Alimentary System

The alimentary system (Lt. alimentum :nourishment ) includes all the body structures involved in preparing food for absorption into the body and excretion of waste products ie. The gastrointestinal system is (strictly speaking) composed of only the stomach and small and large intestines but is commonly deemed to include the oesophagus as well.

A knowledge of the normal microanatomy of the gastrointestinal system is vital to the understanding of:

Normal function of the gastrointestinal system

The main functions of the gastrointestinal system are:

Absorption of nutrients from ingested food occurs in four main phases within defined regions of the gastrointestinal tract

1. fragmentation

2. digestion 3. absorption 4. elimination of waste products In order for food to be properly absorbed, it must be propelled smoothly from one segment to another. This occurs via: Protection against ingested pathogens occurs as a result of

Structure of the gastrointestinal system
The gastrointestinal system (or tract) is essentially a muscular tube which has the same basic structure throughout. Four layers are recognised, from inside out:

  1. mucosa
  2. submucosa
  3. muscularis propria
  4. serosa

The mucosa varies greatly in the different regions of the gastrointestinal tract while the other layers are essentially the same.


Structure of GI tract

Mucosa

Submucosa Muscularis propria Serosa
1.Colonic mucosa
2. Whole colon

The oesophagus

3.Oesophageal mucosa

The oesophago-gastric junction

4. Oesophago-gastric junction

Case history: A 52 year old woman complained of a retrosternal burning sensation for six months, particularly after food and on lying down. This is a picture of her lower oesophagus taken at endoscopy.
5. Barrett's oesophagus

The stomach


Anatomical regions of the stomach

 

Region Secretion Cell type
Cardia Mucus Mucous neck cells
Fundus & body Gastric juice containing
  • hydrochloric acid (pH 0.9-1.5)
  • intrinsic factor
  • pepsin
  • gut hormones

 

Parietal cells
Parietal cells
Chief cells
Endocrine cells
Pylorus Mucus
Gastrin
Mucus secreting cells
G cells

6. Gastric body mucosa
7. Gastric body mucosa

The small intestine

Epithelial cells of the small intestine
Cell type Location Function
Enterocyte Lining most of the villus Digestion
Absorption
Goblet cell Scattered between the enterocytes Produce mucin
Paneth cell Bases of the crypts of Lieberkuhn Unknown

Life cycle of epithelial cells

8. Ileum
9.
10.Villus small intestine
11. Electron micrograph Villus small intestine
12. Villus small intestine
13.Electron micrograph Villus small intestine
14. Crypt of Lieberkuhn

Case history :A 3 year old girl presented with a history of poor appetite and failure to thrive. On examination, her height and weight were both on the third centile. A duodenal biopsy was carried out
15. Enteropathy

The large intestine

16. Colonic mucosa

The Liver and Pancreas

The liver, biliary system and pancreas are derived from the foregut during embryogenesis and have important roles in digestion and metabolism of many substances.

Exocrine functions: Both the liver and pancreas are exocrine glands emptying secretions via ducts into the duodenum.

Endocrine functions:

The Liver
The liver is the largest internal organ and gland of the body. Hepatocytes are the main functional cells of the liver. In which primary tissue type would you classify hepatocytes?

Liver architecture The connective tissue 'stroma' of the liver is very important in that it maintains the ordered architecture of the liver and thereby the close relationship between the hepatocytes, the vasculature and the biliary ducts. In cirrhosis of the liver the normal architecture is disrupted by excess fibrocollagenous tissue resulting in impaired liver function.

  • The liver is surrounded by a fibrocollagenous capsule.
  • Fibrocollagenous connective tissue continuous with the capsule, extends into the liver as septa in which the vasculature and ducts of the liver travel. The liver can be considered to be divided into lobules by this connective tissue. The supplying vascular components (the portal veins and hepatic arteries) and ducts for biliary outflow (bile ducts) are located in portal tracts or the connective tissue at the periphery of the lobules.
  • A fine network of reticular fibres (type III collagen) supports the hepatocytes and sinusoids of the hepatic lobule as well as the central vein that carries blood away from the sinusoids.

 

17.Liver, hepatic lobules
The pig has especially prominent connective tissue septa surrounding hepatic lobules and is used here to illustrate the lobules.
18.Liver, reticulin stain
19.Liver, reticulin stain
20.The portal tract, H&E

The Pancreas

The pancreas is a highly lobulated gland with a delicate capsule and fibrocollagenous septa dividing the lobules. The exocrine portion is formed by closely packed acini which drain their secretions into a highly branched duct system. Hence, the pancreas is referred to as a compound acinar gland. Each acinus is surrounded by the basement membrane of its epithelial cells and supporting tissue.

The acinar cells are the most abundant cells in the pancreatic. These are typical protein-secreting cells which package the enzymatic precursors they produce into many zymogen granules which stain brightly red in H&E preparations. The initial segments of the pancreatic ducts are formed by more lightly staining centroacinar cells which along with other portions of the duct system secrete bicarbonate ions and water into pancreatic juice.

21.Note the division of the pancreas into lobules.
22.High magnification view of the pancreas


Systems