Monogastric Stomach - Anatomy & Physiology

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Introduction

The enlarged swelling of the gastrointestinal tract between the oesophagus and duodenum is called the stomach. It is a simple structure in carnivores and a compound structure in ruminants.

The stomach functions as a reservoir of food where digestion occurs through chemical and mechanical processes. This allows food to be broken down further and absorbed.

Development

Stomach Development- Copywright RVC 2008

• The gut tube is formed from the folding of splanchnopleure (mesoderm and endoderm)

• The mesoderm forms the skeletal muscle (oesophagus and anus), smooth muscle (lateral plate mesoderm) and connective tissue layers around the inner endoderm.

• The endoderm is the inner layer forming the epithelia and glands.

1. Region enlarges. Swelling indicates where the stomach will form.

2. Dorsal surface becomes convex to form the greater curvature

3. Ventral surface becomes concave to form the lesser curvature

4. 2 rotations of 90 degrees occur along the longitudinal axis and then then the dorso-ventral axis

5. The dorsal mesogastrium becomes elongated (with the spleen) and expands into a large fold along the ventral abdominal wall. This becomes the greater omentum which covers all the abdominal organs. It is a superficial structure which is free to move.

6. The ventral mesogastrium becomes the lesser omentum. It is inbetween the stomach and the liver. The rest of the ventral mesentry degenerates.

Functional Anatomy

• Stomach split into regions: cardia, fundic, body and pyloric parts

• Entire stomach is motile

• pH 0.9 to 1.5

• The larger part of the stomach lies to the left of the midline, under cover from the ribcage and contacts the liver and diaphragm. The oesophagus opens into it at the cardiac sphincter.

• The smaller part of the stomach has thicker walls and passes to the right of the midline into the duodenum at the pyloric sphincter.

• The fundus is a blind dome rising above the cardia

• The body extends from the cardia ventrally

• The pyloric part is on the right divided into a more muscular and a less muscular half

• The serosa (external peritoneum) covers the entire organ

• Contractions start near the cardia and spread distally, accelerating and becoming more vigorous as they reach the pylorus region.

• The pyloric sphincter is open for 1/3 of the time during contractions

• The empty stomach lies completely within the rib cage and does not contact the abdominal floor. Little secretion is produced and only small peristaltic contractions occur.

• Once food is offered or anticipated secretions begin

Histology

Glandular Stomach Histology Dog- Copywright RVC 2008

• Columnar epithelium

• Folded mucosa forming longitudinal rugae

• Invaginations called gastric pits which are continuous with gastric glands

• Chief (zygomatic) cells secreting pepsinogen

    -Secreted in response to vagus and gastrin
    -Basophilic 
    -More abundant in base of glands

Fundic Region of Glandular Stomach (Parietal and Chief cells)- Copywright RVC 2008

• Goblet cells secreting mucous

    -Protects against autodigestion

• Parietal (oxyntic) cells secreting hydrochloric acid in gastric pits

    -Aids digestion
    -Activates gastric enzymes, e.g. pepsinogen
    -Kills microorganisms that enter with food
    -Secreted in response to vagus or pepsin  
    -Large, pyramis shaped
    -More abundant in upper region of glands

Lamina Muscualris 3 Regions - Copywright RVC 2008

• 3 layers of lamina muscularis: the outer longitudinal, middle circular layers and inner oblique.

• Glands are short, coiled, branched tubular. Need to be replaced due to wear and tear. Only in mucosal layer.

• Cardia is a narrow muscle strip

• Pyloric sphincter is thickened tunica muscularis from the middle circular smooth muscle layer

• In fundic region, tunica muscularis is thinner, glands are straight and gastric pits are shallow. Abundance of parietal and cheif cells in gland.

• In pyloric region, tunica muscularis is thicker, glands are coiled and gastric pits are deep.

• Lymphatic vessels are present in the submucosa

Blood Supply

• Coeliac artery (which is a branch of the dorsal aorta)

• Coeliac artery splits into the hepatic artery supplying the liver, pancreas and stomach (right gastric and left gastro-epiploic arteries)

• Coeliac artery also splits into the splenic artery which supplies the spleen and the stomach (left gastro-epiploic artery)

• Coeliac artery also splits into the left gastric artery supplying the stomach

• The gastro-epiploic arteries supply the greater curvature of the stomach

• The gastric arteries supply the lesser curvature of the stomach

• The numerous veins join the portal vein

Innervation

• Sympathetic fibres run with the arteries

• Parasympathetic from the vagus nerve (CN X) are within the two vagal trunks. In the proximal region of the stomach, vagal stimulation suppresses muscular contraction (VIP) and in the distal region, vagal stimulation increses muscular activity (ACh).

Digestive Enzymes

• Proteases

   -Inactive zymogen
   -Activated by hydrochloric acid
   -Active pepsin produced
   -Completed near brush border to generate small peptides and individual amino acids

• Carbohydrases

   -e.g. amylase
   -Salivary and pancreatic to produce disaccharides
   -disaccharides converted to monosaccharides near brush border
   -Celluloses from symbiotic micro-organisms (ruminant stomachs)

• Lipases

   -Assisted by bile salts which neutralise stomach acids and emulsifies fats
   -Generates free fatty acids, monoglycerides and diglycerides

Innervation

• Control of gastric secretions under hormonal (gastrin), paracrine (histamine) and neural (ACh) mediators in the cephalic and gastric phases

• Gastric secretions inhibited during the intestinal phase by CCK and secretin

Species Differences

• Horses have a region called the margo plicatus which separates the glandular and non-glandular parts of the equine stomach. The non-glandular area is lined with squamous epithelium (not columnar).

• The equine stomach is relatively small (10% GIT) with a strong cardiac sphincter which prevents the animal from vomiting. The equine stomach is rarely empty and retention time is short as expulsion into the duodenum stops when feeding stops.

• The size of the non-glandular region in the simple stomach varies between species. It is largest in the horse, pig and then smallest in the dog.

• The canid stomach is of variable size ranging from 0.5 to 6L according to breed. A full stomach can touch the bladder.

• The cardia in the pig is thickened, taking up nearly half the area of the stomach

• Carnivores have a subglandular layer of fibroblasts and collagen fibres for protection, e.g. from consuming bones. It is between the glands and the lamina muscularis.