The breasts are paired structures located on the ventral thorax. The female breast consists of 10–20 lobes that are surrounded by connective and adipose tissue. Each lobe contains milk-producing glandular structures and multiple terminal duct lobular units that form the milk duct system and drain via the major milk ducts and the lactiferous sinuses to the nipple. Lactating lobules produce the breast milk that nourishes many newborns and protects them from infections (passive immunity). For more detailed information on lactation, see . The nipple also contains multiple nerve endings, which make it particularly sensitive to stimulation.
- The breasts are paired structures located at the level of ribs 2–6, below the subcutis on the muscles of the ventral thorax.
- Divided into four quadrants: upper lateral, upper medial, lower lateral, and lower medial
- Milk production
- Sexual arousal
- Mammary gland
- Interlobular mammary stroma: surrounds the glands and ducts with connective and adipose tissue
- Nipple-areolar complex
- Pectoral fascia
- Suspensory ligaments of the breast (Cooper ligaments)
Vasculature and innervation
The breasts are supplied by the intercostal and axillary pathways.
- Medial breast: medial mammary branches of the internal thoracic artery from the subclavian artery
- Lateral breast
- Innervation (sensory and autonomic fibers )
Lymph nodes of the breasts
- Three groups
Axillary lymph nodes
- Number of nodes: 30–60
- Provide 75% of the lymphatic drainage of the breast
- Parasternal (internal thoracic) lymph nodes
- Posterior intercostal nodes: (provide 5% of the lymphatic drainage of the breast)
- Axillary lymph nodes
- The skin of the breasts drains to axillary, inferior deep cervical, and infraclavicular nodes.
- Three groups
A tumor infiltrating the breast tissue can deform, shorten, and retract the Cooper ligaments and lead to dimpling of the skin. Tumorous infiltration and blockage of the lymphatics manifest as lymphedema and thickening of the skin, which is known as peau d'orange because of the resemblance to orange peel. Retraction of the nipple can be a sign of a tumor within the ducts (Paget disease of the breast).
Carcinomas of the breast metastasize early on through the lymph pathways. As most carcinomas of the breast develop in the upper lateral quadrant, lymph node metastases often occur in the axilla of the same side.
The lobes of the breasts consist of multiple smaller lobules with milk ducts. The milk duct system of the lobules joins to form one main milk duct per lobe.
Terminal ductal lobular units (TDLU)
- Basic histopathological units of the mammary gland
- Consist of:
Lobule of the mammary gland: (functional unit of the breast)
- Intralobular stroma: loose, cell-rich connective tissue
- Intralobular terminal (milk) duct with multiple outpouchings called acini or ductules (site of milk production)
- Extralobular terminal duct
- Lobule of the mammary gland: (functional unit of the breast)
Milk duct system
- Function: transports the milk from the terminal duct to the endpoint of the nipple
- Structure: two-layered epithelium
- Terminal ducts or lactiferous ducts: excretory duct of a single lobule
- Major ducts: largest part of the milk duct system
- Lactiferous sinus:
After skin cancer, breast cancer is the most common malignancy in women. Depending on the tissue in which malignancy originates, breast cancers can be histologically differentiated into ductal carcinoma (originating from the milk duct epithelium) and lobular carcinoma (originating from the lobules). Invasive ductal carcinoma of the breast is the most common type.
The transition from non-lactating to lactating breasts already begins in the first trimester of pregnancy. During this time, the mammary gland increases in size, and the production of breast milk is initiated.
Changes in the mammary gland
- Enlargement and differentiation of the lobules of the mammary gland
- Tubulo-alveolar glands of the lobules initiate lactation
- ↓ Interlobular stroma
- ↑ Vascularity
Changes in the nipple
- ↑ Size, ↑ pigmentation
- become visible
- Hormonal regulation of the transition of the glands
Lactating breasts have distinct histological features: very large lobules with tubulo-alveolar ends and only small amounts of connective tissue between the lobules! Because of the reduced amount of stroma, histological preparations of lactating breasts can be confused with those of the prostate or of the parathyroid gland!
Menstrual cycle-dependent and age-dependent changes of the breasts
- Menstrual cycle
- Puberty: See “ “ and “ .”
- With age: involution of the breasts with the onset of menopause due to a decrease in estrogen levels
Development of mammary glands 
- Development of the mammary ridges (milk lines) 
- Cutaneous crest from which multiple glandular buds originate
- Develop vertically on both sides from the axilla to the groin from the 5th week on
- Thickening of the ectoderm (milk line)
- Thickening spreads increasingly into the subepithelial connective tissue, at which point it is called the mammary ridge.
- Regression of excess mammary ridges
- The accessory mammary ridge regresses.
- Only the thoracic mammary bud of each side of the body remains.
- Sprouting of two epithelial buds
- Eversion of the nipple
If the regression of the mammary ridge is impaired, more than two epithelial buds may remain. In this case, accessory nipples (polythelia) or breast tissue (polymastia) may develop. Total regression of nipples (athelia) and breast tissue (amastia) can also occur.
Maturation of the mammary gland
- Changes of the female breasts during puberty
Maternal pregnancy hormones can also prepare the mammary glands of newborns for lactation. The mammary glands of newborns may be slightly swollen after birth and produce a milky secretion (i.e., witch's milk or neonatal milk).
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