Download Salivary Glands

Salivary Glands
Saliva is a mixed secretion, which is derived from numerous large and small salivary
glands that all open into the oral cavity. Small salivary glands are situated in the
connective tissue beneath the epithelia lining the oral cavity, and, in the case of the
tongue, they may also be found between the muscular tissue. Depending on the
localisation they are grouped into lingual, labial, buccal, molar and palatine glands.
The large salivary glands form three paired groups:
1. the sublingual glands, which are positioned beneath the tongue and embedded deeply in
the connective tissue of the oral cavity,
2. the submandibular glands
3. the parotid glands, which lie outside the oral cavity.
All of these glands are tubuloacinar glands, i.e. they have secretory acini but the first
part of the duct system originating from the acini also participates in the secretory
process. The salivary glands are divided by connective tissue septa into lobes, which are
further subdivided into lobules.
Functionally the secretory acini can be divided into two groups: those that secrete a
rather liquid product - serous acini, and those that secrete a very viscous product mucous acini. This functional differentiation is reflected in the appearance of these acini
in histological sections.
• The cells forming the serous acini contain a round or slightly ovoid nucleus which is
placed basally in the cell. In an H&E stain, the apical cytoplasm may appear pinkish/red
or, in well-preserved tissue, contain reddish granules. The granules represent the vesicles
which contain the secretory products of the cell. The digestive enzyme α-amylase is also
secreted by the acinar cells.
• The cells forming the mucous acini usually contain flattened nuclei which appear to be
"pressed" against the basal surface of the cell. Secretory vesicles fill much of the apical
cytoplasm. The secretory product has either been dissolved during the staining process or
remains unstained. Small amounts of cytoplasm which remain between the vesicles gives
the apical part of the cell a distinct "spongy" appearance.
Occasionally, and in particular in glands located relatively close to the oral cavity, serous
cells and mucous cells may form compound or mixed acini. The serous cells form in
these cases small half-moon or crescent-shaped structures, which attach to mucus
producing acini and empty their secretory product into interstices between the mucusproducing cell. Following their appearance they are called serous demilunes.
Both serous and mucous acini and parts of the secretory duct system are surrounded by
myoepithelial cells which by their contraction participate in the secretory process. They
are usually difficult to distinguish in histological sections.
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Glands located close to the oral cavity have mainly mucous secretions, whereas glands
located further away from the oral cavity have mainly serous secretions. Following this
general rule, the parotid glands contain almost exclusively serous acini, the
submandibular glands contain both serous and mucous acini, and the sublingual glands
Ducts of the Salivary Glands
The ducts of the salivary glands can, according to their position in relation to the lobes
and lobules of the glands, be divided into two parts. Interlobular and contain mainly
mucous acini or mucous acini with serous demilunes.interlobar ducts are embedded in
the connective tissue surrounding the lobes and lobules of the glands. Intralobular ducts
are located in between the secretory acini within the lobules and, consequently, only
surrounded by scant, if any visible connective tissue.
Interlobar and interlobular ducts function mainly in the conduit of the saliva and are
formed by a stratified cuboidal or stratified columnar epithelium. The epithelium is
replaced by the stratified squamous epithelium as they approach the opening into the oral
cavity.
The product of serous glands is extensively modified by the initial part of the duct system.
Intralobular ducts can on the basis of their function be divided into intercalated ducts and
striated ducts. The secretory acini empty into intercalated ducts which merge into the
striated ducts.
• Cells forming the intercalated ducts add bicarbonate ions to the saliva (buffering
function) and absorb chloride from the saliva. They are typically formed by cuboidal
epithelium.
• Striated ducts are formed by columnar cells. In contrast to many other columnar
epithelia, the nucleus of these cells is located approximately midways between the apical
and basal cell surfaces. The striations of the striated duct are found in the basal part of the
cytoplasm of the cells where numerous mitochondria are found between infoldings of the
basal cell membrane. This specialisation provides the cell with the necessary energy and
surface area to perform its task in the modification of the saliva - the secretion of
potassium and the absorption of sodium. Cells of the striated ducts also take up a
secretable form of antibodies and release them into the saliva.
