Gross Anatomy and Development of the Breast

Functional & Structural Identity of the Female Breast

  • Functional classification

    • • Modified sweat gland ➜ evolutionarily derived from apocrine-type cutaneous glands.

    • • Primary function: production, storage & ejection of milk (lactation).

  • Structural classification

    • • Compound tubulo-alveolar gland (see comparative diagram below).

    • • Contains multiple branched ducts ending in secretory alveoli.

  • Histological composition

    • • Glandular (parenchymal) tissue → active milk secretion.

    • • Fibro-connective stroma → structural support.

    • • Adipose tissue → accounts for the majority of gross size & contour variation among individuals.

Taxonomy of Exocrine Glands (Context)

  • Simple glands

    • • Unicellular (e.g., goblet cell).

    • • Simple tubular/alveolar (straight, branched, or coiled).

  • Compound glands

    • • Compound tubular.

    • • Compound alveolar.

    • • Compound tubulo-alveolar ← Breast belongs here (multiple branched ducts + secretory alveoli).

Topographical Anatomy: Position & Extent

  • Lies entirely within the superficial fascia on the anterior thoracic wall, separated from deep fascia by the retromammary space.

  • Vertical span: from the 2^{nd} to 6^{th} ribs.

  • Horizontal span: from the lateral border of the sternum to the mid-axillary line.

  • Posterolateral projection: Axillary tail of Spence

    • • Passes along the inferolateral edge of pectoralis major toward the axilla (clinically palpable during breast exams).

  • Underlying muscles (deep relations)

    • • Mainly pectoralis major.

    • • Minor contributions from serratus anterior & external oblique (inferolaterally).

External (Superficial) Features

  • Areola

    • • Circular pigmented zone surrounding nipple.

    • • Contains enlarged sebaceous glands called areolar or Montgomery’s glands.

    • – Function: secrete lipid-rich sebum to lubricate & protect nipple/areolar skin, especially during lactation.

    • • Lacks hair follicles & subcutaneous fat → facilitates stretching during feeding.

  • Nipple (papilla mammae)

    • • Cylindrical projection in centre of areola.

    • • Contains no fat, hair or typical sweat glands.

    • • Rich circular & longitudinal smooth-muscle fibres → contraction causes erection/eversion (aids infant latch-on & milk expression).

    • • Multiple (15–20) tiny lactiferous duct openings on apex.

  • Nipple position

    • • Nulliparous female (before childbirth): usually opposite 4^{th} intercostal space.

    • • Multiparous female: position becomes variable due to ligamentous stretching & involutional changes.

Internal (Deep) Features

  • Suspensory (Cooper’s) ligaments

    • • Condensations of fibrous stroma anchoring dermis to deep pectoral fascia.

    • • Superiorly well-developed; maintain breast contour.

    • • Clinical note: tumour infiltration/shortening → skin dimpling.

  • Glandular architecture

    • • ~15\text{–}20 lobes arranged radially around nipple.

    • • Each lobe → 20\text{–}40 lobules → many alveoli.

    • • Lobules embedded in fat & connective tissue.

  • Ductal system

    • • One main lactiferous duct per lobe.

    • • Each duct enlarges beneath areola forming lactiferous sinus (ampulla) that stores milk droplets between feeds.

    • • Duct narrows to individual ostium at nipple tip (facilitates hand-expression when areola compressed).

  • Retromammary (submammary) space

    • • Loose areolar plane + small fat cushions.

    • • Allows relative mobility over pectoralis major; loss of mobility can be an early sign of malignant fixation.

Vascular Supply

  • Arterial

    • • Axillary artery branches

    • – Thoracoacromial artery (pectoral branch).

    • – Lateral thoracic artery.

    • • Internal thoracic artery (aka internal mammary) → anterior perforating & medial mammary branches.

    • • Posterior intercostal arteries (3rd–5th) → lateral perforators.

  • Venous

    • • Deep venous plexus mirrors arteries → empties into:

    • 1. Axillary vein.

    • 2. Internal thoracic vein.

    • 3. Posterior intercostal veins.

    • • Superficial venous plexus located just beneath dermis → drains into deep system; occasionally used for reconstruction flaps.

Lymphatic Drainage (Key in Metastasis)

  • General concept: cancer spreads preferentially via lymphatics; mapping sentinel nodes guides surgical management.

  • Superficial (cutaneous) plexus → drains skin except areola & nipple

    • • Axillary nodes.

