Download Histology of the digestive system (part

Assistant lecturer: Ali Maki Hamed
Dijla University College
Histology lec4 2nd grade
Faculty of Dentistry
Histology of the digestive system (part-1)
Oral cavity. The salivary glands and their connections to the oral cavity,
morphology of the tongue in cross section, and added detail of a taste bud are
illustrated.
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Assistant lecturer: Ali Maki Hamed
Dijla University College
Histology lec4 2nd grade
Faculty of Dentistry
The digestive system is a long hollow tube or tract that starts at the oral cavity and
terminates at the anus. The system consists of the oral cavity, esophagus, stomach, small
intestine, large intestine, rectum, and anal canal. Associated with the digestive tract are the
accessory digestive organs, the salivary glands, liver, and pancreas. The accessory organs
are located outside of digestive tract. Their secretory products are delivered to the digestive
tract through excretory ducts that penetrate the digestive tract wall.
The Oral Cavity
In the oral cavity, food is ingested, masticated (chewed), and lubricated by saliva for
swallowing. Because food is physically broken down in the oral cavity, this region is lined
by a protective, nonkeratinized, stratified squamous epithelium, which also lines the inner
or labial surface of the lips.
1) The Lips
The oral cavity is formed, in part, by the lips and cheeks. The lips are lined by a very thin
skin covered by a stratified squamous keratinized epithelium. Blood vessels are close to the
lip surface, imparting a red color to the lips. The outer surface of the lip contains hair follicles,
sebaceous glands, and sweat glands. The lips also contain skeletal muscle called orbicularis
oris. Inside the free margin of the lip, the outer lining changes to a thicker, stratified squamous
nonkeratinized oral epithelium. Beneath the oral epithelium are found mucus-secreting labial
glands.
2) The Tongue: is a muscular organ located in the oral cavity. The core of the tongue
consists of connective tissue and interlacing bundles of skeletal muscle fibers. The
distribution and random orientation of individual skeletal muscle fibers in the tongue
allows for increased movement during chewing, swallowing, and speaking.
a) Papillae
The epithelium on the dorsal surface of the tongue is irregular or rough owing to
numerous elevations or projections called papillae. These are indented by the underlying
connective tissue called lamina propria. All papillae on the tongue are covered by stratified
squamous epithelium that shows partial or incomplete keratinization. In contrast, the
epithelium on the ventral surface of the tongue is smooth.
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Assistant lecturer: Ali Maki Hamed
Dijla University College
Histology lec4 2nd grade
Faculty of Dentistry
There are four types of papillae on the tongue: filiform, fungiform, circumvallate, and
foliate.
1- Filiform Papillae: The most numerous and smallest papillae on the surface of the tongue
are the narrow, conicalshaped filiform papillae. They cover the entire dorsal surface of
the tongue.
2- Fungiform Papillae: Less numerous but larger, broader, and taller than the filiform
papillae are the fungiform papillae. These papillae exhibit a mushroom-like shape and
are more prevalent in the anterior region of the tongue. Fungiform papillae are interspersed
among the filiform papillae.
3- Circumvallate Papillae: are much larger than the fungiform or filiform papillae. Eight to
12 circumvallate papillae are located in the posterior region of the tongue. These papillae
are characterized by deep moats or furrows that completely encircle them. Numerous
excretory ducts from underlying serous (von Ebner’s) glands, located in the connective
tissue, empty into the base of the furrows.
4- Foliate Papillae: are well developed in some animals but are rudimentary or poorly
developed in humans.
b) Taste Buds
Located in the epithelium of the foliate and fungiform papillae, and on the lateral sides of the
circumvallate papillae, are barrel-shaped structures called the taste buds. In addition, taste
buds are found in the epithelium of the soft palate, pharynx, and epiglottis. The free surface
of each taste bud contains an opening called the taste pore. Each taste bud occupies the full
thickness of the epithelium. Located within each taste bud are elongated neuroepithelial
(taste) cells that extend from the base of the taste bud to the taste pore. The apices of each
taste cell exhibit numerous microvilli that protrude through the taste pore. The cells that are
receptors for taste are closely associated with small afferent nerve fibers. Also present within
the confines of the taste buds are elongated supporting sustentacular cells. These cells are
not sensory.At the base of each taste bud are basal cells. These cells are undifferentiated and
are believed to serve as stem cells for the specialized cells in taste buds.
