Human Anatomy: Breast Structure and Position

Breast Structure

• The breast is mostly comprised of fibrous and fatty tissue.
• The breast tissue is composed of 15-20 lobes.
• Each lobe contains smaller lobules.
• The lobules contain acini that produce milk.
• Ducts drain milk from the acini to the nipple.
• The areola contains modified sebaceous glands that lubricate the nipple during breastfeeding.
• Montgomery's tubercles form in the areola during pregnancy.
• The nipple and areola form the nipple areola complex.
• Cooper's ligaments form a support structure.
• Suspensory ligaments of Cooper divide the breast parenchyma into 15-20 segments.

Breast Position and Extent

• The breast lies in the superficial fascia of the pectoral region.
• The breast extends vertically from the second to sixth rib.
• The breast extends horizontally from the lateral sternal border to the mid-axillary line.
• The axillary tail of Spence is a normal extension of breast tissue in the axilla
• The majority of breast cancers develop in the upper outer quadrant.

Breast Development

• Breast development begins as early as the fourth week from a thickened mammary ridge (milk line) of ectoderm, along a line from the axilla to the inguinal region.
• Supernumerary nipples or glands proper can form at lower levels.
• Lobule formation occurs in the breast only after puberty.

Lactation

• The resting breast is mostly fibrous and fatty tissue.
• During pregnancy, alveoli bud off from the ducts, and the breast significantly enlarges.
• Perforating arteries enlarge during lactation to supply the mammary glands.
• The breast involutes when lactation ceases.
• Lobe and duct atrophy after menopause.

Male Breast

• Male breast resembles a rudimentary female breast.
• Males don't have lobules or alveoli.
• The nipple and areola overlie the fourth intercostal space.
• Breast cancer is rare in males.

Arterial Supply of the Breast

• Most arteries supply the anterior breast surface.
• The posterior surface is relatively avascular.
• The breast's arterial supply comes from the internal thoracic artery (medial aspect), lateral thoracic artery (lateral aspect), thoracoacromial artery, and posterior intercostal arteries.

Venous Drainage of the Breast

• Most breast venous drainage flows to the axillary vein via the lateral thoracic vein.
• Other veins involved include internal thoracic veins and posterior intercostal veins.
• Cancer can spread to the vertebrae through communication with the vertebral venous plexus.

Lymphatics of the Breast

• Most breast lymph drains to axillary nodes (~75%).
• Other lymph nodes include parasternal nodes, infraclavicular/deltopectoral nodes, and supraclavicular nodes.
• Infraclavicular/deltopectoral nodes are located in the deltopectoral groove, draining the anterolateral aspect of the upper limb.
• Supraclavicular nodes are just lateral to the clavicular head of sternocleidomastoid.
• Lymph drainage terminates in the central venous system.
• The axillary lymph nodes are located in the axillary region of the upper limb, encompassing drainage for the breast and the trunk above the umbilicus.
• There are approximately 20-30 axillary lymph nodes, categorized into 5 groups (anterior/pectoral, posterior/subscapular, lateral/humeral, central, and apical/subclavicular). .
• Lymphatic drainage pathways include axillary, internal mammary, and retromammary routes.

Dermatomes

• A dermatome is an area of skin supplied by a single spinal nerve.
• There are 30 dermatomes.
• Sensory innervation of the breast is mainly from T3-T5.

Learning Objectives

• Describe the structure of the breast.
• Describe arterial supply and venous drainage of the breast.
• Describe the dermatomal innervation of the breast.
• Describe the lymphatic drainage of the breast.

Lymph Node Levels

• Lymph node levels are based on pectoralis minor m.
• Level I: Latissimus dorsi to lateral aspect of minor muscle.
• Level II: Posterior to pectoralis minor.
• Level III: Medial margin of pectoralis minor to thoracic inlet.