Gross Anatomy of the Breast
The breasts are paired organs on the anterior chest wall and are composed of modified apocrine sweat glands arranged into lobules and ducts.
Location and structure
- Anterior thoracic wall
- Overlying the deep fascia and pectoral muscles
- At the level of ribs 2–6, from the sternum to the midaxillary line, with the nipple in the 4th intercostal space
- Paired, tear-shaped, and of varying size
- Each composed of a mammary gland surrounded by connective tissue
- Components from internal to external:
- Mammary gland: modified apocrine sweat gland arranged into lobules and ducts
- Adipose tissue: 80% of the breast in nonbreastfeeding woman
- Nipple–areolar complex: glandular (lactiferous) ducts converge and open to the skin in the mammary papilla (nipple), surrounded by the areola (circular area of darker pigmentation), which contains sebaceous Montgomery glands to lubricate and protect the nipple during breastfeeding.
- Ligaments and supporting structures:
- Deep pectoral fascia: attachment of the breast to the pectoralis major muscle. The space between the breast and muscle is the retromammary space.
- Suspensory ligaments of the breast (Cooper ligaments): run between the pectoral fascia and the skin, between the secretory lobules
- Begins as a thickening of the chest wall, leading to the mammary ridge, or milk line
- Nipples and breast duct system are present at birth.
- Further development during puberty is under the influence of female sex hormones.
- Breast size increases during pregnancy and reduces after menopause
Microscopic Anatomy of the Mammary Gland
The mammary gland is composed of modified apocrine sweat glands divided into lobules, which reach the nipple via lactiferous ducts. The ducts are composed of epithelium surrounded by myoepithelial cells.
- Mammary gland: milk-producing modified apocrine sweat gland consisting of 15–20 lobes, further divided into lobules connected to the nipple by lactiferous ducts:
- Milk-producing glands
- Composed of alveoli lined with cuboidal epithelium surrounded by contractile myoepithelial cells
- Lactiferous ducts:
- Passage from alveoli to nipple
- Dilation at the ends (lactiferous sinuses) function as milk reservoirs.
- Stroma: adipose tissue and suspensory ligaments surround and separate the lobules and ducts.
- Ductal system: epithelium surrounded by myoepithelial cells
- Lobule → terminal or lactiferous ducts → major ducts → terminal ducts → lactiferous sinus (milk reservoir) → open into nipple
- Prolactin, progesterones, and estrogens stimulate the proliferation and growth of the breast’s secretory units.
The arterial supply to the breast is via branches of the subclavian, axillary, and intercostal arteries with venous drainage into the subclavian, axillary, and azygos veins. The innervation is from branches of the 2nd–6th intercostal nerves, with hormonal input via prolactin.
- Arterial supply:
- Medial breast: internal thoracic artery, which is a branch of the subclavian artery
- Lateral breast:
- Lateral thoracic/abdominal branches from the axillary artery
- Mammary branches from the intercostal arteries
- Venous drainage:
- Accompany the mentioned arteries
- Drain into the subclavian, axillary, and azygos veins
- Lymphatic drainage: lymph from the breast lobules, nipple, and areola → subareolar lymphatic plexus:
- 75% → pectoral lymph nodes → axillary lymph nodes → subclavian lymphatic trunks; important pathway for metastatic spread of breast carcinomas
- 20% → parasternal nodes → bronchomediastinal trunks
- 5% → intercostal nodes → thoracic or bronchomediastinal lymph trunks
- Sensory and sympathetic fibers: supraclavicular nerves (cervical plexus) and branches of the 2nd–6th intercostal nerves
- Lactation: stimulated by prolactin
- Mammography: allows for early detection of breast cancer or precancerous lesions before they develop into invasive breast cancer. It is recommended to start routine annual mammography screening after the age of 40 years.
- Fibrocystic changes: benign changes that mimic but do not achieve the threshold to be considered malignant conditions of the breast. Fibrocystic changes are those that may be seen even in normal breast tissue.
- Mastitis: inflammation of mammary gland tissue, which can be lactational or nonlactational. Individuals will present with warmth and tenderness of the inflamed area. First-line treatment consists of beta-lactam antibiotics, such as dicloxacillin.
- Fat necrosis: benign condition. Fat necrosis can occur anywhere in the breast and can affect women of any age. This condition is characterized by a firm, round lump, which is usually painless, but it may present with tenderness or pain.
- Gynecomastia: benign enlargement of breasts in males due to glandular proliferation, giving breasts a characteristic rubbery nature. Gynecomastia can be caused by medications that elevate estrogen levels, such as cimetidine.
- Fibroadenoma: benign breast masses that are usually mobile on palpation. Fibroadenomas are commonly identified in young women and represent a diagnostic challenge for surgeons. Fibroadenomas do not typically turn into breast cancer, so it is safe to just monitor these masses.
- Breast cancer: Mastocarcinoma is a malignant tumor of the mammary gland. The point of origin may be either the mammary gland’s ducts or lobules, leading to ductal carcinoma and lobular carcinoma, respectively. Breast cancer is treated with surgical excision +/– radiation and chemotherapy.
- Nipple discharge: In nonlactating women, nipple discharge can be classified as either galactorrhea or nonmilky nipple discharge. Galactorrhea is usually caused by endocrine disorders or medications, whereas nonmilky nipple discharge is usually linked to benign tumors.
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