Lesson 21 - Digestive System (III) (Liver Pancreas) (notes).pdf

Cytology and Histology _____________ Lesson 21 ______________ LIVER AND PANCREAS I. LIVER The liver is the largest gland (exocrine and endocrine) in the body. It is located, in all species except birds, in the upper right quadrant of the abdominal cavity, between the diaphragm (cranial) and the s...

Cytology and Histology _____________ Lesson 21 ______________ LIVER AND PANCREAS I. LIVER The liver is the largest gland (exocrine and endocrine) in the body. It is located, in all species except birds, in the upper right quadrant of the abdominal cavity, between the diaphragm (cranial) and the stomach and intestinal loops. The cranial or diaphragmatic face is convex and is divided into a variable number of lobes depending on the species. In all species except horses and birds, there is a vesicle associated with the liver called the gallbladder. The main functions of the liver are: 1. 2. 3. 4. 5. 6. Exocrine function: Synthesize and secrete bile to the duodenum. Endocrine function: Synthesize and secrete proteins into the blood. Metabolic function: Of proteins, carbohydrates, fats, and haemoglobin. Storage: Of triglycerides, glycogen, vitamins A and B. Defence: Intravascular phagocytosis, drug detoxification. Haematopoiesis: In the embryo and, potentially, in the adult. It is very important to remember that all these functions are carried out by only one type of cell, the hepatocyte. I. 1. General structure of the liver It is a parenchymatous organ with stroma and parenchyma. The stroma, from outside to inside, has the following parts: 1) Capsule: It is made of dense irregular connective tissue. It is called Glisson's capsule, and it is covered by peritoneum on its entire surface except at the level of the hilum, located on the caudal side of the organ. Sometimes it contains smooth muscle fibres. 2) Trabeculae: They are branches of the capsule, therefore of dense irregular connective tissue, that penetrate the thickness of the liver. They are very thin and only clearly visible in pigs, a species in which they are thickest. For this reason, pork liver is tougher, as a food, than beef liver. The trabeculae divide the liver parenchyma into contiguous hexagonal spaces called lobules (only clearly visible in pig liver) (Figure 1). 3) Portal spaces: These are the connective tissue areas located at the corners or diagonals of the lobule that contain an arterial branch, a venous branch and a bile duct. The arterial branch comes from the hepatic artery and the venous branch from the portal vein. These vessels enter the liver through the hilum, which is where the bile duct exits. In each lobule there are six portal spaces, one at each angle of the hexagon, but only 3 are seen at a time in a histological section. Portal spaces are also called Kiernan’s spaces (KS, Figure 1). 4) Finally, the thinnest branches of the hepatic stroma are the fibres that support the hepatocytes, which are reticulin fibres. KS CV Figure 1. Schemes of the general structure of the liver in liver lobules (A) and of a part of the liver lobule (B). KS: Kiernan’s space; CV: central vein. HE. The liver parenchyma is made up mostly of hepatocytes. These hepatocytes are organized into morphological units called hepatic lobules, peripherally delimited by the trabeculae and centrally present a vein called central vein (Figure 1). The cellular elements of the liver lobule are: 1) 2) 3) 4) Hepatocytes (80% of lobule cells). Endothelial cells and Kupffer cells as elements of sinusoids. Hepatic stellate cells or Ito cells. Foveolar cells. 1) Hepatocytes Hepatocytes are polygonal cells, 20 to 30 µm in diameter, which are close together, forming rows (cords) of 1 or 2 hepatocytes separated each row from the other by sinusoids. The cords are arranged radially from the centre to the periphery similar to the spokes of a wagon wheel being the centre the central vein. The plasma membrane of hepatocytes has two types of faces or surfaces depending on whether they are in contact with another hepatocyte or with the sinusoid: A) Perisinusoidal surface in which the cell membrane has microvilli and contacts the space that externally surrounds the sinusoids called the space of Disse. These microvilli facilitate the exchange of materials between the hepatocyte and the plasma in the perisinusoidal space. The hepatocyte discharges its endocrine secretions there, which pass into the sinusoidal blood. In addition, the blood elements enter the hepatocyte through this area, thanks to membrane receptors for mannose-6phosphate, ATPase, adenylate cyclase, Na+ and K+ and adenylate cyclase. B) Lateral surface, the membrane may have different specializations: a) Desmosomes, modes of junction to the adjacent hepatocyte membrane. b) Porous junctions with the adjacent hepatocyte membrane to communicate with each other. c) Labyrinthine intercellular spaces of 1 to 2 µm in diameter called bile canaliculi delimited by occluding areas to prevent leakage of bile into the intercellular space. Hepatocytes poured bile to these canaliculi, which lead it to the periphery of the hepatic lobule in the opposite direction to the blood. At the level of the canaliculus, the membrane of the hepatocyte has microvilli that increase the secretion surface. At light microscopy bile can be observed Cytology and Histology between hepatocytes as yellow or green lines with HE stained sections (especially in pathological situations). The nucleus is rounded and central, with one or more nucleoli. 75% of hepatocytes have a nucleus, the rest are binucleated. The size is variable: 50% are small and 50% large, nuclei that are diploid and polyploid, respectively. A B Figure 2.