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In tercalated ducts are difficult to identify in mucous glands and striated ducts are absent
in purely mucous glands. Following the main secretory product of the major salivary
glands, well-differentiated intercalated and striated ducts are a prominent feature of the
parotid glands, rare in the submandibular glands and absent in the sublingual gland. An
additional feature that may aid in the identification of the parotid gland are fairly large
amounts of adipose tissue which is found between the secretory tissue of the lobules.
Parotid Gland - H&E,
Submandibular Gland - H&E
Sublingual Gland - H&E
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Stomach
The stomach is a J shaped expanded bag, located just left of the midline between the
oesophagus and small intestine. It is divided into four main regions and has two borders called the
greater and lesser curvatures. The first section is the cardia which surrounds the cardial orifice where
the oesophagus enters the stomach. The fund us is the superior, dilated portion of the stomach that
has contact with the left dome of the diaphragm. The body is the largest section between the fundus
and the curved portion of the J.
This is where most gastric glands are located and where most mixing of the food occurs. Finally the
pylorus is the curved base of the stomach. Gastric contents are expelled into the proximal duodenum
via the pyloric sphincter. The inner surface of the stomach is contracted into numerous longitudinal
folds called rugae. These allow the stomach to stretch and expand when food enters. The stomach
can hold up to 1.5 litres of material. The functions of the stomach include:
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The short-term storage of ingested food.
Mechanical breakdown of food by churning and mixing motions.
Chemical digestion of proteins by acids and enzymes.
Stomach acid kills bugs and germs.
Some absorption of substances such as alcohol.
Most of these functions are achieved by the secretion of stomach juices by gastric glands in the body
and fundus. Some cells are responsible for secreting acid and others secrete enzymes to break down
proteins.
The wall is divided into four layers as follows:
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Mucosa
The innermost layer of the digestive tract has specialised epithelial cells supported by an underlying
connective tissue layer called the lamina propria. The lamina propria contains blood vessels, nerves,
lymphoid tissue and glands that support the mucosa. Depending on its function, the epithelium may
be simple (a single layer) or stratified (multiple layers).
Simple columnar (tall) or glandular epithelium lines the stomach and intestines to aid secretion and
absorption. The inner lining is constantly shed and replaced, making it one of the most rapidly
dividing areas of the body! Beneath the lamina propria is the muscularis mucosa. This comprises
layers of smooth muscle which can contract to change the shape of the lumen.
Submucosa
The submucosa surrounds the muscularis mucosa and consists of fat, fibrous connective tissue and
larger vessels and nerves. At its outer margin there is a specialized nerve plexus called the
submucosal plexus or Meissner plexus. This supplies the mucosa and submucosa.
Muscularis externa
This smooth muscle layer has inner circular and outer longitudinal layers of muscle fibres separated
by the myenteric plexus or Auer Bach plexus. Neural innervations control the contraction of these
muscles and hence the mechanical breakdown and peristalsis of the food within the lumen.
Serosa/mesentery
The outer layer of the GIT is formed by fat and another layer of epithelial cells called mesothelium
Small Intestine
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The small intestine is where most digestion takes place. It is structurally divided into three parts: the duodenum,
the jejunum, and the ileum. Among humans over five years old, the small intestine tends to vary in length from
4-7 meters (13-23 feet).
The Duodenum
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The duodenum consists of four parts, with the first three forming a "C" shape.
o The first or superior part of the duodenum begins at the pylors, passing laterally for a short
distance before curving into the superior duodenal flexure.
o The second, or descending, part of the duodenum passes from the superior into the inferior
duodenal flexure, and is where the pancreatic and common bile ducts enter the GI tract.
o The third or inferior horizontal part of the duodenum passes from the inferior flexure, crossing the
aorta (major artery) and inferior vena cava (major vein) as well as the spinal column.
o The forth or ascending part of the duodenum passes over the aorta, and curves past the pancreas to
the duodenojejunal flexure.
o The duodenum is where most of the breakdown of food in the small intestines occurs.
o It is here that Brunner's glands produce an alkaline secretion to protect the duodenum from acidic
chyme entering from the stomach and to activate intestinal enzymes enabling digestion and
absorption.
Duodenum-Brunner's Glands
The Jejunum
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The jejunum begins at the ligament of Treitz in the duodenojejunal flexure and continues to the ileum.
