CHAPTER 3
Epithelium and Glands
47
Glands
Introduction and Key Concepts for Glands
Glands are composed of epithelial tissue and can be classiF
ed
as endocrine and exocrine according to how the secretory prod-
uct leaves the gland.
Endocrine glands
release their products into
interstitial fl uid or directly into the bloodstream (see
Chapter 17,
“Endocrine System”).
Exocrine glands
are discussed in this chap-
ter; these glands secrete their products either through ducts into
the lumen of an organ or directly onto the body surfaces. Exo-
crine glands can be classiF ed into several categories according to
various criteria.
Exocrine Glands ClassiF ed by Product
Exocrine glands can be classiF ed as
serous
glands
,
mucous
glands
,
mixed
glands
(
seromucous
), and
sebaceous
glands
, depending on
what type of secretion is produced. (1)
Serous
glands
secrete a watery
proteinaceous fl uid (±igs. 3-18A and 3-19A). The parotid gland, the
gland of von Ebner of the tongue, the pancreas, and sweat glands are
examples of this type of gland. (2)
Mucous
glands
secrete mucus, a
viscous mixture of glycoprotein and water (±igs. 3-18B and 3-19B).
Goblet cells in the small and large intestines, respiratory epithelium
(±ig. 3-20), some glands in the hard and soft palates, and stomach
epithelium are examples of mucous glands. (3)
Mixed
glands
have
both serous and mucous secretions (±ig. 3-18C) and include the
submandibular gland, sublingual gland, and glands in the trachea
and esophagus. (4)
Sebaceous glands
produce lipids (±ig. 3-18D).
The sebaceous glands in the skin are good examples.
Exocrine Glands ClassiF ed by Mechanisms
of Secretion
Exocrine glands can be classiF
ed into
merocrine
secretion
,
apocrine
secretion
, and
holocrine
secretion
based on the path-
way by which the secretory products are released from the cell.
(1) In
merocrine
secretion
, the secretory product is released from
the cell by exocytosis without the loss of cell material (cyto-
plasm). The release of secretory zymogen granules by pancre-
atic acinar cells is an example of merocrine secretion. Merocrine
mechanism is the most common mode. (2) In
apocrine
secretion
,
the secretory product is released together with part of the apical
cytoplasm of the secretory cell. The lipid secretion by epithelial
cells of the mammary gland belongs to this mode of secretion.
(3) In
holocrine
secretion
, the secretory product is released by
disintegration of the entire cell. The secretory cell dies and a
new secretory cell is formed from a nearby basal cell. The fatty
lubricant secretory product,
sebum
, is released by the cells of
sebaceous glands by holocrine secretion.
Exocrine Glands ClassiF ed by Morphology
The exocrine glands also can be classiF ed into
unicellular
and
multicellular
glands
depending upon the number of cells that
form the gland.
UNICELLULAR GLANDS
are composed of only single cells.
The secretory products are released directly onto the surface of
an epithelium. Goblet cells are an example of this type of gland
(±ig. 3-20A,B).
MULTICELLULAR GLANDS
consist of numbers of secretory
cells arranged in different organizations. The multicellular glands
can be further classiF
ed into several subtypes according to their
morphology. In general, the terms
simple
and
compound
are tied
to their duct shape.
Simple glands
have unbranched ducts or
lack ducts.
Compound
glands
have branched ducts. The cells of
the multicellular glands are arranged into secretory units in the
form of
acini
or tubules.
The multicellular glands also can be classiF
ed using a com-
bination of duct shape and the shape of secretory units. (1)
Simple tubular glands
have no ducts. The secretory cells are
arranged in straight tubules (±ig. 3-21A,B). This type of gland
can be found in small and large intestines. (2)
Simple branched
tubular glands
do not have ducts, and their secretory cells are
split into two or more tubules (±ig. 3-22A,B). This type of
gland can be found in the stomach. (3)
Simple coiled tubular
glands
have a long duct, and secretory cells are formed by coiled
tubules (±ig. 3-23A,B). Sweat glands are examples of this type of
gland. (4)
Simple acinar glands
have a short, unbranched duct;
the secretory cells are arranged in acini form (±ig. 3-24A,B). The
mucus-secreting glands in the submucosa of the penile urethra
are examples of this type of gland. (5)
Simple branched acinar
glands
have a short, unbranched duct, and their secretory cells
are formed into branched acini (±ig. 3-25A,B). The sebaceous
glands of the skin belong to this type. (6)
Compound tubular
glands
have branched ducts. Their secretory cells are formed
into branched tubules as can be found in the Brunner glands
of the duodenum (±ig. 3-26A,B). (7)
Compound acinar glands
have branched ducts, and the secretory units are branched acini
(±ig. 3-27A,B). The pancreas and mammary glands are exam-
ples of this type of gland. (8)
Compound tubuloacinar glands
have branched ducts, and the secretory units are formed by both
an acinar component and a tubular component (±ig. 3–28A,B).
The submandibular and sublingual glands are good examples of
this type of gland.
Duct System
The compound glands often have complex
duct
systems
. The
secretory
acini
or
tubules
are arranged in
lobules
. The secretory
cells empty their products into small ducts called
small intral-
obular ducts
, which are often referred to as
intercalated ducts
.
The small intralobular ducts drain secretory products into
larger
intralobular ducts
, which in salivary glands are called
striated
ducts
. The striated ducts are so named because the basal cyto-
plasm of these cells often appears “striped” under the micro-
scope. The striped appearance is a result of the arrangement of
the basal cytoplasm into deep folds packed with
mitochondria
.
This organization provides the large surface area and genera-
tion of energy needed for extensive pumping of ions across the
basolateral membrane of the cell. Some glands, such as the pan-
creas, have intercalated ducts but not striated ducts. In general,
the ducts located
inside
of lobules are called
intralobular ducts
;
and ducts located
between
lobules are called
interlobular
ducts
.
The large
intralobular
ducts
feed into the interlobular ducts; the
interlobular
ducts
course through the connective
tissue (septa)
between the lobules. The secretory products then pass through
the major ducts, the
lobar
ducts
. ±inally, lobar ducts feed into
the
main
duct
of the gland and the secretory products exit the
organ.
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