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Histology of the digestive tract
General structure


Hollow tube with lumen of variable diameter



Wall composed of four layers: mucosa, submucosa, muscularis, serosa



Submucosal and myenteric nerve plexuses make up the enteric
nervous system

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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Functions of the epithelial lining


Selectively permeable barrier



Transport and digestion of food



Absorption of products of digestion



Hormones affecting digestive function



Mucous for lubrication and protection

Lymphoid tissue


Plentiful lymphoid nodules in the submucosa



Immunological protection since digestive tract (apart from oral cavity,
oesophagus and anal canal) is simple epithelium



Lamina propria rich in macrophages, and lymphocytes, active
secretion of IgA (resistant to proteolytic enzymes)

Clinical note
Plexi of the digestive tract severely injured or destroyed in certain diseases such



as Hirschsprung disease (congenital megacolon) or Chagas disease
(Trypanosoma cruzi)
Disrupts gut motility, dilating in some areas



Oral cavity


Lined with stratified squamous epithelium, sometimes keratinised



Keratin protects oral mucosa during mastication; most developed in
gingivae and hard palate



Non-keratinised epithelium on the soft palate, lips, cheeks and floor of
mouth



Dermal papillae in the lamina propria



Lips have striated muscle and transition from non-keratinised to
keratinised skin epithelium

Tongue


Striated muscle covered by mucous membrane



Muscle fibres cross one another in three planes, and are grouped in
bundles separated by connective tissue



Dorsal surface of the anterior tongue is covered by small papillae



The anterior and posterior surfaces are separated by the V-shaped
groove called the terminal sulcus



The posterior third is the root of the tongue, where the lingual tonsils
form small bulges on the surface and smaller lymphoid nodules



Taste buds located on papillae of the tongue and also present in other
parts of the oral cavity, such as the soft palate, continuously flushed by
numerous salivary glands

Types of papillae


There are four types of papillae on the anterior tongue; filiform,
fungiform, foliate and vallate

Filiform papillae


Numerous, highly keratinised, elongated conical shaped



No taste buds



Mechanical role; rough surface helps food movement during chewing

Fungiform papillae


Less numerous, lightly keratinised, mushroom shaped



Connective tissue core



Scattered taste buds

Foliate papillae


Poorly developed in adults



Parallel ridges and furrows on the side of the tongue



Have taste buds

Vallate papillae


Least numerous, largest



Lots of taste buds



Located just anterior to the terminal sulcus



Ducts from serous salivary glands empty into the deep groove
surrounding each papillae



Provides a continuous flow over the taste buds, washing food particles
onto them



Secrete a lipase preventing formation of a hydrophobic film over the
taste buds

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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Pharynx


Transitional space between the oral cavity and respiratory and
digestive systems



Forms area of communication between nasal region and larynx



Lined by stratified non-keritinised epithelium in the region continuous
with the oesophagus, and by ciliated pseudostratified epithelium in the
region close to the nasal cavity



Contains tonsils and mucous salivary glands in the mucosa, with
surrounding constrictor and longitudinal muscle

Teeth


Adult normally has 32 permanent teeth, arranged in bilaterally
symmetric arches in the maxillary and mandibular bones



Eight teeth in each quadrant; two incisors, one canine, two premolars,
three molars



Each tooth has a crown exposed above the gingiva, constricted neck
at the gumline and one or more roots below the gingiva that hold the
teeth in the alveoli



Crown of the tooth is covered by hard enamel and roots by a bonelike tissue called cementum; two coverings meet at the neck



Most of the tooth composed of dentin, surrounding a soft connective
tissue filled space called the pulp cavity



Pulp cavity narrows in the root as the root canals, which extend to the
tip of each root, where the apical foramen allow entry and exit of
vessels, nerves and lymphatics



Periodontal ligaments are fibrous connective tissue bundles of collagen
fibres inserted into both the cementum and alveolar bone, fixing the
tooth in the alveolus

Dentin


Calcified tissue, harder than bone



Odontoblasts secrete secrete type I collagen and glycosaminoglycans



Mineralisation of predentin involves matrix vesicles in a similar process
to osteoid; odontoblast processes lie within dentinal tubules



Odontoblasts remain active in predentin secretion in adult life



Teeth sensitive to cold, heat and acid pH, all can be perceived as pain



Different stimuli can affect fluid in the dentinal tubules, causing
activation of nerve fibres extending from the pulp

Clinical note




Dentin does not turn over or get remodelled, and persists as a mineralised
tissue after loss of the odontoblasts
It is therefore possible to maintain teeth when pulp and odontoblasts have
been destroyed by infection
In adults, erosion of enamel from erosion or dental caries triggers odontoblasts
to resume activity in dentin remodelling

Enamel


Hardest component of the human body



Contains some unique proteins amolegin and enamelin along with
other organic material and hydroxyapatite (main)



Other ions e
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fluoride can be adsorbed by the hydroxyapatite



Interlocking enamel rods are bound together by other enamel



In developing teeth the enamel matrix is secreted by ameloblasts

Pulp


Connective tissue; odontoblasts, fibroblasts, collagen fibrils, ground
substance



Highly innervated and vascularised



Blood vessels and nerve fibres enter the apical foramen and branch



Some nerve fibres extend into the dentin tubules

Peridontium


Structures responsible for maintaining the teeth in the maxillary and
mandibular bones



Comprises cementum, periodontal ligament, alveolar bone, gingiva



Cementium covers dentin of the root, cementocytes present



Periodontal ligament is connective tissue bundle, connecting
cementum to the alveoli
o Allows limited movement of the tooth in the socket
o Support pressure during mastication
o Highly cellular, vascularised and innervated



