Overview
The lecture revises the generic four-layer plan of the gut tube and then works
along the tract showing how each region modifies that plan: the stomach (an
extra oblique muscle layer, pits and glands with four secretory cell types), the
small intestine (three levels of surface-area amplification, villi and crypts,
absorptive and secretory cell types, Brunner’s glands in the duodenum), the
large intestine (no villi, goblet-rich tubular glands, tenia coli) and the
appendix (a miniature colon dominated by lymphoid tissue). The organising idea
is that the same layers persist throughout, and regional function is read off
the modifications to them.
Revision: the generic gut wall
From the lumen outwards the wall is arranged in concentric layers:
- Mucosa, itself three sublayers:
- Epithelium (dips down into the wall to form gland invaginations)
- Lamina propria
- Muscularis mucosae
- Submucosa, containing blood vessels
- Muscularis externa, made of an inner circular and an outer longitudinal component
- Adventitia, containing vessels; where a serous sheet covers the outside, the outermost layer is serosa
Stomach: wall layers
Regions of the stomach: fundus, cardia, body, pylorus.
- Mucosa
- Epithelium: simple columnar
- Lamina propria
- Periglandular vascular plexus
- Muscularis mucosae: 2 layered, smooth muscle
- Submucosa
- Arterial anastomoses and venous plexus
- Submucosal nerve plexus
- Muscularis externa: 3 layers (the stomach’s distinguishing feature)
- Inner oblique: an additional layer for churning
- Middle circular: forms the pyloric sphincter; myenteric plexus associated here
- Outer longitudinal: more in the upper parts
- Adventitia and serosa
- Large blood vessel anastomoses
On low-power H&E the full thickness reads as M (mucosa, thick and deeply stained
with glands running perpendicular to the surface), SM (paler submucosa), ME
(broad pink smooth muscle band) and S (thin serosa), with blood vessels (V) in
the cores of the mucosal folds and in the submucosa. Gastric rugae are folds of
the inner lining.
Gastric mucosa: pits and glands
- Lined by simple columnar epithelium.
- The epithelium invaginates into the lamina propria to form gastric pits, which continue into gastric glands.
- Gastric glands have a neck / isthmus, a body and a base, stacked in that order from the surface downwards.
- Lamina propria supports the glands and is highly vascular; it is the connective tissue lying between the glands.
- Muscularis mucosae separates mucosa from submucosa.
Pits occupy the upper part of the mucosa and glands the lower part, with only
lamina propria between them.
Gastric cell types
Cells listed for the stomach: surface mucosal cells, mucus neck cells,
chief/principal cells, parietal/oxyntic cells, stem cells, neuroendocrine cells.
Arranged superficial to deep along the pit-gland axis: surface mucosal cells,
then mucus neck cells at the neck, then parietal cells, then chief cells towards
the base.
Surface mucous cells
- Location: cover the surface of the stomach and the gastric pits
- Function: mucus secreting
- Microvilli
- Lots of RER, basal nucleus, vesicles of mucus at the top of the cell (large dark apical secretory vesicles)
Mucus neck cells
- Location: near the neck of the gland
- Function: mucus secreting
- Secretory granules (smaller, distributed through the cytoplasm), basal nucleus, lots of polyribosomes and RER
Chief / principal / zymogenic cells
- Location: towards the base of the gland
- Function: secrete the enzymes pepsinogen and lipase
- Cuboidal cells; round basal nucleus with basophilic granular cytoplasm
- Zymogen granules (large, pale, apical) and abundant basal RER
Parietal / oxyntic cells
- Location: apical half of the gland
- Function: secrete hydrochloric acid; secrete glycoproteins required for uptake of vitamin B12 in the small intestine
- Large round or pyramidal cell; central nucleus, eosinophilic cytoplasm (fried egg appearance)
- Lots of mitochondria, RER and intracellular canaliculi that open at the apex
Stem cells
- Found at the base and neck; can differentiate into the other cell types
Neuroendocrine cells
- Control gut motility and secretions
Important
Parietal cells are the source of both gastric acid and the glycoprotein needed
for vitamin B12 uptake, so loss of parietal cells removes two functions at once.
