The Mammary Gland - Detailed Description

a. Extent

Fig. 1 - The axillary process of the breast passing around the pectoralis major into the axilla (Gray's Anatomy for Students)

b. Relations

Anterior

• Skin with overlying superficial fascia
• Nipple and areola at the 4th intercostal space level

Posterior (Deep surface)

• Retromammary space (loose connective tissue) - allows the breast to slide on underlying structures
• Deep (pectoral) fascia covering pectoralis major (medially), serratus anterior (laterally), and external oblique/rectus abdominis (inferiorly)

Superior

• Clavicle
• Branches of the supraclavicular nerves

Inferior

• Upper anterior abdominal wall

Medial

• Sternum

Lateral

• Axilla (with the axillary tail projecting into it)

Muscles related to the breast:

c. Structure

Fig. 2 - Morphology of the mature breast showing lobes, lobuli, alveoli, connective tissue septa, and Montgomery glands (Creasy & Resnik's Maternal-Fetal Medicine)

Parenchyma (Glandular tissue)

• 15-20 lobes arranged radially around the nipple, separated by connective tissue septa (Cooper's ligaments)
• Each lobe contains several lobules
• Each lobule contains 10-100 alveoli (secretory units)
• Each lobe drains via a lactiferous duct that opens independently at the nipple tip
• Just before opening at the nipple, each lactiferous duct dilates into a lactiferous sinus (ampulla), which acts as a temporary reservoir for milk

Stroma

• Dense irregular connective tissue (interlobular) and loose connective tissue (intralobular)
• Cooper's (Suspensory) Ligaments: Fibrous bands that extend from the dermis of the skin to the deep fascia, providing structural support. Distortion or shortening of these by carcinoma produces characteristic skin dimpling
• Adipose tissue: The predominant component in non-lactating women; fat surrounds the glandular elements and gives the breast its smooth, rounded contour

Nipple and Areola

• Nipple: Conical elevation at the center of the areola; contains smooth muscle fibers (erectile), richly innervated with sensory and pain fibers; contains 4-18 lactiferous duct openings (5-8 main ducts)
• Areola: Circular pigmented zone (avg. 15-16 mm diameter, enlarges in pregnancy); contains Montgomery glands (modified sebaceous/sweat glands) that hypertrophy during pregnancy and lactation to lubricate and protect the nipple
• Verrucous surface of nipple; has sebaceous and apocrine sweat glands but no hair

Histological Stages

In males

• Rudimentary breast consisting of small ducts (often cords of cells) that do not extend beyond the areola

d. Blood Supply

Fig. 3 - Full breast anatomy showing arterial supply (lateral thoracic, internal thoracic), lymphatic drainage to axillary and parasternal nodes, suspensory ligaments, and retromammary space (Gray's Anatomy for Students)

Arterial Supply

Venous Drainage

1. Internal thoracic (mammary) vein - anteromedially
2. Axillary vein - laterally and superiorly
3. Intercostal veins - posteriorly and inferiorly

Note: The intercostal veins connect with the vertebral venous plexus (Batson's plexus), providing a route for haematogenous metastasis of breast cancer to the vertebral column, skull, and pelvis - without going through the pulmonary circulation.

Innervation

• Anterior and lateral cutaneous branches of the 2nd-6th intercostal nerves
• Nipple: specifically innervated by the 4th intercostal nerve
• Efferent nerves are sympathetic adrenergic; no parasympathetic or cholinergic supply
• Sensory stimulation of nipple/areola triggers release of prolactin (adenohypophysis) and oxytocin (neurohypophysis)

e. Lymphatic Drainage

Fig. 4 - Right breast showing axillary lymph nodes, parasternal nodes, internal thoracic artery perforating branches, lateral thoracic artery, and lactiferous sinuses (Gray's Anatomy for Students)

Primary Drainage Routes

1. Axillary Route (~75% of total drainage)

• Drains the lateral, superior, and central parts of the breast
• Drains into the axillary lymph nodes, which are divided into 5 groups:

2. Parasternal (Internal Mammary) Route (~20-25%)

• Drains the medial and central parts of the breast
• Lymphatics accompany perforating branches of the internal thoracic artery
• Drain into parasternal (internal mammary) nodes along the internal thoracic vessels on the deep surface of the anterior thoracic wall
• Parasternal nodes drain into the bronchomediastinal trunks

