Pathology Thread

I. Gross Anatomy and Histology

Objectives

After completing this section you will be able to:

• list and identify the various parts of the breast
• describe the gross anatomy and histology of the breast

1. Anatomy

   The breast is a modified, specialized, cutaneous, glandular structure located in the superficial fascia of the anterior chest wall and contains, in the female, elements that secrete milk for the nourishment of the young. The mature female breast has a distinctive protuberant, conical form with a roughly round base. It covers the area from the 2^nd or 3^rd rib to the 6^th or 7^th rib and from the lateral border of the sternum to the anterior or midaxillary line. The nipple projects from the anterior surface and is hyperpigmented. It consists mainly of dense fibrous tissue covered by skin and contains bundles of smooth muscle fibers, which make it "erectile" and assist with milk expression. The skin immediately surrounding the nipple called the areola is also more pigmented than the rest of the breast skin and becomes more so during pregnancy. The breast parenchyma consists of a branching duct system of 15 to 20 lobes, which radiate from the nipple and bound together by fairly dense connective tissue septa, the interlobular connective tissue. Each lobe is roughly pyramidal in shape with the apex toward the nipple. Bands of connective tissue, the suspensory ligaments, extend from the interlobular connective tissue to attach to the dermis. Each ductal system drains through a separate main duct, the lactiferous duct, which terminates and exits the breast at the nipple via a secretory pore. Immediately deep to the areola, each lactiferous duct dilates to form the lactiferous sinus in which milk can accumulate and beyond the sinus extends through successive branches of diminishing size, the large and intermediate (interlobular) ducts. The ducts are continuous with the functional units of the breast, the terminal duct lobular units (TDLU). The TDLU consists of the intralobular duct along whose lumen and end protrude blunt or round saccules called ductules, which differentiate into the secretory units or acini during lactation. Individual lobes vary greatly in size and typically about less than half enlarge to become functional during lactation. The TDLUs are embedded in specialized, hormonally responsive connective tissue stroma, the intralobular stroma.

   
   Anatomy of the breast

   Question: What is the clinical significance of the suspensory ligaments?

The breast parenchyma always spreads further than the gross outline of the breast, and there is nearly always extension into the axilla, forming the axillary tail. The parenchyma is diffusely distributed within adipose tissue, which varies considerably depending on the age and general features of the woman. The upper half of the breast, particularly the upper outer quadrant, contains more glandular tissue than does the remainder of the breast.

Question: What is responsible for the variability in breast size in women?

 

2. Histology

The epidermis of the nipple and areola has a thick cornified (keratin) layer and contains pilosebaceous units and both eccrine and apocrine sweat glands.

Nipple showing pilosebaceous units.

Question: Why does the epidermis over the nipple and areola have a thick horny layer?

 

Question: In which other areas of the body do you find apocrine glands?

The orifice of the lactiferous duct is lined by squamous epithelium where the duct traverses the epidermis of the nipple. The squamous epithelium joins the columnar epithelium of the duct to form a squamo-columnar junction distal to the lactiferous sinus. The lactiferous ducts in the nipple are surrounded by circular and longitudinal bundles of smooth muscle fibers, which are embedded in a collagenous stroma.

Nipple and lactiferous ducts

Lactiferous duct

The extralobular large and intermediate ducts are lined by columnar epithelium supported by myoepithelial cells, basement membrane and periductal elastic fibers and lie within the fibrous interlobular stroma.

Interlobular ducts leading to TDLUs.

The normal histology of the lobules is not constant in the mature female breast because of changes associated with fluctuations in hormonal levels (during menstrual cycle, pregnancy, lactation and menopause). A single layer of cuboidal epithelium supported by underlying myoepithelial cells lines resting lobular ductules. The intralobular stroma contains more capillaries and is less densely collagenized than the interlobular stroma.

Breast lobules (TDLUs)

TDLU. High power view showing the two-cell layer epithelium.