The inner surface or mucous membrane of the jejunum is covered by villi (small finger-like structures)
much longer than found in the duodenum or ileum, contained in many large circular folds (plicae
circulares) which provide extensive surface area for absorption of nutrients.
o The villi can increase intestinal absorptive surface area by a factor of 30.
o The microvilli, extensions of the villi, increase the surface area by an additional factor of 600.
o Villus capillaries collect amino acids and simple sugars.
o Villus lacteals or lymphatic capillaries absorb dietary fats.
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Microvilli
The Ileum
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The ileum is the final and longest section of the small intestine.
Both the jejunum and the ileum are suspended by mesentery, a double layer of peritoneum that allows
these parts of the intestine to move more freely within the abdomen.
Like the jejunum, the wall of the ileum has many folds and villi to increase both absorption of enzymes
and absorption of nutrients. It also has an increasing number of goblet cells.
The ileum is responsible for the final stages of protein and carbohydrate digestion, as contents are
pushed along by peristaltic waves of smooth muscle contractions.
There is no absolute demarcation between the jejunum and the ileum, but the ileum tends to have more
fat inside the mesentery and has a relatively decreasing diameter.
Unlike the rest of the small intestine, the ileum has abundant Peyer's patches, lymphoid follicles similar
to lymph nodes, which function as an important component of the immune system response to
pathogenic organisms in the GI tract.
Large Intestine
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Also commonly referred to by the name of its longest component, the colon, the large intestine is the
last part of the digestive system.
Its principal function is to absorb remaining water from the waste products of digestion as it compacts
the accumulated waste for periodic elimination by defecation.
While food is not broken down further at this stage, the fluid absorption function of the large intestine
does act to gather in vitamins created by beneficial bacteria or flora inhabiting the colon.
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Instead of the predominance of evaginations of villi found in the small intestine, the large intestine has
increased invaginations of glands and an abundance of goblet cells. The large intestine is structurally
divided into three parts: cecum, colon, and rectum.
The Cecum
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The cecum is a pouch at the beginning of the large intestine, separated from the ileum of the small
intestine by the ileocecal lower right quadrant of the abdomen.
The cecum is host to a large number of bacteria which aid in the final enzymatic processing of material
not completely digested in the small intestine.
The vermiform appendix is a worm-like cul-de-sac attachment of the cecum, until recently considered
entirely vestigial in humans, but now thought to have a role as a haven for the beneficial gut flora, as
well as the site of infection-fighting lymphoid cells.
The Colon
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The colon consists of four parts named for their relative orientation in the abdomen and the rectum:
o The ascending colon (1)
o The transverse colon (2)
o The descending colon (3)
o The sigmoid colon (4)
o The rectum (5)
By the time the chyme has reached the colon, almost all nutrients and most of the water have already
been absorbed by the body.
It is here that the chyme is mixed with mucus and bacteria to become feces. The waste products of
bacterial metabolism include some nutrients used by the cells lining the colon for their own
nourishment.
The colon ends at the junction of the sigmoid colon and the rectum.
o The rectum is the last part of the large intestine, beginning at and continuous with the colon, and
terminating at the anus.
o The rectum provides temporary storage for feces
o Stretch receptors of the nervous system located in the rectal walls stimulate the desire to
defecate. As peristaltic waves propel the feces into the anal canal, external and internal
sphincters allow the final exit of waste material from the GI tract.
Specialized Sections of the Large Intestine
The vermiform appendix
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is a small blind-ending diverticulum from the cecum. The most important features of the appendix is the
thickening of its wall, which is mainly due to large accumulations of lymphoid tissue in the lamina
propria and submucosa. Intestinal villi are usually absent, and crypts do not occur as frequently as in the
colon. There is often fatty tissue in the submucosa. The muscularis externa is thinner than in the
remainder of the large intestine and, the outer, longitudinal smooth muscle layer of the muscularis
externa does not aggregate into taenia coli.
An extreme proliferation of lymphocytes (lymphoid hyperplasia) as a consequence of bacterial or viral
stimulation may lead to the obstruction of the lumen of the appendix and thereby cause appendicitis, but
this is only one of many possible causes.
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