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Trait 4

1. Digestive Tract 4

3. The main functions of the salivary glands are to moisten and
lubricate ingested food and the oral mucosa, to initiate the
digestion of carbohydrates and lipids with amylase and lipase, and
to secrete innate immune components such as lysozyme and
lactoferrin.
Exocrine glands in the mouth produce saliva, which has digestive,
lubricating, and protective functions. With a normal pH of 6.5 to
6.9, saliva also has an important buffering function and in some
species is also important for evaporative cooling.

4. Major salivary glands
About 90% of saliva is produced by three bilateral pairs of salivary
glands: the parotid, submandibular, and sublingual glands.
Locations and relative sizes of these glands are shown here
diagrammatically. These glands plus microscopic minor salivary
glands throughout the oral mucosa produce 0.75-1.50 L of saliva
daily.

5. Three epithelial cell types comprise the salivary
secretory units:
■ Serous cells are polarized protein-secreting cells, usually
pyramidal in shape, with round nuclei, well-stained RER, and
apical secretory granules. Joined apically by tight and
adherent junctions, serous cells form a somewhat spherical
unit called an acinus (L. grape), with a very small central
lumen. Serous acinar cells secrete enzymes and other
proteins.

6. ■ Mucous cells are somewhat more columnar in shape,
with more compressed basal nuclei. Mucous cells contain
apical granules with hydrophilic mucins that provide
lubricating properties in saliva but cause poor cell staining
in routine preparations. Mucous cells are most often
organized as cylindrical tubules rather than acini. Mixed
salivary glands have tubuloacinar secretory units with both
serous and mucous secretion.

7. ■ Myoepithelial cells are found inside the basal lamina
surrounding acini, tubules, and the proximal ends of the
duct system. These small, flattened cells extend several
contractile processes around the associated secretory unit
or duct and their activity is important for moving secretory
products into and through the ducts.

9. ❖Intralobular duct system
❖Intercalated ducts
❖Striated ducts
❖Excretory ducts

10. The secretory portions are composed of pyramidal serous (violet) and mucous (tan) cells.
Serous cells are typical protein-secreting cells, with rounded nuclei, accumulation of rough ER
in the basal third, and an apex filled with protein-rich secretory granules. The nuclei of mucous
cells, flattened with condensed chromatin, are located near the bases of the cells. The short
intercalated ducts are lined with cuboidal epithelium. The striated ducts are composed of
columnar cells with characteristics of ion-transporting cells: basal membrane invaginations
with mitochondrial accumulations. Myoepithelial cells are shown around the serous acini.

11. ■ Parotid glands, located in each cheek near the ear, are branched acinar
glands with exclusively serous acini. Serous cells of parotid glands secrete
abundant α-amylase that initiates hydrolysis of carbohydrates and
proline-rich proteins with antimicrobial and other protective properties.
■ Submandibular glands, which produce two-thirds of all saliva, are branched
tubuloacinar glands, having primarily serous acini, but with many mixed
tubuloacinar secretory units. Serous cells of these mixed units are mostly in
peripheral crescentshaped groups called serous demilunes, aggregates
formed at least in part artifactually when the adjacent mucous cells swell
during routine tissue preparation. Basolateral membrane in foldings of the
serous cells facilitate electrolyte and water transport. In addition to
α-amylase and proline-rich proteins, serous cells of the submandibular gland
secrete other enzymes, including lysozyme for bacterial wall hydrolysis.
■ Sublingual glands, the smallest of the major glands, are also considered
branched tubuloacinar glands, but here secretory tubules of mucous cells
predominate and the main product of the gland is mucus. The few serous
cells present in demilunes add amylase and lysozyme to the secretion.

12. The large parotid gland consists
entirely of serous acini with cells
producing amylase and other proteins
for storage in secretory granules.
Micrograph of a parotid gland shows
densely packed serous acini (A) with
ducts. Secretory granules of serous
cells are clearly shown in this plastic
section, as well as an intercalated
duct (ID) and striated duct (SD), both
cut transversely.
Parotid gland

13. Striations of a duct (SD) are
better seen here, along with
A septum (CT) and numerous
serous acini (A). The connective
Tissue often includes adipocytes.

14. Ultrastructure of serous and mucous cells
A micrograph of a mixed
acinus from a submandibular
gland shows both serous
and mucous cells surrounding
the small lumen (L).
Mucous cells (M) have large,
hydrophilic granules like
those of goblet cells, while
serous cells (S) have
small, dense granules.
Small myoepithelial cells
(My) extend contractile
processes around
each acinus. X2500.

