Gastrointestinal System II
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System I | Main Anatomy
Index | Liver,
Gallbladder and Pancreas
Last updated 30 March 2006
This page was contributed by David
Gastrointestinal System II
Structural organisation of the
- The wall of the alimentary canal is formed by 4 distinct layers:
- Muscularis externa
- Serosa or adventitia
The mucosa of the G.I. tract has a barrier
function, secretory function, and an absorptive
It is composed of epithelium, lamina propria, and a muscularis mucosa.
The epithelium differs throughout the
alimentary canal, adapting specifically for one or more of these functions.
The lamina propria has glands, lymphatic and
fenestrated vascular vessels to receive the absorbed substances and an immunologic barrier
- Diffuse lymphatic tissue, including lymphocytes and plasma cells,
which, together with lymphatic nodules, are called gut-associated lymphatic tissue (GALT)
- Eosinophils and macrophages
- The muscularis mucosa has smooth muscle
cells in inner circular and outer
- There are 4 basic mucosal types in the G.I. tract:
- Protective - stratified
squamous epithelium found in the oral cavity, pharynx, oesophagus and anal canal.
- Secretory - found in the stomach, where the mucosa consists of long, closely packed
tubular glands which may be simple, or branching,
depending on the region of the stomach.
- Absorptive - found in the small intestine, where the mucosa has finger like projections
called villi, with intervening short glands called crypts (of Lieberkuhn).
- Absorptive/Protective - in large intestine, where the mucosa is arranged as closely
packed, straight tubular glands with cells specialised
for water absorption and mucous-secreting goblet cells for lubrication of the intestine.
- This is composed of dense connective tissue, containing larger blood vessels,
lymphatics, and sensory, parasympathetic, sympathetic and enteric nerves.
- ganglion cells of postganglionic
parasympathetic neurons and enteric neurons
form the submucosal plexus (Meissners plexus)
- Glands are present in this region in the oesophagus and
- This is generally arranged as 2 thick layers of smooth muscle, with an inner spiral layer of circular fibres
and outer, looser spiral layer
of longitudinal fibres.
- Between these layers lies the myenteric plexus
(Auerbachs plexus) of the enteric nervous system.
- Synchronised rhythmic contractions of these layers,
controlled by the enteric nervous system, forms waves of peristalsis,
propelling the gut contents distally.
- The circular muscle layer thickens along several points of the G.I. tract to form
sphincters or valves, including:
- The pharyngeoesophageal sphincter
- The pyloric sphincter
- The ileocaecal valve
- The internal anal sphincter
Serosa and Adventitia
The serosa, consisting of
mesothelium and loose connective tissue, is the visceral peritoneum of the abdominal
cavity, continuous with the mesentery.
Those portions of the tract that are retroperitoneal or lie outside the
abdominal cavity attach to adjacent structures via loose connective tissue called adventitia.
- This is a muscular tube extending from the oropharynx to stomach, lined
by stratified squamous nonkeratinising
- The dense lamina propria has diffuse
lymphatic tissue and lymph nodules, as well as
oesophageal cardiac glands in the terminal part, and are
similar to the cardiac glands of the stomach.
- Their mucous secretions protect the oesophagus from gastric reflux juices.
- The muscularis mucosa is composed of longitudinal
- The submucosa forms loose,
longitudinal folds, allowing for distension during
swallowing, and has oesophageal glands proper along its
length, mostly found in the upper half.
- These small, compound tubuloalveolar glands are
invaginated from the epithelium, and their mucous secretions lubricate the lumen.
- The muscularis externa is composed of striated muscle in the upper third,
striated and smooth muscle in the middle
third, and smooth muscle in the distal third.
- It is innervated by both somatic and visceral motor fibres of the vagus
- Most of the oesophagus is covered by adventitia, but
the abdominal portion is covered by serosa.
Here the epithelium undergoes abrupt change from protective, stratified squamous epithelia to tightly packed, glandular secretory mucosa
Other layers are continuous, although there is a physiological
sphincter mechanism that helps prevent reflux of gastric juices.
- This is a very distensible organ with rugae
on its internal surface in the collapsed state.
- It has a scant lamina propria, between gastric glands.
- Its muscularis externa is described as having an extra inner layer of oblique fibres.
- It is histologically divided into 3 portions, based on the nature of the glands present
in its mucosa:
- The cardia, near the oesophageal
orifice, with cardiac glands
- The fundus (including the fundus and
body as defined in gross anatomy), containing fundic glands
- The pylorus, proximal to the pyloric
sphincter, with pyloric glands.
