Lobules of mammary gland

The lobules of the mammary gland are the fundamental secretory units of the breast. Each lobule consists of a cluster of small, rounded alveoli (also called acini) lined by secretory epithelial cells surrounded by myoepithelial cells and a basement membrane. These alveoli are grouped together and drain into a common terminal ductule (intralobular terminal duct), which then connects to the larger ductal system of the breast.

Structural Organization

The mammary gland is organized into 15–20 lobes, each of which is subdivided into multiple lobules. The lobules are embedded in loose, hormone-responsive intralobular stroma, which is distinct from the denser interlobular connective tissue. Together, the terminal duct and its associated lobule form the terminal duct lobular unit (TDLU), which is considered the functional unit of the breast and the site of origin for most breast pathologies, including the majority of breast carcinomas.

Functional Significance

• During the resting (non-lactating) state, lobules are small with few acini and a predominance of stromal tissue.

• During pregnancy and lactation, lobules undergo marked proliferation and differentiation under the influence of estrogen, progesterone, prolactin, and other hormones, resulting in enlarged, milk-producing acini.

• After menopause, lobules undergo involution and are progressively replaced by adipose tissue.

Clinical Relevance

The lobule is the origin of lobular neoplasia (atypical lobular hyperplasia and lobular carcinoma in situ), while the TDLU as a whole gives rise to the majority of both ductal and lobular breast cancers.