Small intestine: surface-area amplification
Three successive levels of amplification are shown, with the multiplication
factors given on the slide:
- Plica circularis (“Pica circularis” as written on the slide): a fold of mucosa plus submucosa thrown up from the wall, with villi projecting from its surface. Seen endoscopically as concentric circular mucosal folds.
- Villi: finger-like projections lined by columnar epithelium with numerous goblet cells, crypts lying between the villus bases. Density approximately 10-40 per mm². Amplification x7.
- Microvilli: a dense parallel array forming the brush border on the apical surface of the enterocytes. Amplification x13.
A villus in longitudinal section shows tall columnar enterocytes covering a core
containing a central lacteal and vessels.
Warning
The micrograph of enterocytes on the villus carries letter labels (G, BV, A,
LV, E, L) with no key given anywhere on the slide, so what they stand for is
not recorded.
Small intestine: wall and mucosa
Layers seen in the labelled plate of the small intestine wall, from lumen
outwards: lining epithelium with goblet cells; lamina propria (villus core)
containing the lacteal; intervillous spaces between villi; intestinal glands;
muscularis mucosae; submucosa carrying artery and vein; muscularis externa made
of inner circular and outer longitudinal smooth muscle with the myenteric nerve
plexus between them; adipose cells and serosa outermost. A lymphatic nodule with
a germinal centre sits in the mucosa/submucosa. The plica circularis is the fold
carrying all of this, with a submucosal core.
Epithelium
- Simple columnar
- Covers the luminal surface and the villi
- Cells: enterocytes and goblet cells
- Dips into the lamina propria to form crypts of Lieberkühn
Lamina propria
- Forms the core of the villi
- Contains blood vessels (capillary network, arteriole, venule) and lacteals
- Lymphoid aggregations / Peyer’s patches
Muscularis mucosae
- Extends between the crypts, compressing the glands
- Extends through the villous core, propelling lacteal contents
Paneth cells sit at the base of the intestinal gland and are labelled as
functioning in innate immunity.
Warning
The full-page numbered plate of the small intestine wall carries no slide title
or explanatory text; labels are transcribed as printed, including the apparent
typos “Laminal propria” and “Lympatic nodule”.
Small intestinal lining cells
Enterocytes (tall columnar cells)
- Function: absorptive
- Basal nucleus
- Surface microvilli forming the brush border
- Glycocalyx, a glycoprotein coat on the apical cell surface, which:
- protects from autodigestion and from micro-organisms
- carries digestive enzymes for disaccharides and oligopeptides
On electron microscopy of the epithelium: Mv microvilli apically, M
mitochondria, Ly a lymphocyte within the epithelium, IC intercellular clefts
between adjacent cells, C a capillary below the epithelium, and BM the basement
membrane [BM is labelled on the micrograph but is not in the slide’s key box].
Goblet cells
- Function: produce mucin for lubrication of intestinal content and protection of the epithelium
- Few short microvilli, in contrast to the dense brush border of the neighbouring absorptive cells
- Mucinogen granules in the apical part, bulging between the adjacent absorptive cells
- Mucus spreads over the glycocalyx
Enterocytes and goblet cells sit side by side within the same simple columnar
layer.
Crypt (of Lieberkühn) cells
- Crypt enterocytes: secrete ions and alkaline fluids to dilute chyme; also aid absorption.
- Paneth cells: acidophilic granules (strong stain); secrete antibacterial enzymes that help control the normal bacterial flora. Located at the crypt base.
- Stem cells: in the basal region of the crypt, above the Paneth cells.
- Neuroendocrine / enteroendocrine cells: produce locally acting hormones which regulate GI motility. Sit in the crypt wall; on H&E they show fine basal granules.
