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DIGESTIVE SYSTEM
FUNCTIONS OF THE ALIMENTARY MUCOSA:
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1. Barrier function: The mucosa serves as a barrier to the
entry of noxious substances, antigens, and pathogenic
organisms.
2. Immunologic function: Lymphatic tissue in the mucosa
serves as a first line of defense of the body by the immune
system.
3. Secretory function: The lining of the alimentary canal
secretes, at specific sites, digestive enzymes, hydrochloric
acid, mucin, and antibodies.
4. Absorptive function: The epithelium of the mucosa
absorbs metabolic substrates, i.e., the breakdown products
of digestion, as well as vitamins, water, electrolytes, and
other substances.
Four types of papillae of the tongue:
Filiform papillae are the most numerous
in humans and the smallest. They are
conical,
elongated
projections
of
connective tissue that are covered with
partly keratinized stratified squamous
epithelium. They are distributed over entire
anterior dorsal surface of the tongue.
Fungiform papillae are mushroom
shaped projections located on the dorsal
surface of the tongue. They are more
numerous near the tip of the tongue.
Foliate papillae occur on the lateral
edge of the tongue. In aged humans,
the foliate papillae may not be
recognized. In younger individuals, they
are easily found.
Circumvallate papillae are the large,
dome-shaped structures that reside in
the mucosa just anterior to the sulcus
terminalis. Each papilla is surrounded
by a moat-like invagination lined with
stratified squamous epithelium.
GINGIVA
The gingiva is firmly attached to the teeth and
underlying bony tissue.
 It is composed of stratified squamous epithelium
and numerous connective tissue papillae.
 This epithelium is bound to the tooth enamel by
means of a cuticle that resembles a thick basal
lamina and forms the epithelial attachment of
Gottlieb.
 Between the enamel and the epithelium is the
gingival crevice a small deepening surrounding the
crown.
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CLASSIFICATION OF THE SALIVARY GLANDS
The major salivary glands
 parotid
 submandibular
 sublingual
The minor salivary glands
 lingual
 labial
 buccal
 molar
 palatine
SALIVARY DUCTS
Intercalated ducts are located between a secretory acinus and a larger duct
and are lined by low cuboidal epithelial cells. Several of these ducts join
to form an intralobular duct, the striated duct.
Striated duct cells have numerous infoldings of the basal plasma
membrane with numerous elongated mitochondria. Striated ducts are
lined by a simple cuboidal epithelium that gradually becomes columnar.
The infoldings of the basal plasma membrane are seen in histologic
sections as “striations”. The striated ducts of each lobule converge and
drain into the connective tissue septae separating the lobules, where
they become interlobular or excretory.
Excretory ducts travel in the interlobular and inter lobar connective tissue.
Excretory ducts constitute the principal ducts of each of the major
glands. They connect with oral cavity. The epithelium of small excretory
ducts is simple cuboidal. It gradually changes to stratified cuboidal or
pseudostratified columnar.
Diagram comparing the components of the salivon
in the three major salivary glands
The salivon is the salivary secretion unit, consisting of the terminal acini,
the intercalated duct, the striated duct, and the excretory duct
SUBMANDIBULAR GLAND
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The submandibular glands are branched tubuloacinar
gland.
Its secretory portion contains mainly serous and some
mucous cells.
The paired, large, mixed submandibular glands are
located under either side of the floor of the mouth, close
to the mandible.
A duct from each of the two glands runs toward and
medially to a papilla located on the floor of the mouth
just lateral to the frenulum of the tongue.
Intercalated ducts are less extensive than in the parotid
gland
ALIMENTARY CANAL STRUCTURE
I.
Mucosa consists of a lining epithelium, an underlying
connective tissue called lamina propria which
contains glands, vessels and elements of the
immune system, and also of muscularis mucosae,
composed of smooth muscle cells arranged as an
inner circular and an outer longitudinal layer.
II. Submucosa consists of dense irregular connective
tissue which contains the larger blood vessels and
the nerve network
DIAGRAM OF GENERAL ORGANIZATION OF THE ALIMANTARY
CANAL
ESOPHAGUS
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The esophagus is lined with a nonkeratinized stratified
squamous epithelium.
The underlying lamina propria and the muscularis mucosae
are not unique.
The submucosa along with the muscularis mucosae forms a
number of longitudinal folds and creates a highly irregular
luminal profile.
Upper one-third of the muscularis externa is striated muscle,
the middle third is striated and smooth muscle and the distal
third consists of smooth muscle.
The outer layer of esophagus in the thoracic cavity is
composed of adventitia.
