The breast undergoes dramatic changes in size, shape, and function in association with puberty, pregnancy, and lactation. These changes are critical to successful breastfeeding.
The physiology of lactation is reviewed here. The stages of breast development and breast anatomy are discussed elsewhere. (See "Breast development and morphology".)
The breast is composed of a stroma consisting of fat and connective tissue that supports a tubuloalveolar parenchyma (figure 1) . The normal breast tissue of adult women contains three types of lobules, known as types 1, 2, and 3. Formation of type 1 lobules (Lob 1) begins with puberty (picture 1). The changing levels of estrogen and progesterone during menstrual cycles stimulate Lob 1 to sprout new alveolar buds and evolve to more mature structures called type 2 and type 3 lobules (Lob 2 and 3) (picture 2 and picture 3 and picture 4). After puberty is completed, further maturation of the breast does not occur except during pregnancy. (See "Breast development and morphology".)
Pregnancy — The alveolar development and maturation of the epithelium that occurs in response to the hormonal changes of pregnancy is known as stage II mammogenesis . Progesterone plays an important role in stimulating alveolar development during this phase [2,3].
The maximum branching capability of the breast is expressed during this period. The secretory acinus that is formed represents a terminal outgrowth that marks the full extent of glandular differentiation.