Fig. 4.

During pregnancy, mammary alveolar epithelial cells express the gene reporter driven by the adipocyte-specific aP2 promoter. Mammary gland of aP2-Cre/R26R mouse, β-gal histochemistry. (a) In virgin mice only adipocytes stain for X-Gal. Note the absence of staining in the epithelial portion of the gland (asterisk). Ad cyt, adipocyte cytoplasm. (b) On day 18 of pregnancy, adipocytes (Ad) as well as many alveolar epithelial cells (Gl) are positive for X-Gal. d, Main duct. (Insets) Enlargements of the corresponding boxed areas. [Scale bar: 10 (a), 8 (Insets), and 30 (b) μm.]