In the present study we provide evidence that the cytodifferentiation of primary mouse mammary epithelial cells within the alveolar-like structures formed after culture on a reconstituted basement membrane resembles development in vivo during late pregnancy and early lactation. During the first two days in culture on a basement membrane gel in the presence of lactogenic hormones, epithelial cells isolated from mid-pregnant mice are disorganized and central lumina are largely absent. Levels of mRNA for the milk proteins, beta-casein and transferrin, are dramatically reduced. By the second or third day in culture, cytoplasmic polarization becomes evident and prominent apical junctional complexes are formed. Synthesis of both mRNA and milk protein is reinitiated at this time. By day 4, well-defined lumina appear, and abundant synthesis and secretion of casein and lipid is observed. A striking feature of this differentiation in culture is the specific localization of milk protein gene expression (beta-casein mRNA) to luminal epithelial cells in the alveolar-like structures. At the ultrastructural level, increased milk protein synthesis and secretion are paralleled by a fourfold increase in rough ER that resembles the dramatic increase in the ER observed in vivo following parturition. One indication of tissue-specific differentiation observed in later cultures (days 4–11) is the synthesis and secretion of abundant casein micelles. A second characteristic of lactating mammary epithelial cells in vivo that has not previously been observed in culture is the secretion of milk fat globules. Taken together, these observations indicate that mammary epithelial cells plated onto a reconstituted basement membrane differentiate to the lactating phenotype in culture.

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