The Pancreas and Salivary Glands
- Recognize and distinguish the pancreas and salivary glands.
- Distinguish each of the 3 salivary glands.
- Understand the structure of the glandular elements and the relationship
between structure and function.
The glands of the oral cavity may be classified as serous, mucous or mixed
according to the nature of their secretory product(s). The basic unit of structure
is a saccular or tubular arrangement of mucous and serous secretory epithelial
cells enclosing a lumen. One or several of these secretory units may empty into
a small intralobular duct which joins others to form progressively larger intralobular,
the interlobular ducts and finally the interlobar and excretory ducts. Portions
of the intralobular ducts are involved in altering the ionic composition of
the secretion. The glands are usually well vascularized, and are under control
of the autonomic nervous system.
In H & E stained sections, the cells making up a mucous alveolus are columnar
or pyramidal, with colorless or mildly basophilic non-granular cytoplasm. The
nucleus is very flattened at the extreme base of the cell. Cell boundaries between
adjacent cells are usually clear. In a serous acinus, the cell boundaries are
not distinct, and the component pyramidal cells are usually smaller and more
basophilic than those of mucous acini. The nucleus lies in the basal cytoplasm,
but is oval or round and not flattened against the base of the cell. The apical
cytoplasm is less abundant, and ideally contains an accumulation of acidophilic
secretory granules although these are often lost in preparation.
Since the precise chemical nature of mucous and serous secretions of various
organs can differ, mucous and serous acini from various locations are not necessarily
identical in histological appearance, although the above descriptions include
overall characteristics generally observed.
Secretory acini are scattered throughout the oral cavity, but three groups
of acini form named anatomical entities, the three paired salivary glands. Of
these, the parotids and submandibulars (submaxillaries) are more clearly defined
as distinct organs, while the sublinguals are embedded within the tongue.
The salivary glands produce about 12 liters of saliva per day.
Study the parotid gland (slide #38). Under very low magnification
note the separation of the gland into lobules by septa of
connective tissue containing large, pink interlobular ducts as well as
blood vessels and nerves. Often the septa appear to be embedded within
the lobules as the architecture is not completely regular. With the 10x
lens, note that this gland is composed predominantly of dark-staining
serous acini and ducts. The latter are the acidophilic (pink) structures
with round nuclei in the center of tall cells. Examine serous
acini and note the rounded basally located nuclei and dark granular
staining of the cytoplasm. Usually the gland lumen is not easily
seen. The acini lie in a fine network of connective tissue. The absence
or scarcity of mucous acini and the presence of adipose tissue
between acini, i.e., in intralobular locations, aid in the identification
of the parotid gland. The lumen of each individual acinus opens into an
intercalated duct (first portion of the intralobular duct) which
is lined with low cuboidal, pale-staining cells. In cross-section they
are smaller than secretory acini, are more acidophilic, and tend to have
more centrally located nuclei. Try to find longitudinal sections showing
the continuity between secretory acini and intercalated ducts. More often,
the continuity between intercalated ducts and the next portion of the
duct system, the striated ducts, can be seen. Striated ducts are
lined by cuboidal to columnar acidophilic cells and are so-named
because of characteristic striations in the basal portions of the cells
which are due to the alignment of mitochondria between deep infoldings
of the basal cell membrane. The nucleus is in the central part of the
cell above the striations. The latter are sometimes difficult to discern
because of postmortem changes.
|Striated & Inter- calated ducts
EM of basal region of striated duct
With the high-dry or oil immersion lens, note myoepithelial
cells with long, flattened nuclei (small and triangular in cross section)
and little cytoplasm, visible in some acini and ducts; they are located between
the bases of adjacent secretory cells within the basement membrane. In contrast,
the nuclei of the serous epithelial cells are round and lie above the basement
membrane more or less in the center of the acinar or duct cell. Myoepithelial
cells are not found in the pancreas.
The striated ducts empty into interlobular ducts in the connective
tissue septa where they become larger in diameter. Interlobular ducts
fuse to form progressively larger structures. Those with walls consisting
of multiple layers of cells are called excretory ducts. The cells bordering
the lumen in these ducts usually have rounded apical surfaces.