After entering the abdominal cavity it is covered by serosa.
PHOTOMICROGRAPH OF THE ESOPHAGUS
STOMACH
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The stomach is an expanded part of the digestive tube
that lies under the diaphragm.
The stomach has mucosae, submucosa, muscularis
externa and a serosa.
The inner surface of the empty stomach has a number
of longitudinal folds or ridges called
In the mucosal surface is present numerous openings.
These are
or
.
The smaller regions of the mucosa are formed by
grooves or shallow trenches that divide the stomach
surface into bulging irregular areas
PHOTOGRAPH OF A HEMISECTED HUMAN
STOMACH
DIAGRAM OF GASTRIC GLANDS
FUNDIC GLANDS ARE COMPOSED OF
that give rise to the mature cells.
secrete soluble mucus compared
with insoluble or cloudy mucus produced by the
surface mucous cells.
secrete pepsin in an inactive precursor form
designated pepsinogen and a weak lipase
secrete HCL and intrinsic factor. Intrinsic
factor, a glycoprotein that is essential for the
absorption of vitamin B12.
secrete gastrin, one of the
gastrointestinal polypeptide hormones, is the principal
effective agent for stimulating the secretion of HCL.
DIAGRAM OF CHIEF CELL
Chief cells are located in the deepest part of the fundic
glands. Chief cells are typical protein-secreting cells.
DIAGRAM OF ENTEROENDOCRINE CELL
Enteroendocrine cells may be
found at any level of the
fundic gland but they tend to
be more prevalent in the
base. They are small and rest
on the basal lamina and do
not always reach the lumen.
Enteroendocrine cells secrete
their product into the lamina
propria
SMALL INTESTINE
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Mucosa consists of:
Plicae circulares are permanent transverse folds that contain a core
of submucosa.
Villi are finger-like and leaf-like projections of the mucosa that extend
into the intestinal lumen.
Microvilli of the enterocytes give the apical region of the cell a
striated appearance, the so-called striated border.
The intestinal glands or crypts of Liberkühn are simple tubular
structures. They open on to the luminal surface of the intestine of the
base of the villi.
The lamina propria contains numerous cells of the immune system
and nodules of lymphatic tissue that represent a major component of
the Gut Associated lymphoid tissue
The muscularis mucosae consist of two thin layers of smooth muscle
cells, an inner circular and an outer longitudinal layer.
ENTEROCYTES
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Structure:
They are tall columnar cells with a basally positioned
nucleus.
Microvilli of the enterocytes increase the apical surface
area as much as 600 times.
Functions:
Enterocytes are specialized for the absorption and
transport of substances from the lumen of the intestine
to the circulatory system.
Enterocytes are also secretory cells producing
glycoprotein enzymes needed for terminal digestion and
absorption.
PANETH CELLS
Structure:
 The acidophilic secretory granules contain the
antibacterial enzyme lysozyme, other
glycoproteins, an arginine rich protein and zinc.
 Functions:
 Lysozyme digests the cell walls of certain groups of
bacteria.
 This antibacterial action and the phagocytosis of
certain bacteria and protozoa by Paneth cells
suggest that they have a role in regulating the
normal bacterial flora of the small intestine
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ENTEROENDOCRINE CELLS
Functions:
 Cholecystokinin, secretin and gastric inhibitory
peptide are the most active regulators of
gastrointestinal physiology that are released in
this portion of the gut.
 These three hormones increase pancreatic and
gallbladder activity and inhibit gastric secretory
function and motility.
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INTERMEDIATE CELLS
Structure:
 . These cells have short, irregular microvilli and
small mucin-like secretory droplets which form a
column in the center of the supranuclear
cytoplasm.
 Intermediate cells have characteristics of both
immature absorptive cells and goblet cells.
 Function:
 These cells are still capable of cell division
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SUBMUCOSA CONSISTS OF:
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A dense connective tissue and localized sites that contain
aggregates of adipose cells
Submucosal branched tubuloalveolar glands (of Brunner)
The secretion of these glands contains neutral and alkaline
glycoproteins and bicarbonate ions.
Function:
Secretion of these glands serves to protect the proximal
small intestine by neutralizing the acid-containing chime that
is delivered to it
Secretion of these glands serves to bring the Ph of the
intestinal contents close to the optimal pH for the pancreatic
enzymes that are also delivered to the duodenum.
Muscularis externa consists of:
 an inner layer of circularly arranged smooth
muscle cells
 an outer layer of longitudinally arranged
smooth muscle cells
 Serosa
 Serosa is not unique.
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