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Alimentary Systeme

  1. Oral Structures
    Pseudostratified, columnar, ciliated epithelium covers the pharyngeal side, and stratified squamous the oral surface.
  2. Gastrointestinal Tract

General Plan

Structural organisation of the alimentary canal
The wall of the alimentary canal is formed by 4 distinct layers :
  1. Mucosa
  2. Submucosa
  3. Muscularis externa
  4. Serosa or adventitia
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 specialized 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 (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.
  • Synchronized 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 pharyngoesophageal 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.
  • Mesothelial cells bear microvilli, are well attached, and secrete lubricants to allow viscera to move freely.
  • To avoid knots and obstruction, the plan for the GI tract is fasten, loosen, fasten, and so forth, so that only the small intestine and transverse colon have long stretches of mobile tube: fastening requires an adventitia, mobility, a serosa.


  1. Mucosa has stratified squamous epithelium ending sharply, but along a jagged line, at the gastric junction, creating a white-red distinction between proximal and distal sides of the Z-line in endoscopy. Here, abnormalities of the oesophageal epithelium and the position of the epithelial junction are quite common - Barrett’s oesophagus, where the stratified squamous epithelium is replaced metaplastically by simple columnar epithelium with some or all of the small-intestinal cell types.
  2. Muscularis mucosae - longitudinal smooth muscle.
  3. Cardiac glands - make neutral mucus and are branched tubular, in the mucosa near the gastric cardia, and in mucosa of the upper oesophagus; inconsistently present.
  4. Oesophageal glands - acidic mucous, compound, tubulo-alveolar, and lying in the submucosa, less numerous in the middle segment of the oesophagus.
  5. Circular and longitudinal external muscle coats of skeletal muscle in the upper fifth or so give way progressively to only smooth muscle in the lower half.
  6. Outermost coat is CT adventitia, except on a small piece below the diaphragm.
  7. Function - rapid passage of food to (and from) the stomach.


