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

Digestive System
Middle and Posterior Segments of the Digestive System
Pancreas

The Pancreas is a mixed gland comprising both exocrine and endocrine components.

The exocrine portion produces pancreatic juice rich in digestive Enzymes—such as Trypsin, lipase, and amylase—which travel through the excretory duct into the duodenum, where they facilitate the Breakdown of Proteins, fats, and CARBOHYDRATES into their end products.

The endocrine portion synthesizes several Hormones, including Insulin, Glucagon, Somatostatin, and Pancreatic Polypeptide, which regulate carbohydrate, protein, and Lipid METABOLISM IN Tissues.

Externally, the pancreas is enveloped by a thin Connective Tissue capsule fused with the visceral Peritoneum.

The parenchyma is divided into lobes separated by connective tissue septa. These lobes contain both the exocrine and endocrine elements of the gland, with the exocrine tissue accounting for approximately 97% and the endocrine tissue for 3% of the total glandular mass.

The Structural and functional unit of the Exocrine Pancreas is the pancreatic acinus.

The pancreatic acinus (acinus pancreaticus) consists of a secretory unit—comprising 8–12 large exocrine pancreatocytes (acinocytes) resting on a basement membrane—and an intercalated duct, which marks the beginning of the gland's duct system. The wall of this duct is lined by small Cells known as centroacinar epitheliocytes.

Exocrine pancreatocytes perform a secretory function. They are cone-shaped, with a narrow apex and a broad base resting on the basement membrane of the acinus.

The apical Regions of the cells are referred to as zymogenic zones, whereas the opposing basal regions are termed homogenous zones. The zymogenic zone is packed predominantly with large secretory granules, while the homogenous zone is rich in granular Endoplasmic reticulum.

The intercalated ducts empty into interacinar ducts, the walls of which are lined with a simple cuboidal epithelium.

Interacinar ducts merge into larger intralobular ducts, whose walls are covered by a simple cuboidal epithelium.

Intralobular ducts continue into interlobular ducts, which course through the connective tissue septa between the lobes.

Interlobular ducts empty into the main pancreatic duct, which opens into the duodenum. These ducts are lined by a mucosa consisting of a tall columnar epithelium and a lamina propria. The duct epithelium contains goblet exocrinocytes as well as endocrinocytes that produce the hormones pancreozymin and cholecystokinin.

The pancreatic islets (insulae pancreaticae) lie between the pancreatic acini and represent the Endocrine portion of the gland.

The islets consist of endocrine cells—insulocytes (insulocyti)—interspersed with fenestrated Blood capillaries surrounded by a pericapillary space.

In their Cytoplasm, insulocytes contain a moderately developed granular endoplasmic reticulum, a well-developed Golgi apparatus, Cell/35.html">Mitochondria, and secretory granules.

The following types of insulocytes are distinguished:

— ß-cells constitute the bulk (70–75%) of the islets, contain granules with the hormone insulin, and are located in the center of the islet;

— a-cells make up about 20–25% of the total mass of islet cells, are located at the periphery of the islets, and contain the hormone glucagon in their granules;

— D-cells (5–10%) are located peripherally, are pear-shaped, and secrete the hormone somatostatin;

— Dl-cells secrete vasoactive intestinal polypeptide, which lowers blood pressure and stimulates the secretion of Pancreatic juice and hormones;

— PP-cells (2–5%) produce pancreatic polypeptide, which stimulates the secretion of gastric and pancreatic juice.

Slide 22. Section of the human pancreas (Fig. 60).

Low power. Examine the General structural plan of the gland. Note that pink connective tissue septa divide the gland into lobules. Within the septa, locate the interlobular excretory duct, lined with simple columnar epithelium, and Blood Vessels, which are easily distinguished from the duct because their endothelial lining cells are flat. Inside the lobule, intralobular and intercalated ducts are present. The latter have a small diameter and are lined with low columnar epithelium. Among the intensely stained terminal portions of the exocrine part of the gland, pale islets representing The Endocrine Part can be found (Micrograph 34).

