Fig. 1. Experimental design. The basic principle of the genetic labeling of differentiating cells in the developing mammary gland using the Cre-lox technique. In this experimental setting, the WAP-Cre transgene is used solely to monitor the differentiation process of alveolar precursor cells in response to lactogenic hormones. The transient upregulation of Cre recombinase in differentiating epithelial cells during pregnancy permanently activates a ubiquitously expressed reporter transgene (Rosa-lacZ), whose expression is not dependent on the differentiation status of a given cell. The reporter gene remains active in cells that no longer require high systemic hormone levels to maintain a functionally differentiated state (i.e. WAP gene expression). Hence, the permanent activation of the reporter gene (blue X-Gal staining) genetically labels differentiating cells that bypass apoptosis and remodeling at the conclusion of the reproductive cycle. The labeled cells in the remodeled (involuted) gland represent a new epithelial subtype, which is not present in nulliparous animals.
