Download Digestive System: Oral Cavity and Salivary Glands

Digestive System: Oral
Cavity and Salivary Glands
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 intes­
tine, 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 ( Overview Figure 1 l.l: Oral Cavity).
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.
The Lips
The oral cavity is formed, in part, by the lips and cheeks. The lips are lined by a very thin skin cov­
ered 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.
The Tongue
The tongue is a muscular organ located in the oral cavity. The core of the tongue consists of con­
nective 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 dur­
ing chewing, swallowing, and speaking.
Papillae
The epithelium on the dorsal surface of the tongue is irregular or rough owing to numerous ele­
vations 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.
There are four types of papillae on the tongue: filiform, fungiform, circumvallate, and foliate.
Filijorm Papillae
The most numerous and smallest papillae on the surface of the tongue are the narrow, conical­
shaped filiform papillae. They cover the entire dorsal surface of the tongue.
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PART II - ORGANS
Fungijorm Papillae
Less numerous but larger, broader, and taller than the filiform papillae are the fungiform papil­
lae. 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.
Circumvallate Papillae
Circumvallate papillae are much larger than the fungiform or filiform papillae. Eight to 12 cir­
cumvallate papillae are located in the posterior region of the tongue. These papillae are charac­
terized 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.
Foliate Papillae
Foliate papillae are well developed in some animals but are rudimentary or poorly developed in
humans.
Taste Buds
Located in the epithelium of the foliate and fungiform papillae, and on the lateral sides of the cir­
cumvallate 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 (Overview Figure 11.1, Oral Cavity).
Lymphoid Aggregations: Tonsils (Palatine, Pharyngeal, and Lingual)
The tonsils are aggregates of diffuse lymphoid tissue and lymphoid nodules that are located in the
oral pharynx. The 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. The pharyngeal
tonsil is a single structure situated in the superior and posterior portion of the pharynx. It is cov­
ered by pseudostratified ciliated epithelium. The 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 squa­
mous nonkeratinized epithelium. Each lingual tonsil is invaginated by the covering epithelium to
form numerous crypts, around which are found aggregations of lym phatic nodules.
CHAPTER 11 - Digestive System: Oral Cavity and Salivary Glands
239
6 Filiform papillae
7 Skeletal muscle
8 Blood vessels:
�����',M.�-- a. Artery
b. Vein
""""�����'*""I---
10 Anterior lingual gland:
a. Interlobular ducts
�fl"---- b. Mucous acinus
.,,.,____ c. Serous acinus
12 Excretory duct of the
lingual gland
FIGURE 11.2 • Anterior region of the tongue (longitudinal section). Stain: hematoxylin and eosin.
Low magnification.
1 Stratified
squamous
epithelium
7 Secondary
papillae
2 Lingual
epithelium
8 Blood
vessels
3 Lamina propria
9 Taste buds
4 Taste buds
10 Furrow
5 Furrow
11 Serous
(von Ebner's)
glands:
a. Excretory
ducts
b. Serous
secretory
acini
6 Serous
(von Ebner's)
glands:
a. Excretory
ducts
b. Serous
secretory
acini
•
12 Skeletal
muscles:
a. Longitudinal
b. Transverse
FIGURE 11.3 • Posterior tongue: circumvallate papilla, surrounding furrow, and serous (von Ebner's)
glands (cross section). Stain: hematoxylin and eosin. Medium magnification.
FUNCTIONAL CORRELATIONS: Tongue and Taste Buds
The main functions ofthe tongue during food processing are to perceive taste and to assist with
mastication (chewing) and swallowing ofthe food mass, called a bolus. In the oral cavity, taste sen­
sations are detected by receptor taste cells located in the taste buds ofthe fungiform and circum­
vallate papillae of the tongue. In addition to the tongue, where taste buds are most numerous,
taste buds are also found in the mucous membrane ofthe 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 continu­
ously washed by watery secretions produced by the underlying serous (von Ebner's) glands.
This secretion enters the furrow at the base ofthe 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 sen­
sations are various combinations of the basic four tastes. The tip of the tongue is most sensi­
tive to sweet and salt, the posterior portion ofthe tongue to bitter, and the lateral edges ofthe
tongue to sour taste sensations.
CHAPTER 11 - Digestive System: Oral Cavity and Salivary Glands
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The Major 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.
Cells of the Salivary Gland Acini
Cells that comprise the secretory acini of salivary glands are of two types: serous or mucous
(Overview Figure 11.2, Salivary Glands).
Serous cells in the acini are pyramidal in shape. Their spherical or round nuclei are dis­
placed basally by secretory granules accumulated in the upper or apical regions of the cytoplasm.
Mucous 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 gran­
ules flatten the nucleus and displace it to the base of the cytoplasm.
