Stomach

A muscular organ which lies between the oesophagus and duodenum in the left upper abdomen

Embryology

• Derived from tubular embryonic foregut
• Begins as a dilatation during the 5th week of gestation, towards the cranial portion
• By the 7th week of gestation, it descends, rotates and dilates into its usual shape and position
• If descent does not occur, a diaphragmatic hernia results
• Rotates 90 degrees clockwise around its longitudinal axis, so that its left side faces anteriorly (hence the left vagus becomes anterior)
• The posterior wall grows faster than the anterior, hence forming the greater curvature
• Invested by the dorsal mesentery (becomes the gastrosplenic, gastrocolic and gastrophrenic ligaments) and the ventral mesentery (becomes the hepatoduodenal, gastrohepatic and falciform ligaments)

Divisions

• Cardia - most proximal region, attaches to the oesophagus
  • Junctional mucosa - mucus cells and glands, usually <2cm in length
  • Most fixed location
• Fundus - superior-most part, above the level of entry of the left side of oesophagus, meaning the fundus is usually 2-5cm in height
  • Floppy and distensible
  • Anteriorly covered in peritoneum
  • Angle of His - anatomic angle formed by the fundus with the left margin of the oesophagus
  • Parietal cells and chief cells
• Body - largest part - sometimes called 'corpus'
  • Bounded by lesser and greater curvatures, as far down as incisura on lesser curvature; no fixed point on greater curvature where this ends, but maybe the 'sump'
  • Angularis incisura - where the lesser curvature abruptly angles to the right, and vessels and nerves of lesser omentum are seen to fan out as the 'crow's foot'
  • Parietal cells and chief cells
• Antrum - between angularis incisura and pylorus (I think that division is at the pre-pyloric vein?)
  • Gastrin-producing cells
  • Thickest muscle layer
• Pylorus
  • Proximal and distal pyloric sphincters, with a sheet of circular muscle between them. They are fused on the lesser curvature side with a muscular knob known as the pyloric torus.
    • Distal sphincter is quite distinct, at the narrowest point of the gastroduodenal junction
    • Proximal sphincter is less prominent, occurring over a longer section of greater curvature
    • Marked by pre-pyloric vein (of Mayo)

Relations

• Anteriorly - left lobe of liver, anterior abdominal wall, left hemidiaphragm
• Posteriorly - lesser sac, pancreas, transverse mesocolon
• Medially - abdominal aorta, coeliac trunk, lymph nodes
• Laterally - spleen

Arteries

• Rich anastomosis in mucosa, submucosa, and intra-muscular layers
• Four main branches, mostly from coeliac trunk:
  • Left gastric artery (direct branch of coeliac trunk) supplies the proximal part of lesser curvature. It ascends a short distance beneath the peritoneum to reach the lesser curve at the gastro-oesophageal junction (accompanied by left gastric (coronary) vein and the coeliac branch of the posterior vagus nerve). To reach the stomach, it raises a fold of peritoneum called the gastropancreatic fold, best seen from the lesser sac. This is the biggest gastric artery normally. Gives off oesophageal branches, and usually divides into anterior and posterior branches. 15-20% of patients have an aberrant accessory left hepatic artery, originating from the left gastric artery. 10% have an accessory left gastric artery from the splenic artery.
  • Right gastric artery (branch of hepatic artery or sometimes gastroduodenal artery) supplies the distal part of lesser curvature
    • Arises from CHA before or after GDA branch and descends towards pylorus, contained in leaves of the lesser omentum
    • Can give off the small artery above the pylorus, the supraduodenal artery
    • Small in size but adequate to support a conduit
  • Left gastroepiploic artery (branch of splenic artery) supplies the proximal greater curvature. Arises from splenic artery, and passes to stomach via gastrosplenic ligament, to reach the greater curvature at about the level of the inferior pole of spleen. It runs in greater omentum about 1cm from greater curvature, as the gastro-omental arcade. This arcade runs between the two layers of the greater omentum. Note that it only runs about two-thirds of the way along the greater curvature, only as far superiorly as its feeding vessel in the gastrosplenic ligament. Left omental branch.
  • Right gastroepiploic artery (branch of gastroduodenal artery) supplies the distal part of greater curvature. Arises as one of the two terminal branches of GDA, behind D1. Mirror image to the left gastro-epiploic.
• Additionally, the fundus and upper left part of the great curvature receive about six short gastric arteries from the splenic artery in the gastrosplenic ligament, which supply fundus normally, but can preserve a gastric pouch.
• Some supply to proximal stomach can come from oesophageal arteries
• In most cases, the stomach will survive if 3 out of the 4 arteries are ligated, provided the anastomotic connections are preserved.
• 40-97% of patients have a posterior gastric artery supplying the posterior portion of the gastric fundus that arises from the middle splenic artery, which can preserve a gastric pouch if care is taken to identify it. Could be conceptualised as the most proximal of the short gastric arteries.

