Pancreas: Histology
pancreas refers to the organs of the endocrine system and produces a variety of enzymes.
Its tissue, depending on function and morphological structure, is divided into two types:
- exocrine;
- is endocrine.
The main task of the exocrine gland is to develop enzymes that pass through the special ducts into the lumen of the duodenum and are involved in the digestion of food components( fats, proteins and carbohydrates).In the endocrine department, hormones are produced that first enter the bloodstream, where they affect protein, fat and carbohydrate metabolism.
Let us consider in more detail what the pancreas is, the histology of which can vary significantly within one organ.
Exocrine tissue
It consists of acini( terminal sections) and excretory ducts. Directly in the acinus( acinous cells), the production of enzymes involved in the digestion process occurs. Among them, lipase, amylase, trypsin, chymotrypsin, and others are worth mentioning. The acini within the lobules themselves are quite dense, and between them is a small layer of connective loose tissue in which the capillaries pass. Looking at a histological preparation, it often turns out to be difficult to identify individual acini, as many of them are in an oblique cut.
The shape of the cells of the exocrine part is triangular. The nucleus of cells is located almost in the center( but slightly shifted to the basal part), it has a rounded shape, large nucleoli are noticeable in the nucleus itself.
In this case, acini can be divided into two parts - apical and basal. In the basal part, membrane structures of the endoplasmic granular network are concentrated. If you prepare a histological preparation, this part of the acinous cells is stained with the basic dyes, while the color will be uniform. Therefore, the basal part is also called homogeneous. The apical part of the cell is directed toward the lumen of the cell, it is stained with acidic dyes, another name is the zymogenic cell division. In the acinus itself, there is a well-developed Golgi complex, as well as a large number of mitochondria, which can be seen by considering the prepared histological preparation.
The excretory ducts of the exocrine gland tissue are represented in different ways. Among them, the following types can be distinguished:
- insertion channels;
- intra-lobular( interazinous);
- interlobular;
- common duct.
The intruding excretory duct begins its production in the intercalary part of the acinus. Here it is represented by a flat single-layered epithelium. As you progress, a gradual change of the planar epithelium into a cubic epithelium is noticeable. It is these cells that can be seen if the preparation is prepared in the study of intra-lobular ducts. Gradually the interacinous ducts merge and form interlobular excretory ducts, which are covered with a single-layered cylindrical epithelium, which is clearly visible when looking into the preparation with a microscope. The common duct is formed from interlobular cells, which merge with each other.
It should be noted that epithelial cells of all these ducts( intercalary, interazinous, interlobular and general) are secreted by bicarbonates, due to which the pancreatic juice has an alkaline environment.
Enzymes released by the exocrine part of the gland, and involved in the digestion process, are themselves subject to regulation. This is required to ensure that at the right time a certain amount of enzymes is released into the lumen of the duodenum. Such regulation is carried out with the help of two hormones - secretin and pancreosimin. Under the influence of secretin, there is a release of non-enzyme components of the pancreatic juice, therefore its influence mainly extends to the cells of small ducts. The effect of pancreosimin is more pronounced, since it affects directly the acinous cells and under its influence stimulates the production of pancreatic juice.
Endocrine tissue
Endocrine pancreatic tissue is represented by clusters of cells that form the islets of Langerhans. The islets themselves can have an oval or round shape. Arelets are located in various lobes of the gland and consist of insular cells, which are supplied with blood by an abundant capillary network. These cells can be arranged as branched strands, which have irregular contours, and compact islands. Between them pass the connecting layers with sinusoids - the capillary network. If you prepare a histological preparation, then these cells are poorly colored. Here we can distinguish the following cell types:
- A-cells;
- B cells;
- D-cells;
- D-1 cells;
- PP cells.
Their difference is determined by the characteristic of secretory granules. Let us consider them in more detail.
A-cells are located on the periphery and produce glucagon, which is an insulin antagonist. The secretory granules of these cells are soluble in water, treated with acidic dyes and can be fixed with alcohol.
B cells are located in the center of the islets and are a larger group. They produce insulin, due to which there is a delay in glycogen in the liver cells, which leads to a decrease in the level of sugar in the blood. These cells stain poorly and have a graininess that does not dissolve in water, but is able to dissolve in alcohol.
D-cells can be seen in the drug on the periphery of the islets. They produce somatostatin - a hormone that causes inhibition of the synthesis of A and B-type cells, as well as acini, located in the exocrine gland. The secretory granules of these cells have a moderate density and average size.
D1 cells are represented by a small amount and produce an intestinal polypeptide. This hormone causes increased secretion in the pancreas and leads to a decrease in pressure. The secretory granules of these cells have a bright bezel and intense density.
PP cells can be located both in the peripheral part of the islets and in the exocrine part of the gland. They produce a pancreatic polypeptide, which causes an increase in the production of pancreatic and gastric juice.
The islets of Langerhans have no excretory ducts, because the hormones they produce go directly to the blood. The largest number of these islets is located in the caudal part of the gland. Throughout life, the number of islets can vary, since they are able to form again. But there is also the revealed regularity, according to which the number of islands grows to about 25 years, and then begins to decline.
As you can see, the pancreas is represented by two types of tissue that perform a different function. To study their glandular cells in more detail, one can look into a histological preparation stained with special dyes.
