Oxytocin enhances myoepithelial cell differentiation and proliferation in the mouse mammary gland.

Using immunocytochemistry and electron microscopy, we demonstrate that oxytocin (OT) exerts a trophic effect on its target myoepithelial cells in the mammary gland. In vitro, in organotypic cultures of mouse mammary gland, we examined proliferation and differentiation of the different cell types induced by OT added to the medium. In vivo, we studied the effect of OT on the structure and cell composition of developing glands. Uptake of 5-bromo-2'-deoxyuridine was used as proliferation marker, while antibodies to smooth muscle alpha-actin (specific for myoepithelial cells) and keratin (MoAb AE1; selective for epithelial cells) were used to identify differentiated cell types. By electron microscopy, we studied structural modifications induced by OT on the extreme projections of the developing gland (sc end buds). The results indicate that OT induces myoepithelial cell differentiation and proliferation, enhancing the effect of mammotrophic hormones in nonlactating mouse mammary gland. A less marked effect was observed in luminal epithelial cells. No significant effect of OT alone was detected in cultured glands from unprimed animals.