3. Intercostal Route (minor)

• Some drainage from the posterior and inferior breast
• Via lymphatics that follow lateral branches of posterior intercostal arteries
• Drain into intercostal nodes near the heads and necks of ribs
• Intercostal nodes drain into the thoracic duct or bronchomediastinal trunks

4. Inferior (Subdiaphragmatic) Route

• Small amount of drainage from the inferior breast
• May communicate with subdiaphragmatic lymphatics and hepatic lymphatics (via the falciform ligament)

Summary Diagram (Lymphatic Routes)

                  BREAST LYMPHATIC DRAINAGE
                         |
         ┌───────────────┼───────────────┐
         ▼               ▼               ▼
   AXILLARY (~75%)  PARASTERNAL    INTERCOSTAL
   (lateral/sup.)   (~20-25%)      (minor, post.)
         |          (medial)            |
   Pectoral nodes    Parasternal     Intercostal
   → Subscapular     nodes           nodes
   → Humeral              |               |
   → Central    Bronchomediastinal  Thoracic duct /
   → Apical     trunk              Bronchomediastinal
         |                          trunk
   Subclavian trunk
         |
   Venous junction
   (subclavian + IJ vein)

f. Applied Anatomy

1. Breast Carcinoma

• Acini (terminal duct lobular units - TDLU) - lobular carcinoma
• Lactiferous ducts - ductal carcinoma (most common)

• Lymphatic: To axillary nodes (most common), parasternal nodes, supraclavicular nodes, and contralateral breast
• Haematogenous: Via Batson's vertebral venous plexus (through intercostal veins) → vertebral column, skull, pelvis; or via axillary/internal thoracic veins → lungs, liver, bone
• Direct invasion: Into pectoral muscles, chest wall, skin

• Size of primary tumor
• Exact site
• Number and sites of lymph node spread
• Distant metastasis (lungs, liver, bone - detected by CT, bone scan, PET-CT)

2. Skin Changes in Carcinoma

3. Lymphoedema After Mastectomy

• Axillary lymph node clearance (ALND) or radiotherapy to the axilla disrupts lymphatic drainage of the upper limb
• Results in chronic lymphoedema of the arm (brawny, non-pitting oedema)
• Clinicians now favor sentinel lymph node biopsy (SLNB) to reduce this complication

4. Sentinel Lymph Node Biopsy (SLNB)

• The sentinel node is the first lymph node to receive lymphatic drainage from the primary tumor
• Identified by injecting blue dye or radioactive tracer around the tumor
• If sentinel node is clear of metastasis, full axillary clearance is avoided
• Reduces risk of lymphoedema significantly

5. Axillary Tail (Spence's Tail)

• Clinicians must examine the axillary tail when assessing breast lumps; pathology (carcinoma, fibroadenoma) can occur in this extension
• It may be mistaken for an axillary lymph node

6. Mammography

• The primary imaging modality for breast cancer screening
• Uses low-dose X-rays; the breast is compressed between plates
• Can detect microcalcifications (early sign of ductal carcinoma in situ - DCIS) and soft tissue masses

7. Paget's Disease of the Nipple

• Eczema-like change of the nipple/areola due to underlying intraductal carcinoma
• Paget's cells (large clear cells) invade the nipple epidermis
• Associated with subjacent ductal carcinoma in situ (DCIS) or invasive ductal carcinoma

8. Gynaecomastia

• Benign proliferation of male breast glandular tissue
• Can occur physiologically (neonatal, pubertal, elderly) or due to hormone imbalance, drugs (spironolactone, digitalis, cimetidine, exogenous estrogens), liver cirrhosis, or testicular tumors (hCG-secreting)

9. Supernumerary Breasts / Nipples (Polymastia / Polythelia)

• Due to failure of involution of the milk line (mammary ridge - a bilateral ectodermal thickening extending from axilla to groin during fetal development)
• Accessory nipples most commonly appear along the milk line, particularly below and medial to the normal breast

10. Abscess of the Breast

• Most common during lactation (puerperal mastitis)
• Superficial abscess: In the subcutaneous tissue; drains anteriorly
• Deep (retromammary) abscess: In the retromammary space; presents with deep fluctuance; may point inferiorly