The breasts

Abstract

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 lactation and breastfeeding. The nipple also contains multiple nerve endings, which make it particularly sensitive to stimulation.

Gross anatomy

Overview

Location

  • 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

Function

  • Milk production
  • Lactation
  • Sexual arousal

General structure

Ligaments and fascia

The mammary glands are attached to the skin and chest wall by strong bands of connective tissue.

Vasculature and innervation

The breasts are supplied by the intercostal and axillary pathways.

  • Arteries
    • Medial breast: medial mammary branches of the internal thoracic artery from the subclavian artery
    • Lateral breast
      • Lateral thoracic and thoracoacromial branches from the axillary artery
      • Lateral mammary branches of the posterior intercostal arteries (2nd–5th ICS) from the thoracic aorta
      • Mammary branch of the anterior intercostal artery
  • Veins
  • Innervation (sensory and autonomic fibers )
    • Supraclavicular nerves of the cervical plexus
    • Lateral and medial branches from intercostal nerves 2–6

Secretion of milk is not mediated by the nerves but by the hormone prolactin.

  • 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
        • Drain most of the medial parts of the breast
        • Lie along the internal thoracic arteries
      • Posterior intercostal nodes: (provide 5% of the lymphatic drainage of the breast)
    • The skin of the breasts drains to axillary, inferior deep cervical, and infraclavicular nodes.

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).

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.

References:[1]

Microscopic anatomy

Non-lactating breasts

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)

Milk duct system

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.

Lactating breasts

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
  • 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

The size and perfusion of the mammary glands vary during the menstrual cycle, but they also change in the course of a woman's lifetime.

Embryology

The mammary glands initially develop as mammary ridges from the embryonic surface ectoderm. The development of the breasts is identical in all sexes until puberty. Glandular tissue only begins developing at puberty.

  • Development of mammary glands

    1. 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.
    2. Regression of excess mammary ridges
      • The accessory mammary ridge regresses.
      • Only the thoracic mammary bud of each side of the body remains.
    3. Sprouting of two epithelial buds
      • The two remaining epithelial buds sprout beneath the epidermis → development of first glandular lumina
    4. Eversion of the nipple
      • Initially, the nipple is level with the skin.
      • Excretory ducts of the glandular lumina are everted → The nipple surpasses the level of the skin.
      • Development and growing pigmentation of the nipples and areolas

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

In puberty, the development of the mammary gland is sex-specific. Breast development in women during puberty is also called thelarche.

  • Changes of the female breasts during puberty
    • Increase of tissue in the area of the areola with slight protrusion
    • Estrogen stimulates the development of breast tissue.

Because men do not have the same hormonal influxes of estrogen, their mammary glands remain rudimentary. Nevertheless, men can also develop carcinoma of the breast!

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).

Clinical signifcance

  • 1. Dutta DC, Konar H. Textbook of Obstetrics. New Delhi, India: Jaypee Brothers Medical Publishers; 2015.
last updated 12/04/2018
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