pituitary-grafted) hosts, the specimens were analysed histologically and immunocytochemically for the expression of milk proteins, smooth muscle actin, and cytokeratins. Mesenchymal effects on adult mammary epithelium varied with the source of the mesenchyme and the hormonal status of the host. In normal female hosts preputial gland mesenchyme induced extensive mammary epithelial growth and ductal branching morphogenesis with epithelial differentiation and ductal pattern being comparable to that observed in homotypic recombinants composed of mammary gland mesenchyme plus adult mammary epithelium. Other mesenchymes (from foot, tail, genital tubercle and uterus) preserved ductal morphology and normal epithelial differentiation, but elicited minimal epithelial growth and branching morphogenesis in adult mammary epithelium. In association with urogenital sinus, vaginal or bladder mesenchymes ductal branching morphogenesis of the mammary epithelium was absent or greatly distorted and the epithelium exhibited a stratified cuboidal phenotype even though considerable epithelial growth had occurred. In hyperprolactinaemic hosts (which received a pituitary graft) epithelial growth, alveolar morphogenesis, and synthesis of casein and milk fat globule protein was stimulated in all tissue recombinants although to different extents. Alveolar morphogenesis and milk protein expression were extensive in tissue recombinants prepared with mesenchyme from embryonic mammary gland, preputial gland, tail skin and urinary bladder, but were minimal in tissue recombinants prepared with foot or genital tubercle skin. Few milk-protein-positive alveoli formed in tissue recombinants composed of mammary epithelium combined with urogenital sinus or vaginal mesenchyme, even following growth in pituitary-grafted hosts. These findings demonstrate: (1) that adult mammary epithelial cells are responsive to the growth-promoting influences of heterotypic embryonic and neonatal mesenchymes; (2) that mammary growth and branching morphogenesis are induced to variable extents by different mesenchymes; (3) that fibrous (non-adipose) mesenchymes are effective inducers of mammary epithelial development; and (4) that the ability to form alveoli and produce milk proteins in adult mammary epithelial cells is critically dependent upon the nature of the connective tissue environment.