Study the submandibular gland (slide #39). The duct system is
similar to that of the parotid. Note that this gland is composed mostly
of dark-staining (purple) serous acini, but also contains a substantial
number of pale (pink) staining mucous acini. In general, the nuclei
of mucous cells are flattened and lie close to the basement membrane.
Contrast those with the rounded, more central nuclei of serous acini.
Groups of dark-staining serous cells form caps called "serous
demilunes" often seen in sections as crescents which partially
surround some of the mucous acini and empty their secretion into a common
lumen. Other features are as described for the parotid gland.
The sublingual gland (slide 48), even, is a mixed gland but
predominantly mucous. It has very short striated ducts which are extremely
difficult to find and lacks intercalated ducts. The intralobular ducts
present are called "mucous tubules" and cannot be distinguished
from the secretory end-pieces in conventional sections. Excretory ducts
of varying sizes may be seen as well as some skeletal muscle.
The pancreas is a compound tubular-alveolar gland composed of serous acini
(exocrine portion), and islets of Langerhans (endocrine portion). The islets
of Langerhans are collections of cell cords separated from the exocrine acinar
tissue by a thin connective tissue sheath.
The acini and their ducts resemble a bunch of grapes. The ducts lie in dense
connective tissue septa which provide the main support of the gland. The vascular
and nervous supply of the acini (and islets) are also located in these septa.
Branches of these vessels terminate as capillary networks around the acini and
form dense networks within the islets. The capillaries, in both cases, are of
the fenestrated type.
The alveoli (acini) are groups of cells which vary in shape from spheres to
elongated tubes and may appear quite irregular in some sections. The cells are
pyramidal in shape and surround a central lumen which is usually difficult to
visualize. During secretory activity, acidophilic granules fill the apical portions
of the cells, while mitochondria and the endoplasmic reticulum with its associated
basophilic RNA are located basally. The acini and ducts are enclosed by a basement
Examine the human pancreas (slide #41) at low magnification
and note its lobular nature. Some regions may be poorly preserved. The
connective tissue capsule may or may not be present but note the
thin septa penetrating into the gland, dividing it first into lobes
and then lobules. Distinguish between basophilic exocrine
and the slightly acidophilic endocrine tissue. The Islets of
Langerhans vary in size and usually appear as round cords of light
pink cells. At 40x magnification note the different appearance of islets
and exocrine cells. The islets make up only one percent of the total pancreatic
mass and are not uniformly distributed within the organ. It is possible
to have sections lacking islets. Try to find a more or less round cluster
of exocrine cells forming an acinus. Note the details of the secretory
cells such as the dense basal basophilia and the brightly eosinophilic
secretory granules in the apical half of the cells. A capillary network
surrounds the acini but is difficult to distinguish.
Locate centroacinar cells of the exocrine pancreas. These
are duct cells which form the junction between the secretory endpiece
and the duct. The ease of identification will depend upon the staining,
so study both slides #40 and #41. Find well-defined acini in a well preserved
region of the slide. The nuclei of centroacinar cells appear to be located
in the center of the acini and mark the beginning of the intralobular
ducts. Occasionally longitudinal sections of the first, narrow, free portions
of the intralobular ducts can be observed emerging from the acini. Find the intralobular ducts among the closely packed acini.
The pancreas lacks striated ducts and the intralobular duct is similar
to the intercalated duct of salivary glands. It often has a collapsed
lumen and the cells are cuboidal with little cytoplasmic staining in
contrast to acinar cells. They are responsible for secretion of a bicarbonate-rich
fluid. The pancreas also lacks myoepithelial cells. Finally,
the interlobular ducts can be seen in the connective tissue septa
along with some blood vessels.
||The different types of secretory cells in the islets of Langerhans are difficult
to differentiate in H & E sections. Examine a section of guinea pig pancreas
(slide #40), which has been especially fixed and stained for this purpose.
The acinar cells stain a deep purple and the cords of secretory cells in the
islets include alpha (A)-cells filled with red-staining granules containing
glucagon and beta (B) cells filled with blue-green staining granules that contain