    • • Inferior deep cervical nodes (along internal jugular vein).

    • • Infraclavicular & deltopectoral nodes.

    • • Parasternal (internal mammary) nodes → possible contralateral spread across midline.

  • Deep (parenchymal) plexus → drains glandular tissue + areola + nipple

    • • Axillary nodes receive ~75\% total drainage.

    • – Primarily anterior/pectoral group.

    • – Others: lateral, posterior, central, apical.

    • • Interpectoral (Rotter’s) nodes, supraclavicular, inferior deep cervical.

    • • Parasternal nodes (medial quadrants).

    • • Inferior phrenic (abdominal) nodes communicate with subdiaphragmatic pathways (important for inferior quadrant tumours).

Nerve Supply

  • Origin: 4th–6th intercostal nerves (anterior & lateral cutaneous branches).

  • Fibre types & targets

    • • Sensory ➜ skin of breast (pain, temperature, tactile input necessary for neuroendocrine reflexes e.g., suckling → prolactin/oxytocin release).

    • • Sympathetic (post-ganglionic) ➜ vascular smooth muscle & areolar/nipple smooth muscle; mediate nipple erection & vasomotor changes.

  • No parasympathetic fibres.

Embryologic Development

  • Mammary ridges (milk lines)

    • • Ectodermal thickenings at week-4 extend axilla → inguinal region in both sexes.

    • • Multiple primordial glands appear along line; normally only one pair (pectoral region) persists in humans — reflects comparative anatomy (e.g., cats, pigs have multiple functional pairs).

  • Primary bud (week-5)

    • • Down-growth of ectoderm into mesenchyme.

  • Branching (week-10 onwards)

    • • Secondary buds (week-12) → continue branching throughout gestation.

  • Canalisation (2nd trimester)

    • • 15\text{–}25 epithelial ingrowths form lactiferous ducts by apoptosis of central cells.

  • Nipple/areola formation

    • • Ducts open into superficial mammary pit.

    • • Mesoderm proliferation everts pit into nipple few weeks post-birth.

    • • Surrounding skin differentiates → areola.

  • Pubertal & reproductive maturation (Tanner stages I–V)

    • • Oestrogen stimulates ductal elongation; progesterone promotes alveolar budding.

    • • Reference QR code in slide for detailed staging (I – preadolescent; V – mature adult morphology).

Comparative & Evolutionary Notes

  • Many placental mammals maintain the entire milk line, hence multiple mammary glands (e.g., dog 5 pairs, sow 7 pairs).

  • Functional placement correlates with typical litter size & nursing position.

Congenital & Developmental Anomalies (Clinical Anatomy)

  • Supernumerary structures along milk line

    • • Polymastia (accessory breast tissue).

    • • Polythelia (accessory nipple); most frequent just inferior to normal breast, may mimic melanocytic nevus.

  • Other variants

    1. Amastia – complete absence of breast.

    2. Micromastia – underdeveloped breast.

    3. Macromastia – excessive hypertrophy (may require reduction surgery).

    4. Athelia – absent nipple.

    5. Microthelia – diminutive nipple.

  • Embryological basis: failure of ridge regression, abnormal bud development, or endocrine dysregulation.

Clinical & Practical Implications

  • Breast cancer

    • • Lymphatic mapping essential (sentinel node biopsy; axillary clearance).

    • • Cooper’s ligament tethering → skin puckering; invasion of retromammary space → fixation to chest wall.

  • Inflammatory conditions (mastitis) exploit lactiferous sinus reservoirs.

  • Retromammary space exploited surgically for implant placement (subglandular vs submuscular).

  • Trauma/seat-belt injuries may disrupt vascular supply causing fat necrosis.

  • Ethical considerations: informed consent for cosmetic & oncologic procedures, inclusivity in gender-diverse care.

Numerical & Statistical Summary

  • Lobes: 15\text{–}20

  • Lobules per lobe: 20\text{–}40

  • Intercostal span: 2^{nd} – 6^{th} ribs.

  • Axillary node drainage proportion: \approx 75\% of parenchymal lymph.

  • Posterior intercostal arterial supply: 3^{rd},4^{th},5^{th} spaces.

  • Lactiferous duct count: 15\text{–}25 (mirrors future lobes).

These notes integrate gross anatomy, embryology, comparative context, vascular-neural relationships and clinical correlations to serve as a stand-alone study resource for examinations and applied medical practice.