Mammary Gland: Anatomy, Lymphatics, and Clinical Significance

Overview

The mammary gland is a specialized modified apocrine sweat gland derived from ectoderm. In the adult female, it is the dominant structure of the breast, designed to produce and deliver milk. In males, it remains rudimentary. The breast overlies the pectoralis major, serratus anterior, and external oblique muscles, with its base extending from the 2nd to 6th rib and from the lateral sternal border to the midaxillary line.

Gross Structure

Lobes and Lobules

• Each breast contains 15–20 lobes arranged in a radial pattern around the nipple
• Each lobe is subdivided into lobules containing acini (secretory alveoli)
• Each lobe drains via its own lactiferous duct to the nipple
• Lactiferous sinuses: small ampullae where ducts widen just beneath the nipple; sites of milk storage during lactation

Nipple and Areola

• Nipple: projects from the areola at the level of the 4th intercostal space
• Areola: circular pigmented area containing sebaceous Montgomery's glands (which produce a lubricant during lactation)
• Inverted nipple: may indicate congenital variant or, if newly inverted, underlying carcinoma

Stroma and Support

• Cooper's (suspensory) ligaments: fibrous strands connecting the lobules to the overlying skin and the underlying deep fascia; loss of these ligaments causes drooping (ptosis)
• Skin dimpling or peau d'orange (orange-peel appearance): produced by tumor invasion and shortening of Cooper's ligaments or dermal lymphatic blockade respectively

Retromammary Space

• Loose areolar tissue between breast and deep (pectoral) fascia
• Allows breast mobility
• Preserved in subglandular implant placement

Blood Supply

• Perforating branches of internal thoracic (internal mammary) artery: supply medial breast (most significant)
• Lateral thoracic artery: supplies lateral breast
• Thoracoacromial artery (pectoral branch): supply superior breast
• Posterior intercostal arteries (4th–6th): supply posterior breast

Venous Drainage

Follows arterial supply: internal thoracic, axillary, and posterior intercostal veins.

Lymphatic Drainage (Critically Important)

The lymphatic drainage of the breast determines the routes of breast cancer metastasis:

Axillary Nodes (~75% of drainage)

• Pectoral (anterior) nodes: receive most of the breast lymphatics; primary drainage
• Then → Central nodes → Apical nodes → Subclavian lymph trunk

Internal Mammary (Parasternal) Nodes (~25% of drainage)

• Along internal thoracic vessels, levels 1–5
• Drain medial and inferior breast
• Important for staging and radiation planning

Other Pathways

• Interpectoral (Rotter's) nodes: between pectoralis major and minor
• Posterior drainage: to posterior intercostal nodes

Breast Quadrants

Clinically divided into four quadrants + central subareolar region:

• Upper outer quadrant (UOQ): contains most glandular tissue (axillary tail of Spence extends into axilla); site of ~50% of breast cancers
• Upper inner
• Lower outer
• Lower inner

Hormonal Influence

• Estrogen: ductal development
• Progesterone: lobular and acinar development
• Prolactin: milk production (lactogenesis)
• Oxytocin: milk ejection (let-down reflex)

Changes through life: thelarche (puberty), cyclic changes, pregnancy-related hypertrophy, involution (menopause)

Clinical Relevance

• Breast cancer: most common cancer in women; upper outer quadrant most common location; spreads via lymphatics to axillary nodes (Level I, II, III); sentinel node biopsy maps the first draining node
• Mastitis/abscess: lactational mastitis in 2–10% of nursing mothers; Staphylococcus aureus most common; abscesses may require drainage
• Fibrocystic change: most common benign breast condition; cyclic pain and nodularity
• Fibroadenoma: most common benign tumor in young women; mobile, firm, well-defined
• Skin dimpling: pulling of Cooper's ligaments by carcinoma
• Paget's disease of nipple: eczematous nipple change indicating underlying ductal carcinoma

Study Points

• Describe the structure of a mammary lobe from alveolus to lactiferous sinus to nipple.
• Explain the role of Cooper's ligaments and what happens clinically when they are involved by tumor.
• Trace the lymphatic drainage of the breast and explain the concept of the sentinel node.
• Name the main arterial supply to the breast and explain the clinical relevance of internal mammary supply.
• Describe the hormonal control of breast development and lactation.