Histology of Glands
Basic Terminology
Examples of glands
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Introduction to Glandular Tissue
Glands are organized arrangements
of secretory cells. All exocrine glands (and also most
endocrine glands), are composed of epithelial
tissue. Even the lung and the kidney,
although not properly glands, have a glandular pattern of tissue organization.
Although
most glands give the appearance of being "solid" tissue, their
epithelial nature is expressed by the organization
of their cells, with each cell attached laterally to its neighbors. Every
exocrine secretory cell has some portion of its plasma
membrane exposed to an external surface, communicating with the outside
of the body by way of a system of ducts.
In most glands, the secretory cells are organized
into secretory units, which are described according to their shape as tubules,
acini, or cords.
In the diagrams here and below, secretory
units are colored orange and ducts are
colored blue.
The basic organization of epithelial tissue into a gland can be best seen in a
very simple gland, like the one from frog skin shown here.
Click on here or on the image to open an enlarged
view.
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BASIC TERMINOLOGY
Histologically, glands are described using some standard vocabulary.
Simple / Compound
The simple / compound distinction is based on on duct shape.
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Acinus / Tubule
/ Cord
Each secretory unit of a gland consists of cells arranged into an acinus,
a tubule, or a cord.
Each of of these arrangements has a different and characteristic appearance
when viewed in section.
Acinus (or alveolus)
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An acinus (from Latin, grape) is a small ball of secretory
epithelial cells containing a tiny central lumen. Acini are usually
formed by serous cells. [Acini are sometimes
called alveoli, from L., small cavity.]
A typical acinar cell is shaped like a pyramid. Its basal
surface, located at the periphery of the acinus, rests on the basement
membrane, which separates the acinus from the underlying stroma.
Its lateral surfaces (the sides of the pyramid) are attached
to adjacent secretory cells. Its apical surface is free
and faces the acinar lumen, which communicates by duct
with the outside. The acinar cell's cytoplasm is also
visibly polarized, usually with basophilic basal cytoplasm and variously-staining
secretory granules concentrated in apical cytoplasm. For more,
see serous cells.
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A compound acinar gland can be quite accurately
modelled as a bunch of grapes embedded in Jello™. The grapes
are the acini, the branching stems are the ducts,
and the Jello™ represents the rest of the stroma.
Major and minor branches of the bunch represent lobes and lobules,
respectively, separated by greater amounts of connective tissue.
In routine tissue sections, most acini are cut in
random planes and look like solid lumps, made of cells having various
sizes and shapes. The lumen of an acinus is typically
quite tiny (i.e., much smaller than a cell) and so is visible only when
an acinus is sliced neatly across the middle. In such a slice,
the cells look like slices of pie, with the lumen in the center,
as in the upper thumbnail image at right.
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Tubules
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Secretory cells may also arrange themselves
into secretory tubules (in contrast to the small balls of cells
which comprise secretory acini).
Sweat glands are probably the most
familiar tubular glands. Other tubular glands include gastric
glands (in the lining of the stomach), uterine
glands, and various mucous glands of the
GI system.
Because tubules are elongated, random sections
commonly include the lumen as well as the secretory cells themselves
(in contrast to the situation with acini).
But interpretation of the sectioned appearance of tubular
glands will depend on whether the tubules are simple or branched,
on whether they are straight or twisted, and on whether or not adjacent
tubules lie parallel to one another.
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Cords
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Cords are epithelial cells arranged into sheets separated
by vascular sinusoids. In
section, the predominant pattern appears linear (hence, "cord"),
even though the lines may twist and branch.
Cords are a common arrangement for endocrine
secretion. The cells retain an epithelial character, attached
to neighboring cells, even though they may no longer comprise a
surface barrier between interstitial space and a secretory lumen
that leads to the outside. Examples of endocrine cells arranged
into cords include the epithelial cells of pancreatic
islets, parathyroid, adrenal
cortex, and liver.
The liver
(in the thumbnails) is notable for having cells arranged into cords
in spite of its major exocrine function. In
order to maintain communication with ducts, the liver
cords contain a network of intercellular channels called bile
canaliculi.