- The secretory gastric mucosa consists of simple
columnar epithelial cells, called surface mucous cells.
- These cells have apical cups of mucinogen granules, and
their cloudy mucous forms a thick, protective coating against acid and abrasion.
- Gastric secretions come from gastric glands that empty
into gastric pits opening onto the mucosal surface,
- Pepsinogen, the inactive precursor of pepsin
- Hydrochloric acid (HCl)
- Intrinsic factor, which is essential to the absorption
of Vitamin B12
- Hormones such as gastrin,
that stimulates HCl secretion
Fundic glands (Gastric glands)
These simple, branched,
tubular glands produce the digestive juice of the
Several glands may extend from one gastric pit to the muscularis mucosa,
with a relatively long neck segment and a shorter, wider base.
Cells of the fundic glands include:
- Mucous neck cells, localised to the neck region, with
less prominent mucous cups, that secrete a more soluble mucous compared to that of the
surface mucous cells.
- Chief cells, located deep within the glands, with
round, basophilic, basal nuclei and apical zymogen granules containing their secretory product, pepsinogen.
- Large parietal (oxyntic) cells,
mostly in the middle and upper portions of the gland, with round
nuclei and an eosinophilic cytoplasm
with an intracellular canalicular system, secreting HCl and Intrinsic Factor.
- Small enteroendocrine (APUD) cells, mostly at the base,
with a clearer staining cytoplasm, that secrete the hormone gastrin into the lamina propria.
- Undifferentiated stem cells.
These tubular, somewhat tortuous glands, occasionally branched, are composed
mainly of mucous secreting cells, with occasional enteroendocrine cells.
They have a short duct segment, entering
into relatively shallow gastric pits.
These coiled, branched
tubular glands lie in the pyloric antrum and the pylorus, composed of mucous cells, enteroendocrine cells,
and occasional parietal cells.
They empty into very deep gastric pits,
half the size of the thickness of the mucosa.
Cell replacement is rapid at the epithelial surface, with the mucous secretory sheet being
replaced over 3-5 days.
Cells of the gastric
glands are renewed at a much slower rate of about once a year.
The APUD cells are endocrine cells found throughout the gastrointestinal mucosa, secreting hormones that act on the
same or functionally related parts of the GI Tract.
Food in the stomach leads to secretion of gastrin into blood, causing secretion
of pepsin and acid from the gastric glands and
increasing gastric motility.
The pyloric sphincter is a
notable increase of circular smooth muscle in muscularis externa
The mucosa changes from the secretory glandular
arrangement of stomach to the absorptive, villous
arrangement of the small intestine with plicae circulares.
The duodenum has unique mucosal glands
extending into submucosa called submucosal (Brunners)
The Small Intestine
- This is the longest component of the digestive tract, whose main function
is absorption, and is comprised of the duodenum,
jejunum and ileum.
- The characteristic feature of the small intestine is the absorptive mucosa, whose surface area is increased by:
- Transverse folds of mucosa and submucosa called plicae circulares,
most prominent in the duodenum and jejunum.
- Finger like projections of mucosa, villi, with a core
of lamina propria that contains a blind ending lymphatic capillary, a lacteal,
surrounded by myofibroblasts.
- Many, many microvilli of the enterocytes,
the intestinal absorptive cells, creating a striated border
in their apical regions.
- The duodenum has a unique
feature of branched, tubuloalveolar submucosal (Brunners)
- Intestinal glands (crypts of Lieberkuhn)
are simple, tubular glands, lined with simple columnar
epithelium, extending from the muscularis mucosa and opening into the lumen at the bases
of the villi.
- The lamina propria surrounds the glands,
containing cells of the immune system as part of the GALT.
- Aggregated lymph nodules are also found
in the lamina propria, and are especially prominent in the ileum, called Peyers patches.
- Strands of smooth muscle extend from the
muscularis mucosae into the lamina propria of the villi.
- Cells of the mucosal epithelium are found both on the mucosal surface and
in the crypts, including:
- Tall columnar enterocytes, with basal nuclei and a striated border
on the apical surface, reflecting their primary function of absorption.
- Mucous secreting goblet cells, most
numerous in the terminal ileum, with basal nuclei and mucinogen granules in the apical cytoplasm.
- Paneth cells, found in the bases of the
intestinal glands, characterised by strongly eosinophilic secretory
granules containing the antibacterial enzyme lysozyme.