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Assistant lecturer: Ali Maki Hamed
Dijla University College
Histology lec4 2nd grade
Faculty of Dentistry
FUNCTIONAL CORRELATIONS: Tongue and Taste Buds
The main functions of the tongue during food processing are to perceive taste and to assist with mastication
(chewing) and swallowing of the food mass, called a bolus. In the oral cavity, taste sensations are detected by
receptor taste cells located in the taste buds of the fungiformand circumvallate papillae of the tongue. In
addition to the tongue, where taste buds are most numerous, taste buds are also found in themucous
membrane of the soft palate, pharynx, and epiglottis. Substances to be tasted are first dissolved in saliva
that is present in the oral cavity during food intake. The dissolved substance then contacts the taste cells
through the taste pore. In addition to saliva, taste buds located in the epithelium of circumvallate papillae are
continuously washed by watery secretions produced by the underlying serous (von Ebner’s) glands. This
secretion enters the furrow at the base of the papillae and continues to dissolve different substances, which
then enter the taste pores in taste buds. The receptor taste cells are then stimulated by coming in direct
contact with the dissolved substances and conduct an impulse over the afferent nerve fibers. There are four
basic taste sensations: sour, salt, bitter, and sweet. All remaining taste sensations are various combinations
of the basic four tastes. The tip of the tongue is most sensitive to sweet and salt, the posterior portion of the
tongue to bitter, and the lateral edges of the tongue to sour taste sensations.
3) The tonsils: The tonsils are aggregates of diffuse lymphoid tissue and lymphoid
nodules that are located in the oral pharynx.
a) Palatine tonsils are located on the lateral walls of the oral part of the
pharynx. These tonsils are lined with stratified squamous nonkeratinized
epithelium and exhibit numerous crypts. A connective tissue capsule
separates the tonsils from adjacent tissue.
b) Pharyngeal tonsil is a single structure situated in the superior and
posterior portion of the pharynx. It is covered by pseudostratified ciliated
epithelium.
c) Lingual tonsils are located on the dorsal surface of the posterior one third
of the tongue. They are several in number and are seen as small bulges
composed of masses of lymphoid aggregations. The lingual tonsils are lined
by stratified squamous nonkeratinized epithelium. Each lingual tonsil is
invaginated by the covering epithelium to form numerous crypts, around
which are found aggregations of lymphatic nodules.
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Assistant lecturer: Ali Maki Hamed
Dijla University College
Histology lec4 2nd grade
Faculty of Dentistry
4) The Salivary Glands
There are three major salivary glands: the parotid, submandibular, and sublingual. Salivary
glands are located outside of the oral cavity and convey their secretions into the mouth via
large excretory ducts. The paired parotid glands are the largest of the salivary glands,
located anterior and inferior to the external ear. The smaller, paired submandibular
(submaxillary) glands are located inferior to the mandible in the floor of the mouth. The
smallest salivary glands are the sublingual glands, which are aggregates of smaller glands
located inferior to the tongue. Salivary glands are composed of cellular secretory units
called acini (singular, acinus) and numerous excretory ducts. The secretory units are small,
saclike dilations located at the end of the first segment of the excretory duct system, the
intercalated ducts.
• Salivary Gland Acini : Cells that comprise the secretory acini of salivary glands are
of three types:
1- Serous acini: cells are pyramidal in shape. Their spherical or round nuclei are
displaced basally by secretory granules accumulated in the upper or apical regions
of the cytoplasm.
2- Mucous acini: cells are similar in shape to serous cells, except their cytoplasm is
completely filled with a light-staining, secretory product called mucus. As a result,
the accumulated secretory granules flatten the nucleus and displace it to the base
of the cytoplasm.
3- Mixed acini: both mucous and serous cells are present in the same secretory
acinus. In these mixed acini, where mucous cells predominate, serous cells form a
crescent or moonshaped cap over the mucous cells called a serous demilune. The
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Assistant lecturer: Ali Maki Hamed
Dijla University College
Histology lec4 2nd grade
Faculty of Dentistry
secretions from serous cells in the demilunes enter the lumen of the acinus through
tiny intercellular canaliculi between mucous cells.
≡ Myoepithelial cells are flattened cells that surround both serous and mucous
acini. Myoepithelial cells are also highly branched and contractile. They are
sometimes called basket cells because they surround the acini with their
branches like a basket.Myoepithelial cells are located between the cell
membrane of the secretory cells in acini and the surrounding basement
membrane.
