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Anatomy and Physiology of the Mammary Glands / 507

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Figure 29-4. Nonlactating mammary gland. A, gland lobules with inactive alveoli; L, intralobular duct; C,
connective tissue. Source: Bacha and Bacha, 2012. Reproduced with permission of John Wiley & Sons, Inc.

between lobules) emerge. These interlobu-
lar ducts unite within a single lobe to form
an intralobar duct. When this duct leaves
the lobe, it is called an interlobar duct; it
may enter the gland cistern directly, or it
may join one or more other interlobar
ducts before entering the cistern. Many of
the ducts have numerous dilations that act
as additional collecting spaces for milk.
The alveoli and ducts are surrounded by
contractile myoepithelial cells, which are
also called basket cells. These cells con-
Figure 29-5. Lactating mammary gland. Star tract to eject milk (called milk letdown) in
indicates a single alveolus. Source: image courtesy of response to oxytocin release (discussed
Gretchen Delcambre, Colorado State University, later).
Fort Collins, Colorado, USA. In addition to the epithelial parenchyma
and the myoepithelial cells, the mammary
of the smallest ducts; milk‐secreting struc- gland possesses an interstitium of white
tures also empty directly into the larger fibrous connective tissue and yellow elastic
ducts and directly into the gland cistern connective tissue. Blood vessels, lymph
and the teat cistern. vessels, and nerves ramify throughout the
A group of alveoli surrounded by a interstitium to reach the epithelial struc-
connective tissue septum form a more or tures. The veins of the mammary gland are
less distinct unit called a lobule (Fig. 29‐1). valveless and form a rich network through-
A group of lobules within a single connec- out the gland and within the wall of the
tive tissue compartment forms a lobe. teat. The vascular layer of the teat is called
Correspondingly, the ducts are classified as a corpus cavernosum (and indeed has a
intralobular, interlobular, intralobar, resemblance to the erectile tissue of the
and interlobar as they increase in size. penis in the male as the name suggests).
The alveoli making up the lobule empty Lymphatic plexuses are found throughout
into small ducts within the lobule, the the udder just deep to the skin and scat-
intralobular ducts. These intralobular tered throughout the parenchyma of the
ducts drain into a central collecting space gland. Nerves are primarily sensory and
from which the interlobular ducts (ducts vasomotor.