The pancreas is an elongated gland which extends from the curve of the duodenum across the midline of the body toward the spleen. It has a head (expanded part lying near the duodenum), body and tail. A thin layer of moderately dense connective tissue forms an incomplete capsule around the organ. Septa extending from the capsule divide the pancreas into poorly defined lobules. A stroma of loose connective tissue surrounds the lobules. Larger blood vessels, nerves and ducts lying between the lobules are surrounded by more abundant connective tissue.
The pancreas has both an exocrine and an endocrine component. The exocrine part consists of serous acini that make up most of the organ. The endocrine part consists of distinct masses of cells called islets of Langerhans scattered among the serous acini. The islets vary greatly in size, from a few cells to hundreds of cells.
The islets of Langerhans make up about 2% of the pancreas, and are most numerous in the tail. There are three principal cells types in the islets. B cells, which make up 60-70% of the islets, secrete insulin. A cells (15-20%) secrete glucagon, and D cells (5-10%) secrete somatostatin. Minor cell types, which secrete a number of other peptides, make up about 5% of the islets. The structure of the endocrine pancreas and the functions of its hormones are discussed in the Endocrine Block.
The cells that make up the serous acini of the pancreas are pyramidal in shape with a broad base and a narrow luminal surface. In the apical cytoplasm, they contain acidophilic zymogen granules. These granules contain a number of digestive enzymes in their inactive form, including trypsinogen, chymotrypsinogen, procarboxypeptidase (all for digesting proteins), ribonuclease, deoxyribonuclease, triacylglycerol lipase, phospholipase A2, elastase, and amylase. These products are conveyed by ducts to the small intestine, where enterokinases from the glycocalyx activate trypsinogen by converting it to trypsin. Trypsin in turn activates all the inactive enzymes, including trypsinogen. The activation of trypsinogen within pancreatic cells is inhibited by trypsin inhibitor.
The duct system in the pancreas begins within the acini themselves. Cells of the smallest ducts, the intercalated ducts, penetrate right into the center of the acinus. In sections, they can be identified as centroacinar cells or CA cells. The cells of the intercalated ducts are flattened and take up little stain. They have elongated nuclei whose long axis is oriented in the direction of the duct.
The intercalated ducts are short and drain into the intralobular collecting ducts, which have a cuboidal epithelium. The intralobular collecting ducts form a complex, branching network and eventually drain into larger interlobular or excretory ducts, which are lined with low columnar epithelium. Enteroendocrine cells and an occasional goblet cell can be found in these ducts. THERE ARE NO SECRETORY (STRIATED) DUCTS IN THE PANCREAS. The interlobular ducts drain directly into the main pancreatic duct, which runs the length of the pancreas parallel to its long axis and joins the common bile duct before entering the duodenum.
The intercalated ducts secrete a large volume of fluid that is rich in sodium and bicarbonate. The bicarbonate serves to neutralize the acidic chyme that enters the duodenum from the stomach. This establishes the optimum pH in the duodenum for the activity of the major pancreatic enzymes.
Two hormones secreted by enteroendocrine cells in the duodenum are major regulators of exocrine pancreatic activity. Secretin stimulates the release of the bicarbonate rich fluid from the intercalated ducts, while cholecystokinin (CCK) stimulates the acinar cells to release their proenzymes. The release of secretin and CCK is stimulated by the entry of acidic chyme into the duodenum.
Figure 7 - Low power view of the pancreas shows a low power view of the pancreas made from slide #53 of your collection. Several lobules, separated by delicate connective tissue septa, can be identified. The connective tissue is not much in evidence, but its location is discernible by the artefactual gaps between lobules. The islets of Langerhans appear as pale-staining islands scattered throughout the organ. Two islets can be easily identified near the top of the field of view. A much smaller one is seen some distance below them. The organization of neither the exocrine acini nor the endocrine islets can be seen at this magnification.
Figure 8 - High power view of pancreatic acini and isletA higher power view of the pancreas is seen in figure 8 (also from slide #53). The serous acini are closely packed together and do not appear very distinct. However, the red, refractile zymogen granules at the apical ends of their cells stand out. Usually, no lumen is identifiable in the centre of the acini on this slide.
Most of an islet of Langerhans is seen at the left, its boundary indicated by asterisks. Islets consist of cords of cells through which numerous fenestrated capillaries course.
The images of the pancreas shown in Figures 9 to 13 are all made from slide S49.
Figure 9 - high power view of centroacinar cells shows a high power view of centroacinar cells lying within pancreatic acini. Near the middle of the field of view, the boundary of a somewhat longitudinally sectioned acinus is indicated by asterisks. An extended section of the intercalated duct lying within that acinus (the pale-staining centroacinar cells) is readily identified. The duct leaves the acinus at the left. CA cells are also seen in many of the surrounding acini.
Figure 10 - High power view of an intralobular collecting duct cut in longitudinal sectionThe intercalated ducts are short and drain into intralobular collecting ducts, which form a complex, branching network within a lobule. The epithelium is cuboidal. Figure 10 shows a longitudinal section of an intralobular collecting duct extending across most of the field of view. The length of the duct in this section is fortuitous. It lies just above an islet of Langerhans. Note the pale staining of both the islets and the ducts. The apical surfaces of the cells of the surrounding acini appear slightly brighter than the basal surfaces due to the presence of zymogen granules.
Figure 11 - Intercalated ducts entering an intralobular duct shows some intercalated ducts entering an intralobular collecting duct. The intercalated ducts are not very distinct, they appear as pale cells squeezed among the acini. The one indicated by the lower leader of "icd" can be followed for some distance to the left and probably originates from the acinus at the far left of the field of view. It is joined by a duct from the acinus just below the lower leader. The intercalated duct indicated by the upper leader of "icd" appears to originate from the acinus just to its right. The intraloubular duct appears as a distinct oval profile with a prominent lumen containing secretory material (arising from the acini and intercalated ducts). It has cuboidal cells whose boundaries are not distinct, but their round nuclei can be clearly seen.
Figure 12 - Excretory duct in the pancreas shows an excretory duct. Excretory ducts are also called interlobular ducts, and as this name implies, they lie in the connective tissue septa between lobules. (The CT is more abundant when ducts or other structures, such as blood vessels and nerves, are present.) Excretory ducts are lined with low columnar epithelium which becomes taller as the duct progresses. The cell boundaries of the duct in Figure 12 cannot be distinguished but their slightly oval nuclei are easily seen. Secretory material is present in the lumen.
Figure 13 - Low power view of large excretory duct shows a low power view of a larger excretory duct lying within connective tissue surrounded by pancreatic parenchyma. This duct will eventually join the main pancreatic duct.
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