In some salivary glands, both mucous and serous cells are present in the same secretory aci­
nus. In these mixed acini, where mucous cells predominate, serous cells form a crescent or moon­
shaped cap over the mucous cells called a serous demilune. The 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.
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, exhibit tiny basal striations. The striations corre­
spond to the basal infoldings of the cell membrane and the cellular interdigitations. Located in
these basal infoldings of the cell membrane are numerous and elongated mitochondria.
Excretory lntralobular Ducts
Striated ducts, in turn, join to form larger intralobular ducts of gradually increasing size, sur­
rounded by increased layers of connective tissue fibers.
Interlobular and lnterlobar 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 ( Overview Figure 11.2: Salivary Glands).
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PART II - ORGANS
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 antibod­
ies (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 numer­
ous electrolytes (calcium, potassium, sodium, chloride, bicarbonate ions, and others). A diges­
tive 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 carbohy­
drates 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 pro­
duced 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 aci­
nar 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 inter­
calated 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 mem­
brane 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 eventu­
ally form the main duct for each gland, which ultimately empties into the oral cavity.
CHAPTER 11
Summary
The Digestive System
Toste Buds
• Hollow tube from oral cavity to anal canal
• Salivary glands, liver, and pancreas are accessory organs
located outside of the tube
• Secretory products from accessory organs delivered to the
tube via excretory ducts
• Located in foliate, fungiform, circumvallate papillae, phar­
ynx, palate, and epiglottis
• Contain taste pores and occupy the thickness of the epithe­
lium
• Neuroepithelial cells associated with afferent axons are the
receptors for taste
• Also contain supportive sustentacular cells, whereas basal
cells can serve as stem cells
• Substances that are tasted are first dissolved in saliva and
then enter taste pore
• Serous glands wash peripheral taste buds in the furrows of
circumvallate papillae
• Basic four taste sensations are sour, salt, bitter, and sweet
• Tip of tongue is sensitive to sweet and sour; posterior tongue
to bitter, and lateral to sour taste
The Oral Cavity
• Lined by stratified squamous epithelium for protection
• Food masticated here, and saliva lubricates food for swal­
lowing
The Lips
• Lined by thin skin covered by stratified squamous kera­
tinized epithelium
• Blood vessels close to the surface impart red color
• Contain hairs, sebaceous and sweat glands, and mucus­
secreting labial glands
• Core contains skeletal muscle orbicularis oris
The Tongue
• Consists of interlacing skeleton muscle fibers
• Surface covered by surface elevations, called filiform, fungi­
form, and circumvallate papillae
• Filiform papillae are the most numerous and smallest that
cover tongue; lack taste buds
• Fungiform papillae less numerous, larger with mushroom­
like shape, and contain taste buds
• Circumvallate papillae are the largest, are in the back of
tongue, and have furrows, underlying serous glands, and
taste buds
• Foliate papillae are rudimentary in humans
• Posterior lingual glands in the connective tissue open onto
dorsal surface of tongue
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Lymphoid Aggregations: Tonsils
• Diffuse lymphoid tissue and nodules in the oral pharynx
• Palatine and lingual tonsils are covered by stratified squa­
mous epithelium and show crypts
• Pharyngeal tonsil is single and covered by pseudostratified
ciliated epithelium
• Some lymph nodules contain germinal centers
Teeth
• Developing teeth found in dental alveolus in the jawbone
• Downward growth from oral epithelium forms dental lam­
ina, which gives rise to ameloblasts
• Mesenchyme gives rise to dental papilla and odontoblasts
• Odontoblasts secrete dentin, whereas ameloblasts produce
enamel of tooth
CHAPTER 11 - Digestive System: Oral Cavity and Salivary Glands
The Major Salivary Glands
• Parotid, submandibular, and sublingual are major salivary
glands that produce saliva
Composed of secretory acini and excretory ducts that bnng
saliva from outside into oral cavity
• Cells are either serous or mucous; serous cells form serous
demilunes around mucous acini
• Contractile myoepithelial cells surround serous and mucous
acini and intercalated ducts
• Serous, mucous, and mixed secretory acini empty secretions
into intercalated ducts
• Intercalated ducts merge into larger striated ducts with basal
membrane infoldings
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• Striated ducts form larger interlobular ducts that empty into
interlobar excretory ducts
• Glands produce about I L of saliva per day, which is mostly
water
• Saliva formed after autonomic stimulation
• Saliva contains electrolytes and carbohydrate-digesting
enzyme salivary amylase
• Saliva contains antibodies produced by connective tissue
plasma cells and lysozyme to control oral bacteria
Saliva is modified by selective transport of ions in the inter­
calated ducts and striated ducts
• Sodium is reabsorbed from saliva, and potassium ions and
bicarbonate ions are added to saliva