Fig. 3. X-Gal stained mammary whole mounts (A,D-F) and their histological sections (B,C,G-I) of WAP-Cre/Rosa-lacZ double transgenic females at day 8 of the second gestation period (A-C) and in transplants of parous (D,E,G,H,I) and nulliparous (F) WAP-Cre/Rosa-lacZ epithelia into nulliparous wild-type recipients. Note that the parity-induced epithelial cells in involuted WAP-Cre/Rosa-lacZ animals serve as alveolar precursors in subsequent gestation cycles (A,B) but they remain essentially absent from larger ducts (C). X-Gal positive cells from involuted double transgenic mice contribute to ductal morphogenesis in transplants (D,E), and they are present in large ducts (G), small ducts (H) and terminal end buds (I). Control transplants from nulliparous double transgenic donors into wild-type recipients remain X-Gal negative (F) suggesting that the WAP-Cre transgene is not activated by the transplantation technique itself. LN, lymph node.