Veins

• Veins of the same name accompany the arteries and drain into the PV or its splenic and superior mesenteric tributaries
• Prepyloric vein (unaccompanied by an artery) drains into the right gastric vein.

Nerve supply

• Parasympathetic: vagus nerve
  • Vagal nerve - originates in the vagal nuclei in the floor of the fourth ventricle; traverses the neck in the carotid sheath; enters the mediastinum; divides into several branches around the oesophagus; these branches coalesce above the oesophageal hiatus to form the left and right vagus nerves
  • Anterior vagal trunk (left vagal fibres) comes down through the oesophageal hiatus, in contact with anterior oesophageal wall, most commonly as a single trunk but sometimes as 2-3. Anteriorly at the hiatus, but can be seen slightly to the right below the hiatus. Divides into two branches shortly distal to the hiatus: the larger branch is the greater anterior gastric nerve (anterior nerve of Latarjet); and smaller branch is the hepatic nerve. Anterior nerve of Latarjet runs down in the lesser omentum with the left gastric artery, parallel to lesser curvature but 1cm away, giving off 2-12 branches to the anterior gastric wall and terminating at a variable point on the antrum, but in 5% reaching as far as the pylorus. Hepatic nerve can be double or triple, and passes in the lesser omentum to the hilum of the liver, giving off a branch to the left of the hepatic artery, supplying pylorus and D1.
    • If in doubt about location of the anterior trunk, it can often be felt as a taught bowstring with the oesophagus on tension
    • Anterior nerve of Latarjet can sometimes be absent, with all branches coming from hepatic division
  • Posterior vagal trunk (right vagal fibres) lies in loose tissue a little separated from the posterior wall of the oesophagus, usually a little to the right. Divides into two divisions: coeliac division and posterior gastric nerve/posterior nerve of Latarjet. Coeliac division runs back along left gastric artery to the coeliac plexus. Posterior nerve of Latarjet parallels the anterior nerve but supplies fewer fibres to the antrum, and none to pylorus. The first branch of posterior stomach is referred to as the criminal branch of Grassi. Can sometimes give branch to hepatic nerve.
  • The posterior trunk is usually larger, but not always. There is usually one large nerve and one small one.
• Sympathetic:
  • Originates at T5-T10
  • Travels in splanchnic nerve to coeliac plexus
  • Postganglionic fibres travel with the arterial system to the stomach
• Auerbach autonomic plexus is found in the muscularis propria throughout
• Meissner autonomic plexus is found in the submucosa

Lymphatic drainage

• Four zones of lymph nodes (clockwise from top in diagram below)
  • Superior gastric drains to left gastric and paracardial nodes
  • Pancreaticolienal drains to left gastroepiploic and splenic nodes
  • Inferior gastric and subpyloric drains along the right gastroepiploic vascular pedicle
  • Suprapyloric drains to right suprapancreatic and hepatic nodes at porta hepatis
• All four zones eventually drain into coeliac nodes and then into the thoracic duct
• Note the 'watershed' area along a line drawn parallel to the greater curvature, two-thirds of the way down the anterior surface
• Gastro-omental nodes often occur several cm out into omentum
• Gastric cancer may metastasize to any of the four nodal groups, regardless of cancer location
• To remember the important lymph node stations (for D2 lymphadenectomy):
  • 1, 3, 5 along lesser curvature, proximal to distal
  • 2, 4, 6 along greater curvature
  • Then the vessels: 7 left gastric, 8 common hepatic, 9 coeliac
  • Spleen: 10 splenic hilum, 11 proximal and distal splenic artery
  • 12 hepatoduodenal
• D1: 1-7

Morphology (layers from external to internal)

• Peritoneum, forming the outer serosa
• Muscularis propria - three layers of smooth muscle, thickest in antrum.
  • Outer layer is longitudinal - mostly seen in distal two thirds
  • Middle layer - circular and oblique. The pyloric sphincter is formed from a thickening of the middle layer. Also important around the fundus at point of entry of oesophagus, help to maintain acute angle of entry and hence lower oesophageal sphincter function.
• Submucosa - strongest layer - connective tissue - contains the anastomotic network of blood vessels and lymphatics
• Mucosa
  • Muscularis mucosae - probably responsible for the rugae
  • Lamina propria - small connective tissue layer
  • Surface epithelium
    • Simple columnar epithelium
    • Interspersed gastric pits containing one or more gastric glands