- Enteroendocrine cells, mainly in the
lower parts of the crypts, with their main secretions being cholecystokinin
(CCK), secretin and gastric
inhibitory peptide (GIP), that increase pancreatic and gallbladder activity and
decrease gastric secretion and motility.
- M (Microfold) cells,
modified enterocytes covering the lymphatic nodules,
characterised by microfolds instead of microvilli, that
discharge their absorptions basolaterally, presenting their contents to intraepithelial
lymphocytes to aid stimulation of GALT.
- Intermediate cells, the majority of
cells in the lower half of the crypts, that are still undifferentiated, with
characteristics of absorptive and goblet cells.
- The inner circular layer of the muscularis externa
produces local contractions deemed segmentation, whilst
the outer longitudinal layer produces peristalsis.
- Serosa of the peritoneum lines the small
- All of the mature cells of the intestinal epithelium are derived from stem cells in the lower half of the intestinal crypts, called
the zone of cell replication.
- Epithelial cells migrate from the crypt onto the villus, with the absorptive and goblet cells
lasting about 5-6 days, whilst Paneth
cells can last for up to 4 weeks.
Gut Associated Lymphatic Tissue (GALT)
- Up to 25% of the gut mucosa consists of lymph nodules, lymphocytes, macrophages, plasma cells and eosinophils in the lamina propria, as well as intraepithelial
lymphocytes, as part of the GALT.
- Diffuse lymphatic tissue, in co-operation with
epithelial cells (especially M cells) samples the antigens
in the epithelial spaces, then migrates to the nodules, undergoing transformation into plasma cells and secreting antibodies.
The mucosa changes from
the villiform pattern of the small intestine to
the glandular form of the large intestine.
A thickened extension of the muscularis forms a cone shaped valve emptying into the caecum, the dilated first
part of the large intestine.
The large intestine
- The large intestine includes the caecum,
ascending colon, transverse colon,
descending colon, sigmoid colon,
rectum and anal canal.
- The main functions of the colon are reabsorption
of electrolytes and water
and the elimination of undigested food and waste.
- Distinctive features of the large intestine include:
- The absence of villi
or plicae circulares
- The outer longitudinal layer of muscularis externa
divided into 3 bands, called taeniae coli
- The mucosa has closely packed straight tubular
glands, (crypts of Lieberkuhn), extending down to a prominent
muscularis mucosa, with thin strands of lamina propria between them.
- The glands consist of simple columnar epithelium,
invaginated from the lumen, and have 2 main cell types:
- Columnar absorptive cells, identical to
the enterocytes of the small intestine, are the
predominating cell type.
- Goblet cells, more numerous than in the
small intestine, and are most prominent in the distal part of the large intestine, as they
lubricate the increasingly solid faeces.
- Other cells of the small intestine are also found, although Paneth cells are normally absent.
- As with the small intestine, epithelial cell turnover times are about 6 days for the absorptive cells
and goblet cells, and about 4 weeks
for the enteroendocrine cells.
- The lamina propria has a thick collagen layer between the epithelium and its fenestrated
capillaries that is secreted by pericryptal fibroblasts forming a pericryptal
- There are many lymph nodules in the
lamina propria, extending into the submucosa, but there are no lymphatic vessels between
the colonic crypts.
- The muscularis externa consists of the
outer, longitudinal taeniae coli, and inner circular
fibres at irregular intervals, creating saccules
- Adventitia attaches the colon
retroperitoneally, whilst serosa covers those parts of
the colon with a mesentery.
Caecum and Appendix
- The caecum is a small blind pouch just distal to the ileocaecal valve.
- The appendix is a small finger-like extension, with
characteristic features of:
- Diffuse lymphatic tissue diffusely infiltrating the
mucosa and submucosa.
- Lymph follicles bulging into the lumen.
- A complete outer longitudinal layer of muscularis
- The rectum is a short dilated terminal portion of the
large intestine, characterised by transverse rectal folds
in its upper part.
- The rectal mucosa is similar to rest of large bowel, with straight
tubular glands, but with even more goblet cells.
- The mucosa then changes from simple columnar, to stratified columnar, and finally, stratified
squamous epithelium of the anal canal.
- The anal canal has longitudinal folds called anal columns.
- Branched, straight tubular anal glands
in the anal canal can extend into the muscularis externa.
- The submocusa contains the anastomosis between branches
of the superior rectal artery and the rectal
venous plexus, which may enlarge to form internal haemorrhoids.
- The circular layer of the muscularis externa thickens
to form the internal anal sphincter.
- The stratified squamous epithelium changes to skin,
containing sebaceous glands, hair follicles and large apocrine circumanal