Figure 1.

The prenatal stages of mammary gland tissue development. The mammary gland forms as early as day 10 of gestation in mice by the formation of the milk line. This is followed by at least four stages of development. At the time of birth the breast tissue is composed of three distinct layers including the mammary epithelium, mammary mesenchyme. (Which forms the structure of the secondary sprouts), and the dermis. Above are the expression patterns of active signaling and necessary for prenatal mammary gland formation. Illustrated in the top are the hedgehog components associated with those stages of development in mouse embryonic study.