Goblet cells and columnar absorptive cells occupy the villus surface, while
Paneth cells, stem cells and enteroendocrine cells occupy the crypt.
Brunner’s glands
- Duodenal submucosal glands.
- Ducts open into the base of the crypts.
- Alkaline (pH 9) mucoid secretion, which neutralises chyme.
- Abundant near the pylorus, proximal to the duodenal papilla.
- Disappear from the duodenojejunal junction.
Histologically they appear as clusters of pale glandular acini lying deep to the
muscularis mucosae, that is, within the submucosa.
Large intestine
- Mucosa
- Epithelium: no villi; straight tubular glands, heavily populated with goblet cells, opening onto a flat luminal surface
- Lamina propria: more lymphoid follicles
- Muscularis mucosae
- Submucosa
- Vascular and nerve plexus
- Muscularis externa: increasing thickness distally
- Inner circular, with the myenteric plexus controlling motility
- Outer longitudinal, gathered as the tenia coli
- Adventitia
- The serosa carries appendices epiploicae
Vermiform appendix
Described on the slide as the “abdominal tonsil”:
- A miniature colon
- Reservoir for normal gut flora
- Narrow lumen in adults (slit-like and irregular in section)
- Fewer glands
- More lymphoid follicles, forming a ring around the lumen in the mucosa and submucosa
- No absorptive function
- Arteries in the adventitia
Self-test
- List the layers of the generic gut wall from the lumen outwards, including the sublayers of the mucosa.
- Name the three layers of the gastric muscularis externa and give the functional point made about each.
- Describe the depth sequence of the gastric pit and gland, naming the regions of the gland.
- List the six gastric cell types given in the lecture.
- Distinguish surface mucous cells from mucus neck cells by location and ultrastructure.
- Describe the location, secretions and appearance of chief cells.
- Describe the location, functions and ultrastructural features of parietal cells.
- Explain why the parietal cell has abundant mitochondria and intracellular canaliculi, relating structure to its stated function.
- State where gastric stem cells are found and what they do.
- List the three levels of surface-area amplification in the small intestine, with the amplification factors and villus density given.
- Explain what distinguishes a plica circularis from a villus in terms of which wall layers each contains.
- Describe the contents of the lamina propria of a small intestinal villus.
- Give the two actions of the muscularis mucosae described in the small intestine.
- Describe the enterocyte, including the composition and two functions of the glycocalyx.
- Distinguish a goblet cell from an absorptive enterocyte on electron microscopy.
- List the four cell types of the crypt of Lieberkühn with the function of each.
- Describe Brunner’s glands: layer of the wall, where their ducts open, their secretion and its purpose, and their distribution along the duodenum.
- List the features that distinguish large intestinal mucosa from small intestinal mucosa.
- Describe the features of the vermiform appendix given in the lecture.
- A patient has lost the acid-secreting cells of the gastric body. Using the lecture content, predict which two gastric functions are impaired and name the vitamin whose uptake is affected.
- A biopsy shows a mucosa with no villi, straight tubular glands packed with goblet cells and numerous lymphoid follicles, and the outer longitudinal muscle gathered into bands. Identify the region and justify each feature.
- Chyme leaving the stomach is strongly acidic. Explain, using two structures from this lecture, how the proximal duodenum is protected.
Answers
Reveal answers
- Mucosa (epithelium, lamina propria, muscularis mucosae), then submucosa, then muscularis externa (inner circular and outer longitudinal), then adventitia, with serosa as the outermost serous covering where present.
- Inner oblique, an additional layer for churning; middle circular, which forms the pyloric sphincter and is associated with the myenteric plexus; outer longitudinal, present more in the upper parts of the stomach.
- The simple columnar surface epithelium invaginates into the lamina propria as a gastric pit, which continues into a gastric gland; the gland runs isthmus/neck, then body, then base, with the muscularis mucosae and submucosa at the deep border.