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Endocrine / Exocrine
The suffix -crine refers to secretion; the prefix endo- or
exo- tells where the secretory product goes.
The product of exocrine glands leaves the body proper, either by direct
secretion onto the body's surface (e.g., sweat) or into the lumen of an organ
(e.g., gastric juice) or else by flowing through a system of ducts
(e.g., saliva, pancreatic enzymes, bile). The cells of exocrine glands
are generally arranged into secretory units in the form of acini
or tubules (although the liver has a remarkable arrangement
of cords).
The product of endocrine glands is secreted into interstitial fluid
and hence into capillaries and general circulation. The cells of endocrine
glands are often arranged into cords adjacent to capillaries
or sinusoids.
Link to the
endocrine system.
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Serous / Mucous / Mixed
The serous / mucous distinction
is based on the secretory cell's product -- whether it is a clear, watery
solution of enzymes (serous, like serum)
or else a glycoprotein mixture (mucous, like mucin).
These two categories of secretory products come from two distinct categories
of cells, each with a characteristic appearance.
Mixed glands (e.g., most salivary glands) contain both
types of cells. Glands which contain only one of these two cell types
may be described either as serous glands (e.g., parotid
gland or pancreas) or as mucous
glands (e.g., Brunner's glands of the duodenum).
Serous Secretion
Serous cells are specialized
to secrete an enzyme solution. Examples include serous cells of the salivary
glands, exocrine cells of the pancreas,
gastric chief cells, and Paneth
cells of intestinal crypts.
Serous cells of the pancreas and the salivary glands are typically organized
into secretory units called acini.
In routine light microscopy, serous cells
are distinguished by basophilic basal cytoplasm, a centrally-located nucleus,
and variously-staining secretory vesicles (zymogen granules) in apical cytoplasm.
These features are all associated with organized mass production of
protein for export. More.
Mucous Secretion
Cells
which are specialized to secrete mucus are called mucous cells. Examples
include secretory cells of the salivary
glands, esophageal glands, stomach
surface, pyloric glands, and Brunner's
glands of the duodenum. These cells are typically organized into
tubular secretory units.
Goblet cells are mucous cells which stand alone within the intestinal epithelium.
Goblet cells take their name from their characteristic shape, with a
broad opening at the apical end and a narrow, "pinched" base. Cells
with this goblet shape are also characteristic of the respiratory tract
and the female reproductive tract.
In routine light microscopy, mucous cells are most conspicuously distinguished
by "empty"-appearing (i.e., poorly stained) apical cytoplasm and
by densely-stained, basal nuclei. (More
on mucous cells of the gastro-intestinal system.)
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Ducts
Ducts are relatively simple tubular structures which are (usually) easily distinguished
from blood vessels by their conspicuous cuboidal to columnar epithelial lining.
Blood vessels, in contrast, are lined by simple squamous endothelium.
Cells which comprise ducts generally receive much less attention
than those which actually secrete the gland's product. However,
complete understanding of a gland requires some awareness of and attention
to the duct system through which the gland drains. Ducts are not just passive "plumbing."
Some duct segments actively modify the secretory product passing through,
concentrating it by removing water.
In general, cells lining ducts may often be distinguished
by one or more of the following:
- cytoplasm rather pale (relative to most serous secretory
cells);
- nucleus centrally located (as opposed to somewhat basal
for most mucous secretory cells).
- basal and apical cytoplasm not obviously differentiated
(but this isn't always obvious for secretory cells either, and striated
ducts do have specialized basal cytoplasm);
- cells relatively short (cuboidal) relative to many (but
not all) secretory cells (and larger ducts may be lined by columnar cells);
For the purpose of describing duct
structure and function, especially in compound glands which
include branching ducts of various sizes and appearances, some special terminology
can be useful. (By and large, the distinctions that these terms allow
represent minor details rather than essential knowledge.)
Intercalated / Striated
Intercalated ducts are small ducts which drain
individual secretory units. These are usually inconspicuous, lined by
a simple epithelium consisting of low cuboidal cells.
In some glands, intercalated ducts lead to striated ducts lined by a simple
epithelium consisting of conspicuous cuboidal to columnar cells. In
the basal cytoplasm of these cells, fine striations are visible at high magnification.