• Salivary Gland Ducts: Connective tissue fibers subdivide the salivary glands into
numerous lobules, in which are found the secretory units and their excretory ducts.
i.
ii.
iii.
iv.
Intercalated Ducts: Both serous and mucous, as well as mixed secretory, acini
initially empty their secretions into the intercalated ducts. These are the smallest
ducts in the salivary glands with small lumina lined by low cuboidal epithelium.
Contractile myoepithelial cells surround some portions of intercalated ducts.
Striated Ducts: Several intercalated ducts merge to form the larger striated ducts.
These ducts are lined by columnar epithelium and, with proper staining.
Excretory Intralobular Ducts: Striated ducts, in turn, join to form larger
intralobular ducts of gradually increasing size, surrounded by increased layers of
connective tissue fibers.
Interlobular and Interlobar Ducts: Intralobular ducts join to form the larger
interlobular ducts and interlobar ducts. The terminal portion of these large ducts
conveys saliva from salivary glands to the oral cavity. Larger interlobular ducts may
be lined with stratified epithelium, either low cuboidal or columnar
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Assistant lecturer: Ali Maki Hamed
Dijla University College
Histology lec4 2nd grade
Faculty of Dentistry
Salivary
glands.
The
different
types of acini (serous, mucous, and
serous demilunes), different duct
types (intercalated, striated, and
interlobular), and myoepithelial
cells of a salivary gland are
illustrated.
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Assistant lecturer: Ali Maki Hamed
Dijla University College
Histology lec4 2nd grade
Faculty of Dentistry
FUNCTIONAL CORRELATIONS: Salivary Glands, Saliva, and Salivary Ducts
Salivary glands produce about 1 L/day of watery secretion called saliva, which enters the oral cavity via different
large excretory ducts. Myoepithelial cells surround the secretory acini and the intercalated ducts in the salivary
glands. On contraction, these cells expel the secretory products from different acini. Saliva is a mixture of
secretions produced by cells in different salivary glands. Although the major composition of saliva is water, it
also contains ions, mucus, enzymes, and antibodies (immunoglobulins). The sight, smell, thought, taste, or
actual presence of food in the mouth causes an autonomic stimulation of the salivary glands that increases
production of saliva and stimulates its release into the oral cavity. Saliva performs numerous important
functions. It moistens the chewed food and provides solvents that allow it to be tasted. Saliva lubricates the
bolus of chewed food for easier swallowing and assistance in its passage through the esophagus to the stomach.
Saliva also contains numerous electrolytes (calcium, potassium, sodium, chloride, bicarbonate ions, and
others). A digestive enzyme, salivary amylase, is present in the saliva. It is mainly produced by the serous acini
in the salivary glands. Salivary amylase initiates the breakdown of starch into smaller carbohydrates during the
short time that food is present in the oral cavity. Once in the stomach, food is acidified by gastric juices, an action
that decreases amylase activity and carbohydrate digestion.Saliva also functions in controlling bacterial flora
in the mouth and protecting the oral cavity against pathogens. Another salivary enzyme, lysozyme, also
secreted by serous cells, hydrolyzes cell walls of bacteria and inhibits their growth in the oral cavity. In addition,
saliva contains salivary antibodies. The antibodies, primarily immunoglobulin A (IgA), are produced by the
plasma cells in the connective tissue of salivary glands. The antibodies form complexes with antigens and assist
in immunologic defense against oral bacteria. Salivary acinar cells secrete a component that binds to and
transports the immunoglobulins from plasma cells in the connective tissue into saliva. As saliva flows through
the duct system of salivary glands, the different salivary ducts modify its ionic content by selective transport,
resorption, or secretions of ions. The intercalated ducts secrete bicarbonate ions into the ducts and absorb
chloride from its contents. The striated ducts actively reabsorb sodium from saliva, while potassium and
bicarbonate ions are added to the salivary secretions. The numerous infoldings of the basal cell membrane or
striations seen in the striated ducts contain numerous elongated mitochondria. These structures are
characteristic features of cells that transport fluids and electrolytes across cell membranes. The striated ducts of
each lobule drain into interlobular or excretory ducts that eventually form the main duct for each gland, which
ultimately empties into the oral cavity.
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