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AN9.1-3 | Pectoral region — Part 2

The Breast — Location, Extent, and Surface Anatomy

The breast (mamma) is a modified sweat gland. In both sexes it lies on the anterior chest wall, overlying primarily pectoralis major, with the upper outer part overlying serratus anterior.

Location and extent:
• Vertically: from the 2nd rib (above) to the 6th rib (below)
• Horizontally: from the sternal edge (medially) to the mid-axillary line (laterally)
• The nipple is typically at the level of the 4th intercostal space in the upright adult

Axillary tail of Spence:
The breast has a projection of glandular tissue that extends upward and laterally through an opening in the deep fascia into the axilla — the axillary tail of Spence. It may be mistaken for an axillary lymph node, and cancer arising in it can be misdiagnosed.

Retromammary space:
Between the deep surface of the breast and the deep fascia overlying pectoralis major lies the retromammary space — loose areolar tissue allowing the breast to move freely. Submammary implants are placed here.

Areola and Montgomery glands:
The areola is the pigmented area around the nipple. Montgomery glands (modified sebaceous glands) appear as small bumps on the areola — they secrete oil to protect the skin during breastfeeding.

Structure of the Breast — Gland, Stroma, and Cooper's Ligaments

The breast has two main components:
1. Glandular tissue (parenchyma) — the milk-producing apparatus
2. Fibrous and fatty stroma — makes up most of the breast volume

Glandular tissue:
The gland consists of 15–20 lobes, arranged radially like segments of an orange. Each lobe contains smaller lobules with clusters of milk-secreting cells called alveoli.

Each lobe drains via a lactiferous duct converging toward the nipple. Just before the nipple, each duct widens to form a lactiferous sinus (ampulla) — a temporary reservoir for milk.

Cooper's ligaments and clinical importance:
Dense fibrous strands called suspensory ligaments of the breast (Cooper's ligaments) run between the skin and the deep fascia, subdividing the breast and giving it its shape.

In breast cancer, tumour invasion of Cooper's ligaments causes them to shorten — pulling the overlying skin inward to produce classic skin dimpling. This is a key clinical sign of malignancy.

Microanatomy:
• Secretory alveoli lined by columnar epithelium surrounded by myoepithelial cells (contract to expel milk — the milk ejection reflex)
• Most breast cancers arise from ductal epithelium — hence ductal carcinoma is the most common type

Blood Supply and Lymphatic Drainage of the Breast

Blood supply:
The breast receives blood from multiple sources:
• Lateral thoracic artery (from axillary artery) — upper outer quadrant; most important
• Internal thoracic artery — perforating branches through intercostal spaces 2–4, medial breast
• Intercostal arteries — posterior perforators, lateral breast
• Thoracoacromial artery — pectoral branches, upper breast

Lymphatic drainage — the most clinically critical aspect:
Approximately 75% of lymph from the breast drains to the axillary lymph nodes (particularly the anterior/pectoral group). The remainder:
• 25% medial drainage — internal mammary (parasternal) nodes
• Upper breast — infraclavicular nodes, then subclavian nodes

Axillary nodes relative to pectoralis minor:
• Level I (lateral) — anterior/pectoral, lateral, subscapular groups
• Level II (behind) — central nodes
• Level III (medial/apical) — apical nodes then subclavian trunk then thoracic duct

Sentinel lymph node biopsy:
The sentinel node is the first lymph node to receive drainage from a tumour. A blue dye or radiotracer is injected into the breast tumour; the first node that lights up is excised and examined. If negative, no further axillary dissection is needed — sparing the patient from arm lymphoedema.

Troisier sign:
An enlarged hard left supraclavicular node can indicate advanced breast cancer spreading retrogradely into the thoracic duct, or gastric cancer (Virchow's node).