Endoscopic anatomy

• Areae gastricae - small, slightly elevated polygonal areas of the gastric mucosa, 1-5mm in diameter, subdivided by the system of intersecting furrows on the mucosal surface
  • Can become prominent in superficial gastritis

Physiology

Gastric peptides

• • Gastrin
    • Produced by G cells in the antrum
    • Similar in form and function to CCK
    • Release stimulated by gastric distension, gastrin-releasing peptide and protein digestion products. Release inhibited by luminal acid.
    • Primary endocrine regulator of the secretory phase of a protein meal
      • Stimulates enterochromaffin-like (ECL) cells to synthesise and release histamine
      • Exerts direct action on parietal cells to stimulate acid release
      • Trophic effects on parietal cells and ECL cells
    • Hypergastrinaemia is most commonly the result of acid-reducing treatment like PPIs
  • Somatostatin
    • Produced by delta cells - diffuse neuroendocrine cells located in fundus and antrum
    • Stimulated by antral acidification and gastrin-releasing peptide, and inhibited by acetylcholine (from vagal fibres)
    • Acts to inhibit parietal cell acid secretion directly and also inhibits gastrin release from G cells and downregulates histamine release from ECL cells
  • Histamine
    • Histamine release is stimulated by gastrin, VIP, ghrelin, achetylcholine, and adrenaline, via their actions on ECL cells
    • Inhibited by somatostatin
    • Stimulates parietal cells to release acid
  • Ghrelin
    • Produced by enteroendocrine cells of the oxyntic glands in GIT
    • Levels are highest before meals during fasting and decrease after meals
    • Decreased levels have been associated with gastritis, and are seen post-bypass or sleeve gastrectomy (unknown mechanism)
    • Increases gastric emptying and motility, and increases gastric acid secretion, in preparation for digestion

Gastric acid secretion

• • Secreted by parietal cells via the hydrogen-potassium-ATPase pump
  • Three regulators stimulate release:
    • Acetylcholine - from vagal nerve
    • Gastrin - hormonal
    • Histamine - paracrine effects
  • Inhibition by somatostatin
  • There is always a basal level of secretion, approximately 10% of total output, which is reduced by vagotomy and anti-histamine drugs
  • Normal total 24 hour output of gastric fluid is ~1.5L

• Three phases of gastric acid secretion
  • Cephalic
    • Sight, smell, thought or taste of food
    • Probably mediated by vagal nerve
  • Gastric
    • Begins when food enters the gastric lumen
    • Mediated via protein interactions with antral G cells, and mechanical distension of the stomach
    • Accounts for most (70%) of acid secretion
  • Intestinal
    • Initiated by entry of chyme into the duodenum, which stimulates gastric release and suppresses gastric motility
    • Lasts as long as chyme remains in the proximal small bowel
• Pharmacological regulation of acid secretion
  • See 'GORD' under medical management

Other secretion products

• • Intrinsic factor
    • Produced by parietal cells
    • Essential for absorption of B12 in the TI
    • Secreted in great surplus
    • Not inhibited by PPIs
    • Intrinsic factor deficiency can develop in patients with pernicious anaemia or those undergoing total gastrectomy - need B12 supplementation either orally or by injection
  • Pepsin
    • Proteolytic enzyme
    • Secreted by the glands of the gastroduodenal mucosa
  • Mucus and bicarbonate
    • Secreted by surface mucous cells and mucous neck cells
    • Neutralise gastric acid at the mucosal surface
    • Production is stimulated by vagus, cholinergic agonists, prostaglandins, and some bacterial toxins
    • Inhibited by anticholinergic drugs and NSAIDs
    • Helicobacter pylori breaks down mucin

Gastric motility

• • Regulation
    • Extrinsic neural control - PNS and SNS
    • Intrinsic neural control - enteric nervous system and interstitial cells of Cajal
    • Myogenic control - from the gastric smooth muscle cells
  • Fasting gastric motility - primarily driven by intrinsic and myogenic control
  • Post-prandial gastric motility
    • Ingestion - decrease in resting tone of proximal stomach and fundus, primarily mediated by vagal nerve
    • Repetitive forceful contractions of the midportion and antral portion to mix food
    • Emptying - mostly hormonally influenced

Gastric barrier function

• • Blood flow - >50% reduction in blood flow leads to mucosal injury, and >75% reduction will see marked mucosal injury