15. The submandibular gland
is a mixed serous and
mucous gland (serous
cells predominate), and
shows well-stained serous
acini (A) and serous demilunes
(S) and pale-staining mucous
cells (M) grouped as tubules in
this tubuloacinar gland.
Small intralobular ducts (ID)
drain each lobule.

16. The sublingual gland is
a mixed but largely
mucous gland with
a tubuloacinar
arrangement of poorly
stained mucous cells (M).
Small intralobular ducts (ID)
are seen in connective
tissue, as well as small
fascicles of lingual striated
muscle (SM).

17. Striated ducts

18. Pancreas and deodenum
The main regions of the pancreas are shown in relation to the two
pancreatic ducts and the duodenum.
Micrographs show a pancreatic islet and several pancreatic acini.

19. Pancreas
Low-power view of pancreas
includes several islets
surrounded by many
serous acini (A).
The larger intralobular
ducts (D) are lined by simple
columnar epithelium.
The ducts and blood
vessels (V) are located in
connective tissue, which
also provides a thin capsule
to the entire gland and thin
septa separating the lobules
of secretory acini.

20. Pancreatic acini
Micrograph of exocrine pancreas
shows the serous,
enzyme-producing cells arranged
in small acini (A) with very small
lumens. Acini are surrounded
by only small amounts
of connective tissue with
fibroblasts (F).
Each acinus is drained by an
intercalated duct
with its initial cells, the
centroacinar cells (arrow),
inserted into the acinar lumen.

21. The diagram shows the
arrangement of cells more
clearly.
Under the influence of
secretin, the centroacinar
and intercalated duct cells
secrete a copious HCO3
rich fluid that hydrates,
flushes, and alkalinizes
the enzymatic secretion
of the acini.

22. Pancreatic acinar cell ultrastructure
TEM of a pancreatic acinar cell shows
its pyramidal shape and the round,
basal nucleus (n) surrounded by
cytoplasm packed with cisternae of
rough ER (RER). The Golgi apparatus
(G) is situated at the apical side of the
nucleus and is associated with
condensing vacuoles (C) and numerous
secretory granules (S) with zymogen.
The small lumen (L) of the acinus
contains proteins recently released
from the cell by exocytosis. exocytosis
of digestive enzymes from secretory
granules is promoted by CCK, released
by enteroendocrine cells of the
duodenum when food enters that
region from the stomach.

23. Pancreatic tissue is protected against autodigestion by
the following:
▪Restricting protease activation to the duodenum,
▪Trypsin inhibitor, which is copackaged in the secretory
granules with trypsinogen,
▪The higher pH in the acini and duct system due to HCO
3
-
secreted by the centroacinar and intercalated duct cells,
which helps keep all the enzymes inactive.

24. Exocrine secretion in the pancreas is regulated mainly
through two polypeptide hormones produced by
enteroendocrine cells of the small intestine:
■ Cholecystokinin (CCK) stimulates enzyme secretion by the
acinar cells.
■ Secretin promotes water and HCO
3− (
bicarbonate ion)
secretion by the duct cells.

25. Salivary Glands
■ Salivary glands have secretory units of either
protein-secreting serous cells, usually organized in
round or oval acini, or of mucin-secreting mucous
cells in elongated tubules.
■ Parotid glands have only serous acini; sublingual
glands are mixed but have primarily mucous tubules,
some with serous demilunes; submandibular glands
are also mixed but have mainly serous acini.

26. ■ Salivary secretory units are drained by simple
cuboidal intercalated ducts that merge as simple
columnar striated ducts, which merge as larger
interlobular or excretory ducts.
■ Cells of striated ducts have mitochondria-lined,
basolateral membrane folds specialized for
electrolyte reabsorption from the secretion;
excretory ducts are unusual in having stratified
cuboidal or columnar cells.

27. Pancreas
■ Pancreatic islets of endocrine cells are embedded in
exocrine serous acinar tissue, which comprises most of
the pancreas and in which the cells secrete hydrolytic
digestive enzymes for delivery to the duodenum.
■ Each pancreatic acinar cell is pyramidal, with
secretory (zymogen) granules in the narrow apical end
and Golgi complexes, much rough ER, and a large
nucleus at the basal end.
■ Intercalated ducts draining pancreatic acini, including
their initial centroacinar cells that insert into the acinar
lumen, secrete bicarbonate ions (HCO
3−
) to neutralize
chyme entering the duodenum from the stomach.

29. Formation of glands from covering epithelia

32. Mechanisms of exocrine gland secretion

33. Serous cells

34. Mucous cells

35. Myoepithelial cells

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