Special Histology and Embryology: Practical Course - V. K. Napkhanyuk 2001

Digestive System
Middle and Posterior Parts of the Digestive System
Small Intestine

The middle section of the Digestive System includes the small and large intestines.

The Small Intestine (intestinum tenue) is a part of the digestive tract located in the lower Abdominal cavity between The Stomach and the cecum.

The small intestine consists of three parts: 1) the duodenum; 2) the jejunum; 3) the ileum.

The wall of the small intestine is formed by: the mucous membrane (tunica mucosa); the submucosa (tela submucosa); the muscular layer (tunica muscularis); the serous layer (tunica serosa).

The mucous membrane consists of three layers:

1) the epithelial layer, represented by a single-layered columnar brush-border epithelium;

2) the lamina propria of the mucosa, characterized by A large number of reticular fibers forming a dense network. Dendritic Cells, structurally similar to the reticular cells of Hematopoietic Organs, are closely associated with these fibers;

3) the muscularis mucosae, which consists of two layers: an inner circular layer and an outer (looser) longitudinal layer.

The relief of the mucous membrane is characterized by circular folds, villi, and crypts, which significantly increase the total surface area of the mucosa, thereby facilitating the absorption function.

The circular folds (plicae circulares) are formed by the mucous membrane and the submucosa.

Villi (villi intestinales) are finger-like or leaf-like projections of the mucous membrane. Villi are numerous, especially in the duodenum and jejunum. The villi are covered with a single-layered columnar epithelium, in which Three types of cells can be distinguished:

a) columnar (brush-border) cells, which form The basis of the epithelial layer. The brush border consists of numerous microvilli where intensive processes of nutrient breakdown and absorption take place;

b) goblet cells, which produce mucus. Their number increases in the direction from the duodenum to the ileum;

c) endocrine cells, which produce BIOLOGICALLY ACTIVE SUBSTANCES: histamine, serotonin, motilin, substance P, enteroglucagon, secretin, cholecystokinin, and pancreozymin.

Depending on their secretion, EC, A, I, G, D, and Dl endocrinocytes are distinguished.

Crypts (cryptae intestinales) are tubular invaginations of the epithelium into the lamina propria of the mucous membrane (Lieberkühn glands).

The crypt opening opens into the lumen between the bases of the villi. The crypt epithelium contains:

— undifferentiated epitheliocytes;

— columnar epitheliocytes;

— goblet exocrinocytes;

— exocrinocytes with acidophilic granules;

— gastrointestinal endocrinocytes.

The mucous membrane is rich in accumulations of lymphoid tissue in the form of Solitary lymphoid nodules with a diameter of about 0.5-3 mm and Aggregated lymphoid nodules.

The submucosa often contains patches of adipose tissue. It houses Blood Vessels and the submucosal nerve plexus. In the duodenum, complex branched tubular submucosal glands are located in the submucosa.

The muscular layer of the small intestine consists of two layers: an inner circular layer (more robust) and an outer longitudinal layer.

Between the two Muscle layers lies a layer of loose Fibrous Connective Tissue containing the ganglia of the myenteric nerve plexus.

The serosa covers the outer surface of the small intestine entirely, except for the duodenum, which is covered by the Peritoneum only anteriorly, while its other parts possess a connective tissue adventitia.

Slide 17. Section of the duodenum (Fig. 54).

Low magnification. Examine and sketch the slide.

Identify the Tunics of the intestinal wall.

Mucosa: villi (mucosal projections) are clearly visible. Numerous intestinal crypts, appearing as epithelial tubules, are located in the lamina propria beneath the villi. The epithelium covering the villi and forming the crypts is simple columnar absorptive epithelium interspersed with goblet cells. The muscularis mucosae is thin and consists of two smooth muscle layers: an inner circular layer and an outer longitudinal layer.

The submucosa is formed by loose irregular connective tissue and contains complex branched tubular mucous glands known as duodenal (Brunner's) glands. Their excretory ducts (lined with simple columnar epithelium) open into the bases of the crypts. The submucosa also contains Meissner's submucosal nerve plexus, which is a cluster of Nerve Cells and nerve fibers.

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Fig. 54. Cross-section of the duodenum. Hematoxylin and eosin stain. x 140:

1 — villi; 2 — crypts; 3 — muscularis mucosae; 4 — duodenal glands in the submucosa; 5 — muscular coat of the duodenum; 6 — serosa

Muscular coat: a smooth muscle layer forming an inner circular layer and a thin outer longitudinal layer. A thin connective tissue layer containing the intermuscular (myenteric) Auerbach's nerve plexus is situated between the muscle layers.

Serosa: a thin layer of connective tissue covered by mesothelium (simple squamous epithelium).

Label the following on the diagram: 1) villi; 2) crypts; 3) muscularis mucosae; 4) duodenal glands in the submucosa; 5) muscular coat; 6) serosa.

Slide 18. Section of the ileum (Fig. 55).

Low magnification. Examine the slide. In principle, The Structure of this intestinal segment is similar to that of the duodenum. The difference is that the ileum (as well as the jejunum) lacks duodenal glands, and its villi are narrower and taller.

High magnification. Study a Cytology/practical/54.html">Longitudinal section of a villus and an intestinal crypt.

Fig. 55. Cross-section of the ileum. Hematoxylin and eosin stain. x 40:

1 — mucosa; 2 — submucosa; 3 — muscular coat; 4 — serosa; 5 — Simple Epithelium of the villus; 6 — lamina propria of the mucosa; 7 — villi; 8 — crypt; 9 — muscularis mucosae; 10 — blood vessels; 11 — circular layer of the muscular coat; 12 — longitudinal layer of the muscular coat

A villus is formed by all the Components of the mucosa and is covered by simple columnar absorptive epithelium. Three Cell types are distinguished within the epithelium: absorptive cells (epitheliocyti columnares villi), which make up the bulk of the epithelial layer covering the villus; goblet cells (exocrinocyti caliciformes); and gastrointestinal endocrine cells (endocrinocyti gastrointestinales) (micrographs 32, 33). The core of the villus consists of loose irregular connective tissue containing strands of smooth muscle cells running parallel to the long axis of the villus, blood vessels (predominantly capillaries), and a central, endothelium-lined blind-ended lymphatic capillary (lacteal) running along the axis of the villus.

Intestinal crypts are formed by simple columnar epithelium containing the following cell types: absorptive cells, goblet cells, exocrine cells with acidophilic granules or Paneth cells (exocrinocyti cum granulis acidophilis), and undifferentiated epithelial cells (epiteliocyti nondifferentiati). The primary function of the crypts is generative, ensuring the regeneration of the villus epithelium.

Label the following on the diagram: 1) mucosa; 2) submucosa; 3) muscular coat; 4) serosa; 5) simple epithelium of the villi; 6) lamina propria of the mucosa; 7) villi; 8) crypt; 9) muscularis mucosae; 10) blood vessels; 11) circular layer of the muscular coat; 12) longitudinal layer of the muscular coat.