  1. General structure
    1. Anatomical regions - cardia, fundus, corpus, pyloric antrum and pyloric canal: the regions are histologically distinct.
    2. Outer covering is a serosa, from which hang omenta.
    3. Muscular coat of three smooth muscle layers - outer, longitudinal ; middle, circular ; inner, oblique. The middle layer is more developed to form a sphincter at the pylorus. The muscle churns the contents (chyme), and passes them periodically in regulated amounts to the duodenum.
    4. Submucosa - no glands: CT carries vessels and the nerve plexus.
    5. Muscularis mucosae - two layers, with the inner circular one sending a few muscle fibres up towards the lumen.
    6. Mucosa is deep and glandular, with only a little lamina propria tissue ; produces acid and enzymes for digestion, and undertakes some absorption, e.g., of water and alcohol.
  2. Stomach mucosa
  1. Empty stomach’s lining is folded in ridges - rugae.
  2. Surface is pitted by recesses - gastric pits/foveolae gastricae.
  3. Long tubular glands extend from the museularis mucosae up to empty into the pits. A gland has a base, neck and isthmus.
  4. The surface of the stomach and the pits are lined by simple, columnar, special mucous epithelial cells.
  5. Gastric glands throughout the body and fundus of the stomach are simple, branched tubules with these cells :
    1. Chief/zymogenic/peptic serous cells : in the majority ; basophilic, with ‘zymogen’ granules and rich granular ER.
    2. Parietal/oxyntic cells: occur peripherally and singly ; large and eosinophil ; packed with mitochondria and smooth ER; have long secretory canaliculi, lined by microvilli, and opening to the gland’s lumen.
    3. Mucous neck cells: concentrated near the neck of the gland.
    4. Endocrine/enteroendocrine/argentaffin/enterochromaffin/Kultschitsky cells : few in number, seen with EM, silver methods, or cytochemistry, but may be recognized from their empty look with H & E, and their rarity.
    5. In the narrow cardiac region lie cardiac glands - compound tubular, with mucous and a few parietal cells.
    6. In the pylorus, pits are much deeper, and glandular tubules are wider and more branching. The main kind of glandular cell present is pale and resembles fundic mucous neck cells.
  1. Gastric secretions and cell types responsible
    1. Surface mucous cells - mucus, to prevent auto-digestion of the mucosa, and bicarbonate ions held in the mucus.
    2. Chief/zymogenic cells - enzymes, e.g., pepsin, rennin, gastric lipase.
    3. Oxyntic/parietal cells - C1HC03 is exchanged baso laterally to balance the apical Na+/H+ proton pump used to form the hydrochloric acid of the digestive juice.
      (The stimulated active parietal cell has greatly extended canaliculi.)
    4. Mucous neck cells - mucus and enzymes, e.g., dipeptidases.
    5. Endocrine cells - hormones and amines; e.g., a hormone - gastrin - produced by the pyloric antral G cells controls the release and formation of acid from parietal cells, and of digestive enzymes from chief cells.
    6. Parietal cells - intrinsic factor - to assist in the absorption of vitamin B12: this role is upset when the parietal cells’ proton pump is an autoimmune target in pernicious anaemia, leading to the cells’ destruction.
  2. Small intestine
  1. General structure
    1. Three regions – duodenum, jejunum and ileum, anatomically and histologically distinguishable.
    2. Serous coat over all except part of the duodenum and the terminal ileum, which are fixed to the abdominal wall.
    3. Suspended on a mesentery carrying blood and lymphatic vessels, lymph nodes and nerves.
    4. Museularis externa has two complete layers.
    5. Submucosa - occupied by Brunner’s* mucous, compound tubular glands in the duodenum; elsewhere is CT as for the rest of the tract.
    6. Museularis mucosae - inner, circular, and outer, longitudinal smooth muscle.
    7. Mucosa has:
      • Villi - finger- or leaf-like projections.
      • Crypts of Lieberkuhn - simple tubular glands.
      • Lamina propria forming the core of each villus and lying between the gland tubules.
      • Covering of simple columnar epithelium.
  2. Cytology of small-intestinal mucosa
    1. Enterocytes are columnar absorptive epithelial cells on the villi; with a striated border; are held apically by junctional complexes; the many vesicles at the base of the microvilli communicate with agranular ER.
    2. Goblet cells, with the nucleus, GER and Golgi apparatus basally, stored mucigen droplets apically.
    3. Paneth cells, with eosinophil granules holding defensin and enzymes; remain at the base of the crypts.
    4. Enteroendocrine cells with hormone- and serotonin-containing basal granules.
    5. Undifferentiated columnar crypt stem cells: few microvilli; able to divide, migrate, differentiate into the other kinds, function, and be extruded at the villus tip, over approximately four days.
    6. Villus core has the basal lamina for the epithelium, a central lymphatic capillary (lacteal), blood vessels, smooth muscle fibres. The loose stroma of reticular and elastic fibres is heavily infiltrated by WBCs, e.g., CD4+ helper-inducer lymphocytes and eosinophils, and plasma cells.
    7. Ileum has Peyer’s patches of extensive lymphoid tissue, erasing villi, breaking into the epithelium, and interrupting the muscularis mucosae to invade the submucosa. Elsewhere, only solitary lymphoid nodules are to be seen. The epithelium domed over the Peyer’s-patch follicles is specialized, with M cells, which transport antigen and otherwise assist immune functions.
  3. Changes within small intestine during descent:
    1. Goblet cells increase in number.
    2. Villi become more finger-like.
    3. Lymphoid tissue increases.
    4. Plicae circulares diminish.
  4. Large intestine
    1. General features
      1. Crypts, but no villi or plicae circulares.
      2. Columnar epithelial cells are: (i) undifferentiated; (ii) goblet (numerous); (iii) colonocytes, absorbing, with microvilli, for water, electrolytes, and some fatty-acid products of bacterial metabolism of the faeces; (some excretion occurs). Endocrine cells are also present.
      3. Dehydrating faeces need lubrication; hence many goblet cells are present in the simple columnar epithelium.
    2. Regional details of large intestine
      1. Colon and caecum: outer longitudinal muscle coat is gathered into three bands - taeniae coli - which pucker or sacculate the tube, forming haustrations.
      2. Appendix: continuous muscle coats; few crypts; the mucosa is mainly occupied by lymphoid tissue; the muscularis mucosae may be deficient and lymphoid tissue seen in the submucosa. The wall may be thick. With age the lumen may be blocked off/occluded by fibrosis.
      3. Rectum: outer longitudinal muscle is one continuous sheet.
      4. Anal canal
        i. Morgagni’s anal columns* are 6-10 vertical mucosal folds.
        ii. Dentate* line lies at the level of the bases of the columns, where there are tiny flaps and pockets - anal valves and sinuses.
        iii. The histological epithelial anal transitional zone (ATZ) lies between unbroken simple columnar colo-rectal epithelium and lower stratified squamous epithelium.
        iv. The ATZ - the common site of anal cancers - is very variable in its extent and outline, in its kinds of epithelia, and the number of crypts.
        v. Submucosal veins display periodic dilations. Deterioration of their supporting connective tissue permits enlargement and prolapse - haemorrhoids*.
        vi. The complex anal musculature includes external skeletal-muscle and internal smooth-muscle sphincters. (The muscles and their innervation are particularly at risk of stretching and damage in women giving birth.)

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