The breasts

Summary

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.

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

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.

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
    2. Regression of excess mammary ridges
    3. Sprouting of two epithelial buds
    4. 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

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 12/04/2019
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