Mammary gland

The mammary gland is a modified apocrine sweat gland that is the defining characteristic of mammals. It is a complex, bilateral organ located in the anterior chest wall, overlying the pectoralis major muscle, and is responsible for the production and secretion of milk (lactation) to nourish offspring.

Anatomy and Structure

The mammary gland is composed of glandular (parenchymal) tissue and stromal (supportive) tissue:

• Glandular tissue consists of 15–20 lobes arranged radially around the nipple. Each lobe is subdivided into lobules, which contain the milk-producing units called alveoli (acini). The alveoli drain into a system of lactiferous ducts that converge at the nipple.

• Stromal tissue includes fibrous connective tissue (Cooper's ligaments), which provides structural support, and adipose tissue, which largely determines breast size and shape.

Key Features

• Blood supply: Primarily from the internal thoracic (mammary) artery, lateral thoracic artery, and intercostal arteries.

• Lymphatic drainage: Predominantly to the axillary lymph nodes (~75%), with additional drainage to the internal mammary (parasternal) nodes.

• Innervation: Intercostal nerves (T4–T6), with the lateral cutaneous branch of T4 supplying the nipple-areolar complex.

Development and Function

The mammary gland undergoes significant changes throughout life, regulated by hormones including estrogen, progesterone, prolactin, and oxytocin. It remains rudimentary until puberty, when estrogen drives ductal elongation and progesterone promotes lobular development. Full functional maturation occurs during pregnancy and lactation, when prolactin stimulates milk production and oxytocin triggers milk ejection (the "let-down" reflex). After menopause, the glandular tissue involutes and is largely replaced by adipose tissue.