High power. In the exocrine pancreatocytes of the terminal portions, one can observe the basophilic basal part—the homogeneous zone—and the light apical part containing acidophilic granules—the zymogen zone (Micrograph 35).

In the figure, indicate: 1) secretory units; 2) interlobular connective tissue septum; 3) excretory duct; 4) vein; 5) artery; 6) islet of Langerhans; 7) venous capillaries; 8) centroacinar cells.

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Fig. 60. Cross-section of the human pancreas. Hematoxylin and eosin staining. x 280:

1 — secretory units; 2 — interlobular connective tissue septum; 3 — vein; 4 — artery; 5 — islet of Langerhans; 6 — venous capillaries; 7 — centroacinar cells

Electron micrograph 34. Secretory unit of the pancreas. x 3200:

1 — apical parts of pancreatic cells with secretory granules; 2 — lumen of the secretory unit; 3 — centroacinar epithelial cells; 4 — endoplasmic reticulum in the basal part of pancreatic cells; 5 — blood capillary (after Ekholm et al.)

Electron micrograph 35. Cells of the pancreatic islet. x 16,000:

1 — acidophilic insular cell (A cell); 2 — endoplasmic reticulum; 3 — mitochondria; 4 — cell membrane; 5 — secretory granules; 6 — definitive insular cell (D cell); 7 — intracellular reticular apparatus; 8 — lysosome; 9 — low electron-dense granules; 10 — unmyelinated nerve; 11 — blood capillary (from Rhodin's atlas)

Control Questions

1. Development of the Liver.

2. Structure OF THE liver.

3. Structural Features of the hepatic lobule.

4. Circulatory system OF the liver.

5. Hepatic plate. Cellular composition.

6. Biliary tract and its characteristics.

7. Gallbladder and its structure.

8. Age-related changes in hepatocytes.

9. Development of the pancreas.

10. General structural layout of the pancreas.

11. Morphofunctional CHARACTERISTICS OF THE exocrine part of the pancreas.

12. Structure and cellular COMPOSITION OF THE pancreatic acinus.

13. Structural features of various types of pancreatic ducts.

14. Morphological Characteristics of the pancreatic islets.

15. Features of the Blood supply to the pancreas.

Situational tasks

1. Two liver preparations are presented. One of them shows lobules sharply demarcated from each other, while on the other, connective tissue between the lobules is poorly developed. Determine which preparation represents the human liver.

2. An extremely large number of Glycogen inclusions are detected in the cytoplasm of hepatocytes on a liver preparation. What processes in the body are associated with this phenomenon?

3. A person's diet is rich in carbohydrates. Which liver function should be activated? What structures will be visible in the cytoplasm of hepatocytes in this case?

4. A dye (Prussian blue) was injected into the portal System of the liver. Which vessels will be injected with the dye?

5. It is known that animal liver is used as a high-quality food product in dietary Nutrition. What Properties of the liver account for this?

6. A decrease in prothrombin content was detected in the patient's blood. Which liver function is impaired?

7. Normally, Bile does not pass from the bile capillary into the blood stream. What ULTRASTRUCTURAL FEATURES OF hepatocyte Organization contribute to this?

8. On a pancreatic preparation in the field of view, There is a group of cells surrounded by numerous wide capillaries. Some cells have basophilic cytoplasm, others have acidophilic cytoplasm, and still others are weakly basophilic. Determine which cells are in the field of view.

9. An animal was administered a drug that selectively damages the pancreatic a-cells (cobalt salts). Which function of the pancreas is impaired?

10. An animal was administered alloxan, which selectively damages the ß-cells of the pancreatic islets. Which function of the pancreas is impaired?

Sample exam questions

1. Liver. Structure of the classical hepatic lobule. Structural and Functional Characteristics of hepatocytes, lipocytes, and sinusoidal hemocapillaries. Age-related changes.

2. Pancreas. Structure of the exocrine and endocrine parts and their histophysiology. Age-related changes.