Gastrointestinal System II

Gastrointestinal System I | Main Anatomy Index | Liver, Gallbladder and Pancreas

Last updated 30 March 2006

This page was contributed by David Boshell.

Gastrointestinal System II

Structural organisation of the alimentary canal

• The wall of the alimentary canal is formed by 4 distinct layers:

• The mucosa of the G.I. tract has a barrier function, secretory function, and an absorptive function.
• 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 consisting of:

• 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 longitudinal layers.
• There are 4 basic mucosal types in the G.I. tract:

1. Protective - stratified squamous epithelium found in the oral cavity, pharynx, oesophagus and anal canal.
2. 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.
3. Absorptive - found in the small intestine, where the mucosa has finger like projections called villi, with intervening short glands called crypts (of Lieberkuhn).
4. 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.

Submucosa

• 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 (Meissner’s plexus)
• Glands are present in this region in the oesophagus and duodenum.

Muscularis externa

• 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 (Auerbach’s 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

• 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.

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The oesophagus

• This is a muscular tube extending from the oropharynx to stomach, lined by stratified squamous nonkeratinising epithelium.
• 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 smooth muscle.

• 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 (X) nerve.
• 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.

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The stomach

• 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:

1. The cardia, near the oesophageal orifice, with cardiac glands
2. The fundus (including the fundus and body as defined in gross anatomy), containing fundic glands
3. 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, including:

• 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

• These simple, branched, tubular glands produce the digestive juice of the stomach.
• 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:

• 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 (Brunner’s) glands.

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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 (Brunner’s) glands.

• 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 Peyer’s 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:

1. Tall columnar enterocytes, with basal nuclei and a striated border on the apical surface, reflecting their primary function of absorption.
2. Mucous secreting goblet cells, most numerous in the terminal ileum, with basal nuclei and mucinogen granules in the apical cytoplasm.
3. Paneth cells, found in the bases of the intestinal glands, characterised by strongly eosinophilic secretory granules containing the antibacterial enzyme lysozyme.
4. 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.
5. 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.
6. 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 intestine.

• 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.

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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:

1. Columnar absorptive cells, identical to the enterocytes of the small intestine, are the predominating cell type.
2. 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 fibroblast sheath.
• 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 (haustra).

• 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 externa.

Recto-anal junction

• 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 glands.

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