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

Digestive System
Middle and Hindgut
Large Intestine

The Large Intestine (intestinum crassum) is a section of the digestive tract responsible for the Formation and Elimination of feces.

The large intestine is subdivided into the following parts: the cecum, the Appendix, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, and the rectum.

Colon

The wall of the colon is composed of the mucosa, the submucosa, the muscular layer, and the serosa.

The mucous membrane contains numerous circular folds and intestinal glands (crypts), but lacks villi.

Circular folds are formed on the inner surface of the intestine by the mucosa and submucosa. They run transversely across the intestine and have a crescentic shape.

The intestinal glands (crypts) in the colon are better developed than in the Small Intestine; they are more densely packed, larger in size (0.4–0.7 mm), wider, and contain an Abundance of goblet exocrinocytes.

The mucosal epithelium is a simple columnar epithelium consisting of three main Cell types: 1) columnar epithelial Cells; 2) goblet exocrinocytes; 3) gastrointestinal endocrinocytes.

The lamina propria of the mucosa forms thin Connective Tissue partitions between the intestinal crypts. Solitary lymphoid nodules are frequently found within this layer.

The muscularis mucosae is more pronounced than in the small intestine and consists of two layers: an inner, denser layer formed mainly by circularly arranged bundles of smooth myocytes, and an outer layer composed of smooth myocyte bundles oriented partly longitudinally and partly obliquely relative to the axis of the intestine.

The submucosa contains numerous fat cells. It houses vascular and submucosal nerve plexuses, as well as an abundance of lymphoid nodules.

The muscular layer consists of two smooth Muscle layers: an inner circular layer and an outer longitudinal layer, which has a unique Structure. It is not a continuous layer; instead, the bundles of smooth myocytes are gathered into three bands (taeniae coli) that run along the entire length of the colon.

The serosa covers the outer surface of the colon. Occasionally, it features ring-like projections consisting of adipose tissue accumulations covered by the Peritoneum.

Appendix

In the mucosa, crypts are well developed, the epithelium is simple columnar with a brush border and a small number of goblet cells. The muscularis mucosae is poorly developed. Numerous large aggregations of lymphoid nodules are located in the connective tissue of the lamina propria and submucosa. The muscular layer is represented by two layers of smooth muscle cells: an inner circular layer and an outer longitudinal layer.

Unlike other PARTS OF THE large intestine, the longitudinal layer of the muscular coat in the appendix is continuous, meaning it does not form taeniae. The outer surface is covered by a serosa.

Rectum

The wall of the rectum is composed of the same tunics as the wall of the colon.

In the pelvic part of the rectum, the mucous membrane forms three transverse folds. These folds are formed with the participation of the submucosa and the circular layer of the muscular coat. Below these folds lie 8–10 longitudinal folds, between which depressions are visible.

The anal part of the rectum features three zones: the columnar zone, the intermediate zone, and the cutaneous zone.

The rectal mucosa consists of the epithelium, the lamina propria, and the muscularis mucosae.

The epithelium in the upper section is simple columnar; in the columnar zone of the lower section, it is stratified cuboidal; in the intermediate zone, it is non-keratinized stratified squamous; and in the cutaneous zone, it is keratinized stratified squamous.

The lamina propria contributes to The formation of the rectal folds. It contains solitary lymphoid nodules and Blood Vessels. In the region of the columnar zone, There is a network of thin-walled vascular lacunae, from which blood drains into the hemorrhoidal Veins. The intermediate zone contains A large number of elastic fibers, as well as lymphocytes and tissue basophils.

In the cutaneous zone surrounding the anus, Hair follicles are associated with Sebaceous Glands.

The muscularis mucosae, similar to other parts of the large intestine, consists of two layers.

The submucosa contains vascular and nerve plexuses. Lamellated corpuscles are also present here, along with the hemorrhoidal venous plexus.

The muscular coat consists of two layers:

1) an inner circular layer, which forms two thickenings at different levels known as sphincters: internal and external, the latter being striated muscle;

2) an outer longitudinal layer, which, unlike in other parts, is continuous.

The serous coat covers the rectum in its upper part; in the lower sections, the rectum possesses a connective tissue coat.

Slide 19. Section of the large intestine (Fig. 56).

Low magnification. Locate the coats of the intestinal wall. The large intestine lacks villi.

Mucous membrane: simple columnar epithelium with a large number of goblet cells, alongside columnar epithelial cells, undifferentiated cells, and endocrinocytes. The lamina propria contains intestinal crypts. Crypts in the large intestine are more closely spaced and have wider lumens. The muscularis mucosae is thin, consisting of an inner circular and an outer longitudinal layer. Solitary lymphatic follicles are typically found in the lamina propria of the mucous membrane. Often, these follicles enlarge and extend into the submucosa, which is formed by connective tissue.

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Fig. 56. Section of the large intestine. Hematoxylin and eosin staining. x 40:

1 — mucous membrane; 2 — submucosa; 3 — muscular coat; 4 — serous coat; 5 — simple intestinal epithelium; 6 — goblet cells; 7 — crypt; 8 — lamina propria of the mucous membrane; 9 — muscularis mucosae; 10 — Nerve Cells of Meissner's plexus; 11 — lymphatic follicle; 12 — blood vessels; 13 — circular layer of the muscular coat; 14 — longitudinal layer of the muscular coat; 15 — mesothelium

The structure of the submucosa shows no significant differences from that of the small intestine.

