- 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 2nd or 3rd
rib to the 6th or 7th 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?
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.
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