The mammary gland is a dynamic tissue that undergoes epithelial expansion a
nd invasion during puberty and cycles of branching and lobular morphogenesi
s, secretory differentiation, and regression during pregnancy, lactation, a
nd involution. The alteration in the mammary gland epithelium during its po
stnatal differentiation is accompanied by changes in the multiple stromal c
ell types present in this complex tissue. The postnatal plasticity of the e
pithelium, endothelium, and stromal cells of the mammary gland may contribu
te to its susceptibility to carcinogenesis. The purpose of this review is t
o assist researchers in recognizing histological changes in the epithelium
and stroma of the rat mammary gland throughout development.