The muscular coat consists of two layers (inner circular and outer longitudinal) of Cytology/cytology/32.html">Smooth Muscle tissue. In the outer layer, bundles of smooth muscle cells are gathered into three bands (teniae) that run along the entire length of the intestine.

The serous coat is the visceral layer of the peritoneum.

Label on the drawing: 1) mucous membrane; a) epithelium; b) lamina propria, containing: intestinal crypts; lymphoid follicles; c) muscularis mucosae; 2) submucosa; 3) muscular coat: a) circular layer; b) longitudinal layer; 4) serous coat.

Slide 20. Cross section of the appendix (Fig. 57).

Low magnification. Examine and draw the slide.

The STRUCTURE OF THE appendix is very similar to that of the large intestine. The mucous membrane forms folds that project into the lumen. The deep crypts have a typical structure.

Unlike other parts of the large intestine, the lamina propria and submucosa contain numerous large lymphatic follicles. In some cases, the follicles reach the lumen, causing the wall of the appendix to bulge, thereby narrowing the lumen. In these areas, crypts are typically absent.

Light germinal centers are visible within the follicles, indicating follicular reactivity. The outer muscular coat consists of inner circular and outer longitudinal layers of smooth muscle, though they are slightly thinner here than in other sections of the large intestine. Unlike other parts of the large intestine, the outer layer of the muscular coat is continuous, meaning it does not form teniae.

The serous coat presents no special features.

Label on the drawing: 1) mucous membrane: a) simple columnar brush border epithelium; b) crypts; c) lamina propria of the mucous membrane; 2) submucosa; 3) muscular coat; 4) serous coat; 5) lymphoid nodules located in the mucous membrane and submucosa.

Fig. 57. Cross section of the appendix. Hematoxylin and eosin staining. x 40:

1 — fold of the mucous membrane; 2 — Simple Epithelium; 3 — crypt; 4 — lamina propria of the mucous membrane; 5 — muscularis mucosae; 6 — lymphoid follicle; 7 — submucosa; 8 — circular layer of the muscular coat; 9 — longitudinal layer of the muscular coat; 10 — serous coat; 11 — mesothelium; 12 — blood vessels

Review Questions

1. Development of the small intestine.

2. Structural Features of the duodenum.

3. Structure of intestinal villi in the small intestine.

4. Crypts of the small intestine.

5. Features of the Blood supply to the small intestine.

6. Histology of the absorption process in the small intestine.

7. Development of the large intestine.

8. Structure of the colon.

9. Structure of the appendix.

10. Morphofunctional Features of the rectum.

11. Structural features of the epithelium in different segments of the rectum.

12. Structure of the muscular tunic of the rectum.

13. Morphofunctional characteristics of duodenal glands.

Electron micrograph 32. Apical part of an intestinal villus epithelial cell. x 16,000:

1 — cellular microvilli forming the brush border; 2 — zonula occludens; 3 — intercellular boundaries; 4 — Mitochondria; 5 — lysosome; 6 — Endoplasmic reticulum (from Rodin's atlas)

Electron micrograph 33. Apical part of a goblet cell from an intestinal crypt gland. x 16,000:

1 — lumen of the crypt; 2 — cellular microvilli; 3 — mucous secretion (after Trier)

Case Studies

1. In an experiment, the intramural ganglia of the myenteric nerve plexus of the small intestine were removed. What functional disorders will the intestine undergo?

2. The intestinal villi are lined with epithelium containing Three types of cells. Which of these cells are involved in membrane (contact) Digestion?

3. Two unnamed histological slides are presented. One shows broad, short villi, crypts, and glands in the submucosa; the muscular tunic consists of two layers: an outer longitudinal layer and an inner circular layer. The other slide shows deep pits, glands in the lamina propria of the mucosa, and three layers in the muscular tunic: an outer longitudinal, a middle circular, and an inner oblique layer. Which parts of the gastrointestinal tract are represented on these slides?

4. As a result of trauma, the epithelium of the small intestinal mucosa was damaged. Which cells will drive its regeneration? In which structures of the intestine are they located?

5. Nerve plexuses are located within the wall of the gastrointestinal tract. Neurocytes of some plexuses control The activity of glandular and muscle cells, while neurocytes of others control muscle cells exclusively. Is there a difference in their localization? In which layers of the gastrointestinal wall are they situated?

6. At the peak of digestion, active movement of the intestinal villi is observed, resulting in changes to their length. What is the explanation for this phenomenon?

7. As a result of prolonged antibiotic therapy, a patient has impaired Cellulose digestion in the large intestine. What is the reason for this?

8. Slides have been prepared from the duodenum and the small intestine. What structural features can be used to distinguish between them?

9. Slides have been prepared from the jejunum and the colon. How can they be distinguished?

Sample Examination Questions

1. Small intestine. Structural and functional Characteristics of Individual layers. Histophysiology of the villus–crypt system.

2. Large intestine. Vermiform appendix. Structure. Functional significance.