Development of the human breast.

The human breast undergoes a complete series of changes from intrauterine life to senescence. These changes can be divided into two distinct phases; the developmental phase and the differentiation phase. The developmental phase includes the early stages of gland morphogenesis, from nipple epithelium to lobule formation. In lobule formation, both processes, development and differentiation, take place almost simultaneously. For example, the progressive transition of lobule type 1 to types 2, 3, and 4 requires active cell proliferation, to acquire the cell mass necessary for the function of milk secretion. This later process implies differentiation of the mammary epithelium. Therefore, the presence of lobule type 4 is the maximal expression of development and differentiation in the adult gland, whereas the presence of lobule type 3 could indicate that the gland has already been developed. It is important to point out that the presence of proteins that are indicative of milk secretion, such as alpha-lactalbumin, casein, or milk fat lobule type membrane protein, also indicates cellular differentiation of breast epithelium. However, only when all the other components of milk, (such as lactose, alpha-lactalbumin, casein and milk fat) are coordinately synthesized within the appropriate structure can full differentiation of the mammary gland be acknowledged.