Breast

Breast or mammary gland

• Modified apocrine sweat gland whose primary purpose is to produce milk
• lies in the subcutaneous tissue of the anterior thoracic wall.

Embryology

• Begins to develop as early as the 5th week - 'milk lines' form along a line from the axilla to the inguinal region, of ectodermal origin
  • Supernumerary nipples or even glands may form along this line
• Distal aspect of milk lines begin to disappear by week 9
• Ectoderm in pectoral region invaginates into the surrounding mesoderm. These extensions eventually epithelialize, branch and canalize, forming ducts which reach the nipple.

Surface markings of base:

• Medial - sternal edge
• Superior - second rib
• Inferior - sixth rib
• Lateral - near the midaxillary line

Structure and functional anatomy:

• Composed of adipose tissue, connective tissue and glandular tissue
• Glandular tissue is most concentrated in the upper outer quadrant
• Breast parenchyma (glandular tissue) is composed of lobes, which can be further divided into smaller lobules
  • The lobular units are made up of acini within loose intralobular stroma
  • The lobular units are surrounded by denser interlobular stroma, containing blood vessels, larger breast ducts, and fat
• Lactiferous ducts (15-20) converge in a radial direction, each draining a lobule, and each opening individually to the tip of the nipple.
  • Each duct has a dilated sinus at its terminal distal portion in/near the nipple.
  • The ducts end in terminal ductules or acini
    • Milk-forming glands of the lactating breast
    • Form a 'lobular unit' or 'lobule' together with their small efferent ducts or ductules
• Nipple - projection just below centre of breast, surrounded by an area of pigmented skin, the areola. Nipple contains smooth muscle cells which can contract to cause nipple erection.
• Areola contains sebaceous glands, sweat glands and other areolar glands. Areolar glands form small elevations (tubercles of Montgomery), particularly when they enlarge during pregnancy.
• Strands of fibrous tissue, forming the suspensory ligaments of Cooper, connect the dermis of the overlying skin to the ducts of the breast and to the posterior fascia
  • When tumour infiltrates these strands, the skin is tethered to tumour, and dimpling or subtle deformities of shape result
• The male breast resembles the rudimentary female breast and has no lobules or alveoli. The small nipple lies over the fourth intercostal space.
• Breast size varies greatly from <100g to >2kg

Relations:

• Superficial: Sub-dermal layer of adipose tissue
• Deep:
  • Superficial fascia (which is an upward continuation of Scarpa's fascia in the abdomen) condenses posteriorly to breast tissue to form a posterior capsule.
  • Between capsule and the pectoralis fascia lies the loose connective tissue of the retromammary space, also called retromammary fat pad. Relatively avascular.
  • Overlies pectoralis major, overlapping onto serratus anterior and a small part of rectus sheath/external oblique
• Lateral: axillary tail may prolong towards axilla, usually in the subcutaneous fat rather than the deep fascia going into the actual axilla

Lymphatic drainage:

• Abundant lymphatics in the breast parenchyma and dermis
• 'Sappey plexus' located under the NAC. Lymph flows from skin to subareolare Sappey plexus, then into the interlobular lymphatics of breast parenchyma
• Sentinel node location:
  • 75% of breast lymph passes to axillary lymph nodes, mainly to anterior nodes, some to posterior nodes; direct drainage to central or apical nodes is possible.
  • 25% to parasternal nodes along the internal thoracic artery (5% sole pathway, 20% shared with axilla)
• Can drain to posterior intercostal nodes.
• Occasionally, can drain to infraclavicular nodes in the deltopectoral groove
• Direct drainage from breast to supraclavicular nodes is possible
• These minor pathways tend to only convey lymph from breast when the major channels are obstructed by malignant disease.

Blood supply:

• Three sources:
  • Branches of axillary artery
    • Derived mainly from lateral thoracic artery, by branches that curl around the border of pectoralis major, and by other branches that pierce the muscle.
    • Pectoral branch of the thoracoacromial artery supplies the upper part of the breast.
  • Internal thoracic artery (a.k.a. internal mammary) also sends branches through the 2nd-5th intercostal spaces beside the sternum (largest in second and third IC spaces).
  • Small perforating branches also arise from the posterior intercostal arteries.
• Venous drainage is mainly by deep veins that run with the main arteries to internal thoracic and axillary veins. Some drainage to posterior intercostal veins provides a link to the internal vertebral venous plexus veins - hence metastatic spread to bone.
• There is also a circumareolar venous plexus

Nerve supply

• Overlying skin supplied by cutaneous branches of intercostal nerves T4-6
• Sympathetic fibres to blood vessels and glands
• Control of lactation is hormonal

Microscopic anatomy

• Three principal tissue types, with the proportions varying with age, cycle, pregnancy and breastfeeding:
  • Glandular epithelium
  • Fibrous stroma and supporting structures
  • Adipose tissue
• Layers surrounding ducts, from inside to outside the duct:
  • Entire ductal system lined by epithelial cells
    • Stratified squamous epithelium near the nipple
    • Transition to cuboidal epithelial cells in the lactiferous sinuses behind the nipple
  • Epithelial cells surrounded by specialised myoepithelial cells - have contractile properties and serve to propel milk towards the nipple
  • Then a continuous basement membrane containing laminin, type IV collagen, and proteoglycans
    • This layer is the key differentiation between invasive and in situ breast cancer - DCIS does not breach it
  • Stroma - where the lymphatics and blood vessels are located

Physiology

• Puberty
  • Thelarche - onset of adult breast development - ductal elongation, increase in the height of the pithelium and an increase in stromal density
  • Oestrogen from ovaries results in formation of additional lobular units
• Post-pubertal mature
  • Responsive to exogenous hormones - cyclical stimulation. Late luteal (pre-menstrual) phase sees accumulation of fluid and intra-lobular oedema, sometimes producing pain and breast engorgement.
• Pregnancy
  • Diminution of the fibrous stroma
  • Formation of new acini or lobules (adenosis of pregnancy)
  • More alveoli per lobule, and lobular units differentiate into secretory units
• Childbirth
  • Sudden loss of placental hormones, combined with continued high levels of prolactin, triggers lactation
  • Expulsion of milk is under hormonal control by oxytocin - contraction of periductal myoepithelial cells, which do not appear to have any innervation. Occurs in response to stimulation of the nipple.
• Menopause
  • Involution and general decrease in the epithelial elements of the resting breast
  • Increased fat deposition, diminished connective tissue, and the disappearance of lobular units
  • HRT can prevent these changes - retention of breast epithelium and stromal tissue with persisting breast density