Mammary gland (breast)

Glandula mammaria (mamma)

Also known as: breast, lactiferous ducts, nipple, areola, Cooper ligaments, axillary tail

A modified apocrine sweat gland in the subcutaneous tissue of the anterior chest wall, organised into 15–20 lobes drained by lactiferous ducts that open on the nipple; it produces milk and drains lymph chiefly to the axillary nodes.

Show on body map Hypothalamic–Pituitary Axis

Overview

Lobes15–20

ExtentRibs 2–6; sternum to midaxillary line

Lymph to axilla>75 %

Nipple dermatomeT4

In the adult female, the breast extends vertically from the 2nd to the 6th rib and transversely from the lateral border of the sternum to the midaxillary line; an axillary tail (of Spence) passes through the deep fascia into the axilla. About two-thirds of it rests on the deep fascia of pectoralis major and one-third on serratus anterior, separated by a loose retromammary space that lets the breast move on the chest wall (fixation suggests deep invasion by cancer).

The gland consists of 15–20 lobes arranged radially, each drained by a lactiferous duct that dilates beneath the areola into a lactiferous sinus and opens on the nipple. Lobes are divided into lobules whose functional units are the terminal duct lobular units (TDLUs), where most breast cancers arise. Fibrous suspensory ligaments (of Cooper) run from the dermis through the gland to the deep fascia and support it. The areola contains sebaceous areolar (Montgomery) glands that enlarge in pregnancy. The nipple lies over about the 4th intercostal space in men and young women (T4 dermatome).

The breast changes throughout life: ducts grow at puberty under estrogen, lobules under progesterone; alveoli proliferate in pregnancy under estrogen, progesterone, prolactin and hPL; milk is secreted after delivery; and glandular tissue involutes after weaning and at menopause, being replaced by fat.

Location: Superficial fascia of the anterior thoracic wall, from rib 2 to rib 6 and from the sternal edge to the midaxillary line, with an axillary tail.

Parts & subdivisions

Lobes and lobules
15–20 radially arranged lobes; lobules contain the terminal duct lobular units.
Lactiferous ducts and sinuses
One duct per lobe, dilating beneath the areola and opening on the nipple.
Nipple and areola
Smooth muscle (erection of nipple), sensory nerve endings, areolar glands.
Suspensory ligaments (Cooper)
Fibrous septa anchoring the gland to skin and deep fascia; tethering by cancer causes dimpling.
Axillary tail (of Spence)
Extension into the axilla; a site of tumours.

Anatomy

Innervation: Anterior and lateral cutaneous branches of the 4th–6th intercostal nerves (sensory, and sympathetic fibres to vessels and nipple smooth muscle).
Arterial supply: Internal thoracic artery (medial mammary branches of its perforating branches, especially 2nd–4th spaces — the largest supply); axillary artery branches (lateral thoracic, thoracoacromial pectoral branches); lateral branches of the posterior intercostal arteries (2nd–4th).
Venous drainage: Mainly to the axillary vein, also internal thoracic and intercostal veins; intercostal veins connect with the vertebral venous plexus (route for spinal metastases).
Lymphatic drainage: >75 % to the axillary nodes (pectoral/anterior group first, then central and apical; surgical levels I–III relative to pectoralis minor), mainly from the lateral quadrants; medial quadrants drain largely to the parasternal (internal thoracic) nodes; some lymph crosses to the opposite breast or passes inferiorly to abdominal nodes. A subareolar plexus collects lymph from the nipple and areola.
Histology: Ducts and alveoli lined by two cell layers — inner luminal epithelial cells and outer myoepithelial cells (contract with oxytocin) — on a basement membrane; intralobular loose connective tissue and interlobular dense connective tissue and fat.
Development: Ectodermal mammary ridges (milk lines) run from the axilla to the groin in week 4–5; normally only the pectoral segment persists. Accessory nipples (polythelia) or breast tissue may occur along the milk line.

Function

Synthesises and secretes milk (lactation).
Transfers maternal antibodies (secretory IgA) to the infant.
Secondary sexual characteristic and sensory organ.

Clinical significance

Breast cancer

Commonest female cancer; most arise in the upper outer quadrant from TDLUs. Signs: hard irregular lump, skin dimpling (Cooper ligaments), nipple retraction, peau d'orange (lymphatic obstruction), axillary nodes. Triple assessment; sentinel lymph node biopsy for axillary staging.

Benign breast disease

Fibroadenoma (young women; smooth, mobile "breast mouse"), fibrocystic change (cyclical lumpiness and pain), cysts, intraductal papilloma (bloody nipple discharge), duct ectasia, fat necrosis.

Lactational mastitis and abscess

Usually Staphylococcus aureus entering through a cracked nipple; painful, red, wedge-shaped area with fever. Continue breastfeeding or expressing; antibiotics; drain an abscess.

Axillary surgery

Axillary dissection risks lymphoedema of the arm, injury to the long thoracic nerve (winged scapula), the thoracodorsal nerve and the intercostobrachial nerve (numbness of the medial upper arm).

Gynaecomastia

Benign enlargement of male breast glandular tissue from an increased estrogen:androgen ratio (puberty, ageing, cirrhosis, drugs such as spironolactone, Klinefelter syndrome, testicular tumours).

Test yourself

The majority of lymph from the breast drains to which nodes?

Parasternal (internal thoracic) nodes
Axillary nodes
Supraclavicular nodes
Contralateral breast

Answer: Axillary nodes

More than 75 % of breast lymph — especially from the lateral quadrants — drains to the axillary nodes (pectoral group first). Medial quadrants drain partly to parasternal nodes.