The cells of the striated ducts are specialized for concentrating
the secretory product that is flowing through the duct. They do this by pumping
water and ions across the duct epithelium, from the duct lumen and into
interstitial fluid. This striated duct function is carried to extremes
in the proximal and distal tubules of the kidney.
Ultrastructurally, striated duct cells display extensive
infoldings of the basal membrane. These folds are closely associated
with mitochondria that provide ATP for the membrane pumps. In light
microscopy, the basal folds and mitochondria are sometimes visible as basal
striations, hence the name striated duct.
Secretory / Excretory
Both intercalated and striated ducts are sometimes called
secretory ducts. They are located within lobules (intralobular).
More distal ducts (interlobular), sometimes
called excretory ducts, are generally passive conducting tubes. Their
size varies, depending on how many branches have converged proximally. Larger
excretory ducts may be lined by stratified cuboidal epithelium.
It is sometimes convenient to refer to ducts
by location within the gland. The following terms are all directly descriptive.
Intra- means within. Inter-
means between. Lobes and lobules are clusters of
secretory units served, respectively, by major and minor branches of the duct
tree. Within a lobule, individual secretory units are separated from
one another by little more than basement membranes and capillaries. In
contrast, the stroma which separates lobules and lobes consists
of thicker septa of connective tissue. (The distinction between lobes
and lobules is arbitrary; lobes are evident upon gross inspection while lobules
are evident to low power microscopy.)
Intralobular
-- Located within lobules, with no more connective tissue intervening
between ducts and secretory units (i.e., acini or tubules) than between
adjacent secretory units. Intercalated and striated
ducts are intralobular.
Interlobular -- Located between lobules, within the thin
connective tissue septa that separate lobules. All interlobular ducts
are excretory.
Interlobar -- Located between lobes, within conspicuous,
thick connective tissue septa that separate lobes. All interlobar
ducts are excretory.
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Parenchyma / Stroma
The parenchyma of an organ consists of those cells which carry out
the specific function of the organ and which usually comprise the bulk of
the organ. Stroma is everything else -- connective
tissue, blood vessels, nerves, and ducts.
In most glands, the parenchyma consists of secretory epithelial cells. However,
the parenchyma / stroma distinction can be convenient for describing
not only glands but also other organs and even tumors. Examples:
- Cardiac muscle cells comprise the parenchyma of
the heart. Everything else is stroma.
- Nephrons comprise the parenchyma of the kidney.
Everything else is stroma.
- Hepatocytes comprise the parenchyma of the liver.
Everything else is stroma.
- Neurons and glia comprise the parenchyma of the brain. Everything
else is stroma.
- Cancer cells comprise the parenchyma of malignant
neoplasms. Everything else is stroma.
Because parenchyma often seems more interesting, stroma is commonly
ignored as just boring background tissue. But no organ can function
without the mechanical and nutritional support provided by the stroma. In
any gland, connective tissue and capillaries of the stroma envelop every
acinus, tubule, or cord, although they are often inconspicuous.
Pay attention to the stroma. If an organ is inflamed,
the signs of inflammation appear first in the stroma.
Historical note: Ignoring
inconspicuous tissue features can have consequences. Stromal capillaries
are seldom evident in tissue specimens. Nothing calls them to one's
attention, so they are often ignored and forgotten. Unfortunately,
just such inattention may have delayed for decades the realization that
interfering with tumor vasculature might powerfully inhibit tumor growth.
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SOME EXAMPLES of ordinary exocrine glands
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Sweat gland
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Gastric glands
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Esophageal gland
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Esophageal gland
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Pancreas
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Duodenal glands
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Salivary gland
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Salivary glands
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Parotid gland
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Uterine glands
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Note: As with all topics in this website, this sample of glands is not comprehensive.
See Other Tissues for a list of glands (and other anatomical features) which are not described on these pages.
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Comments and questions: dgking@siu.edu
SIUC / School
of Medicine / Anatomy / David
King
https://histology.siu.edu/intro/glands.htm
Last updated: 24 January 2023 / dgk