- Surface mucosal cells, mucus neck cells, chief (principal) cells, parietal (oxyntic) cells, stem cells, neuroendocrine cells.
- Surface mucous cells cover the stomach surface and the gastric pits and have microvilli, abundant RER, a basal nucleus and large mucus vesicles concentrated apically. Mucus neck cells lie near the neck of the gland and have smaller secretory granules distributed through the cytoplasm, a basal nucleus and abundant polyribosomes and RER. Both secrete mucus.
- Towards the base of the gland; secrete pepsinogen and lipase; cuboidal with a round basal nucleus and basophilic granular cytoplasm, containing zymogen granules and RER.
- Apical half of the gland; secrete hydrochloric acid and the glycoproteins required for vitamin B12 uptake in the small intestine; large round or pyramidal cell with a central nucleus and eosinophilic cytoplasm (fried egg appearance), with abundant mitochondria, RER and intracellular canaliculi opening at the apex.
- Acid secretion is an energy-demanding secretory process, so the cell is packed with mitochondria, and the intracellular canaliculi are channels opening at the apical surface that provide the surface through which the secretion is delivered into the gland lumen.
- At the base and the neck of the gland; they can differentiate into the other gastric cell types.
- Plica circularis (a fold of mucosa plus submucosa), villi (x7, about 10-40 per mm²), microvilli forming the brush border (x13).
- A plica circularis is a fold of the wall containing a submucosal core carrying artery and vein, with villi projecting from its surface. A villus is a mucosal projection only, its core being lamina propria containing a lacteal and capillaries.
- It forms the villus core and contains blood vessels (capillary network, arteriole, venule), a central lacteal, and lymphoid aggregations / Peyer’s patches.
- It extends between the crypts and compresses the glands, and it extends through the villous core to propel lacteal contents.
- A tall columnar absorptive cell with a basal nucleus and apical microvilli forming a brush border, coated apically by a glycocalyx of glycoprotein which protects against autodigestion and micro-organisms and carries digestive enzymes for disaccharides and oligopeptides.
- The goblet cell has only a few short microvilli and its apical portion is filled by a mass of pale mucinogen granules bulging between neighbouring cells, whereas the absorptive cell has a dense microvillous border and no such granule mass.
- Crypt enterocytes, which secrete ions and alkaline fluid to dilute chyme and also aid absorption; Paneth cells at the base, with acidophilic granules, secreting antibacterial enzymes that help control the normal bacterial flora; stem cells in the basal region just above the Paneth cells; neuroendocrine (enteroendocrine) cells producing locally acting hormones that regulate GI motility.
- They are submucosal glands of the duodenum, lying deep to the muscularis mucosae; their ducts open into the base of the crypts; they produce an alkaline (pH 9) mucoid secretion that neutralises chyme; they are abundant near the pylorus, proximal to the duodenal papilla, and disappear from the duodenojejunal junction.
- Large intestine has no villi, straight tubular glands rich in goblet cells opening onto a flat surface, more lymphoid follicles in the lamina propria, a muscularis externa that thickens distally with the outer longitudinal layer gathered as tenia coli, and appendices epiploicae on the serosa.
- A miniature colon acting as a reservoir for normal gut flora, with a narrow lumen in adults, fewer glands, more lymphoid follicles, no absorptive function, and arteries running in the adventitia.
- Hydrochloric acid secretion and secretion of the glycoproteins required for uptake of vitamin B12 in the small intestine; the vitamin affected is B12.
- Large intestine. No villi and straight tubular glands with abundant goblet cells are the stated features of colonic mucosa; increased lymphoid follicles are described in its lamina propria; and the banded outer longitudinal muscle is the tenia coli.
- Brunner’s glands in the duodenal submucosa deliver an alkaline (pH 9) mucoid secretion into the base of the crypts to neutralise chyme, and the crypt enterocytes secrete ions and alkaline fluid that dilute chyme; the goblet cell mucus layer spreading over the glycocalyx and the glycocalyx itself also protect the epithelium.