Alveolar bone is immediate contact with the alveolar bone, forming its
periosteum
o Collagen fibres of the periodontal ligament penetrate the bone,
binding it to the cementum



Gingiva is a mucous membrane bound to periosteum of the maxillary
and mandibular bones

o Stratified squamous epithelium
o Junctional epithelium is bound to the cementum at the neck
o The gingival sulcus is a groove between the enamel and
epithelium

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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g
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

Surface epithelial cells form tight line of defence due to the tight
intracellular junctions, mucous protection, and ion transporters



Underlying circulatory bed provides HCO3-, nutrients and oxygen and
removes toxic metabolites

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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pylori can
disrupt the epithelial lining leading to ulceration
Ulceration may heal or may be aggravated by local aggressive agents,
leading to more gastric and duodenal ulcers
Aspirin and ethanol irritate the mucosa partly by reducing mucosal blood
flow
NSAIDs inhibit production of prostaglandin E, important for alkalinisation of
the mucous layer

Regional differences in the mucosa


Cardia is a narrow region at the transition between the oesophagus
and the stomach



Pylorus is the funnel shaped region that opens into the small intestine

Mucosa of the cardia and the pylorus


Tubular, usually branched glands, with coiled secretory portions called
cardial and pyloric glands



Pits are longer in the pylorus



Glands in both regions secrete mucous and lysozyme

Mucosa of the fundus and body


Branches, tubular gastric glands



Each gland has an isthmus, neck and base



Non-uniform distribution of epithelial cells



Isthmus contains
o Differentiating mucous cells that migrate upward to replace
surface mucous cells
o Stem cells
o Parietal cells



Neck contains
o Stem cells

o Mucous neck cells
o Parietal cells


Base contains parietal, chief and ECF cells

Cells of the gastric glands


Mucous neck cells
o Present in the neck of glands
o Mucous secretion less alkaline and different composition to
surface ones



Parietal cells
o Few in base, mostly in upper half of the gland
o Have circular invaginations in the apical membrane, forming an
intracellular canniculus
o Secrete HCl and intrinsic factor (for B12 uptake in the ileum)
o Abundant mitochondria and microvilli
o Activity stimulated by ACh, gastrin, histamine



Chief cells
o Predominately in the lower region of the gland
o Cytoplasmic granules containing inactive pepsinogen
o Pepsinogen rapidly converted to pepsin in acid pH; active pH<5
o Also produce lipase and leptin



Enteroendocrine cells
o Secrete variety of hormones


Ghrelin (throughout stomach) – increased sense of hunger



Gastrin (pylorus) – stimulates gastric acid secretion



Somatostatin (pylorus) – local inhibition of other
enteroendocrine cells

o In the fundus, located in basal lamina of the gland and primarily
secrete 5HT
o In the lower body and pylorus, located in contact with the
stomach lumen as G cells, secreting gastrin (stimulates acid
production by parietal cells and trophic effect on mucosa)



Stem cells
o Found in neck region
o Some daughter cells move upward to replace pit and surface
mucous cells, other migrate deeply into the glands and
differentiate to mucous neck cells, parietal, chief and
enteroendocrine cells

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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Lamina propria to the serosa


Loose connective tissue with blood and lymphatic vessels, nerve fibres
and smooth muscle cells



Penetrates the core of each villus



Smooth muscle fibres inside the villus are responsible for their rhythmic
movements which aid absorption



Muscularis mucosae produces local movements of the villi and plicae
circularis



In the primary part of the duodenum, in the mucosa and submucosa,
are numerous duodenal (Brunner’s) glands
o Excretory ducts open into the intestinal crypts

o Alkaline fluid produced neutralises the chyme entering from the
pylorus
o Protects the mucosa from the acid and provides pH for
pancreatic enzymes


In the ileum, the lamina propria and submucosa contain Payer’s
patches, lymphoid nodule aggregates (MALT)



Muscularis is well developed with an internal circular and external
longitudinal layer



Covered by thin serosa with mesothelium

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition
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Large intestine


Mucosal membrane with no folds except in the distal portion, and no
villi



Mucosa is penetrated with glands containing goblet, absorptive and
enteroendocrine cells



Absorptive cells are colonocytes, with short, irregular microvilli



Stem cells located at the bottom of each gland



Main functions are absorption of water, forming faecal mass from
indigestible material, and mucous production to lubricate the surface



Lamina propria is rich in lymphoid cells and in lymphoid nodules that
often extend into the submucosa



Richness in MALT related to bacterial population



Muscularis has longitudinal and circular strands



Fibres of the outer layer gather in three longitudinal bands called
taeniae coli



Intraperitoneal parts of the colon are covered by serosa, characterised
by small, pendulant protuberances of adipose tissue



At the start of the large intestine (caecum) is the appendix



Appendix has small and irregular lumen, with short and less tubular
glands and no tenae coli; MALT component
In the anal region, the mucous membrane forms a series of



longitudinal folds known as the anal columns
At the recto-anal junction, the mucosal lining is replaced by stratified



squamous epithelium
In this region the lamina propria contains a plexus of large veins



Clinical note
Appendicitis




Appendix can easily become a site of inflammation due to it being a closed
sac and relatively static
With the small lumen and thin wall, inflammation and growth of lymphoid
follicles can cause swelling leading to burst of the appendix
Burst appendix can lead to infection of the peritoneal cavity

Adenocarcinomas



90-95% of malignant tumours of the digestive system are derived from gastric or
intestinal epithelial cells; usually in the large intestine
Usually derived from glandular epithelium (adenocarcinomas)

Reference
1
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Junqueira’s Basic Histology: Text and Atlas
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Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, Twelfth Edition

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