Normal Breast Development and Structure

The breasts are paired structures on the anterior chest wall (!).

• The are, effectively, modified sweat glands that develop on the ‘milk line’- a process that is largely hormone-driven
  • There are different stages of breast development:
    1. Intra-uterine development
    2. Puberty
    3. Adult ‘resting’ period
       1. Pregnancy and lactation
       2. Involution
    4. Post-menopausal atrophy

1. The breast develops from the breast bud which is formed at around 6 weeks gestation in utero.  Further normal breast development doesn’t occur until puberty.

2. At puberty there is:

• Growth of the the breast ducts
• Development of the hormonally responsive specialised periductal stroma
• The beginning of development of the lobules (this continues at pregnancy)

3. The adult premenopausal breast undergoes changes during the menstrual cycle:

• During the proliferative (follicular) phase of the cycle
  • lobules are small; mitosis is infrequent and the specialised stroma is relatively condensed
• During the secretory (luteal) phase
  • the terminal duct lobular units increase in size with a more loose oedematous stroma; there is increased epithelial mitotic activity (and the cytoplasm is vacuolated)
• In the perimenstrual period
  • it can be normal to have some sloughing of the epithelium and associated lymphocytic infiltration (i.e. not indicative of inflammation/disease)

4. After the menopause:

• there is a decrease in glandular tissue (atrophy)- resulting in a proportional increase in stroma
• Glandular units show decreased numbers of acini, decreased amount of specialised stroma and smaller epithelial cells
• There may be areas of dense fibrosis and benign vascular calcification (this is normal)

Breast anatomy

Blood supply

• The blood supply to the skin of the breast varies from person to person but the proportions are usually similar to the vasculature supplying the deep tissue:
  • Predominantly, the perforating arteries from the internal mammary artery (a branch of the internal thoracic- from subclavian) (runs down the sternal edge/ribs)
  • Further supplied by the lateral thoracic, thoracoacromial arteries (from axillary artery) and the posterior intercostals (from aorta)
• The venous drainage is mainly via the axillary vein.

Lymphatics

• The majority (75%)- especially the lateral zones- drain to the axillary nodes
  • the remainder either drain to the parasternal nodes or via the opposite breast (if in the medial zone) OR the inferior phrenic nodes (if in the inferior zone)
• The exception is the nipple and areola complex and lymphatic drainage from the skin, which drain to the cervical nodes (this is because the areolar complex drains to a subareolar lymphatic plexus)

Nerve Supply

• Mainly dermatomal i.e. T3-6
• Although not innervating breast tissue, it is important to remember that the long thoracic and intercostobrachial nerves are in this area

Musculature

• The breast lies of the pectoralis major

Ducts, ductules and lobules

• Each breast has around 10-12 duct systems- each with numerous lobules
  • A lobule consists of a terminal ductule and acini involved with it
  • Terminal ductules empty into the ducts
• Ducts and ductules are lines by a 2-cell layer
  • inner epithelium
  • outer myoepithelial cells (for milk expulsion)
• The largest ducts drain into the the lactiferous ducts and sinus to drain to the nipple.
• The lobules and ducts are surrounded by supportive stroma and adipose tissue.

Aberrations of normal development

• Lumpiness/mastalgia
• Microfocal areas of adenosis; apocrine metaplasia; sclerosis; microcyst formation; etc are all variations of NORMAL

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