- Hepatocytes under light microscope stained with HE (A) and under the electron transmission microscopy (B). The cytoplasm contains: A) Cell-specific organoids that synthesize proteins (both self and export): abundant ribosomes, RER and Golgi complex, preferably close to the bile canaliculi. B) Numerous mitochondria, due to the great energy needs; it is estimated approximately 2,000 in each hepatocyte. C) Peroxisomes or microsomes, which are small spherical organelles surrounded by a membrane unit, very similar to lysosomes in size and structure. However, what they contain are totally different enzymes. Thus, using histochemical techniques, we observed that they contain oxidases (involved in the oxidation of long-chain fatty acids to hydrogen peroxide: bactericidal) and catalase; they also regulate the metabolism of hydrogen peroxide. D) SER constituted by a network of short, branched and anastomosed tubules that present light and, sometimes, dense globules (very low-density lipoproteins that released into the blood are cholesterol transporters). Contains cytochrome P450 for catabolism of drugs and toxins. E) Cytoplasmic inclusions of: a) Glycogen, in the form of electron-dense granules of 20 to 30 nm in diameter (β particles) near the SER. The amount varies with the nutritional status. They can appear in particle aggregates (β) or rosettes (α). b) Lipids: Low-density lipoproteins, especially after the consumption of fatty foods. With the light microscope they are not usually seen in normal conditions, only in pathologies. 2) Endothelial cells and Kupffer cells as elements of the sinusoids Sinusoids are discontinuous/continuous blood capillaries (depending on the species) that run through the lobule, between the cords of hepatocytes, carrying blood from the hepatic artery and portal vein from the Kiernan’s spaces or portal space to the central vein. The cells that line the sinusoids are endothelial cells and Kupffer cells. Endothelial cells are smaller than Kupffer cells. The former has a heterochromatic nucleus and sparse cytoplasm, while the latter have a euchromatic nucleus. The space between endothelial cells and hepatocytes is the space of Disse. Endothelial cells form a single discontinuous layer without a basement membrane. That is, it is a simple squamous epithelium, with spaces between endothelial cells and no basement membrane. There is only one exception: In ruminants it is continuous and with a basement membrane. In addition, the endothelial cells of all species have pores (Figure 3). Therefore, the exchange between blood products and hepatocytes at the level of the microvilli in the space of Disse is direct. Kupffer cells are macrophages of the phagocytic mononuclear system. Together with those at the spleen they participate in the elimination of old erythrocytes and other waste particles existing in the circulation. They also actively participate in the modulation of the immune response and the inflammatory response (synthesis of IL-1, IL-6, TNF-α). They do not form binding modes with endothelial cells, suggesting that they can continually change location. They emit numerous processes called filopodia that enter the space of Disse through the pores of endothelial cells or between discontinuous endothelial cells. They contain numerous secondary lysosomes, many mitochondria, and a small Golgi complex. At the light microscope and by using special histochemical techniques, a high content of peroxidase and myeloperoxidase may be observed. A B C Figure 3.- Electron micrographs showing the endothelial lining of a sinusoid in the liver (A), Kupffer cell (B) and Ito cell (C). 3) Ito cells Ito cells are found in the space of Disse or perisinusoidal space. They are not usually seen at light microscopy. Under electron microscopy, they have a star shape, store lipids and vitamin A and produce type III collagen fibres (reticulin fibres) that provide structural support (stroma) to the sinusoids since their endothelium lacks a basement membrane (Figure 3). They participate in fibrinogenesis when there is an injury. 4) Foveolar cells Foveolar cells or fossa cells are found in the sinusoids and have electron dense granules. They belong to the APUD system: they synthesize polypeptides with a hormonal nature. They capture amines and decarboxylate them. Its secretory granules are stained with silver and chromium salts. I.2. Blood flow The liver has a dual afferent circulation: it receives oxygenated blood from the hepatic artery, and it receives nutrient-rich blood from the portal vein. All nutrients, except chylomicrons, which are absorbed through the digestive tract, are directly transported to the liver through the portal vein. In addition, iron-rich blood from the spleen is directly directed to the liver via the portal vein. Efferent venous blood leaves the liver through the hepatic vein, which drains into the cava caudal. Cytology and Histology I.3. Biliary circulation The bile canaliculi anastomose with each other and form labyrinthine tunnels between the hepatocytes. When they reach the periphery of the lobules, they fuse to form the cholangioli. The cholangioli continue with the canals of Hering and these, in turn, form other larger calibre, interlobular bile ducts that emerge parallel to the arterioles and venules of entry. The cholangioli and the canals of Hering have a simple cuboidal epithelium and join to form larger bile ducts until they reach the common hepatic duct that feeds into the gallbladder. The gallbladder, being a tubular organ, is made up of a mucosa (with simple columnar epithelium with microvilli on the apical border and lamina propria), a thin muscle (one single layer) and a serosa/adventitia. II. PANCREAS The pancreas is the second largest gland attached to the digestive system, and it is located in the region close to the duodenum forming a compact structure in the curvature of the duodenum. Anatomically, it has a head, body, and tail, and a pinkishwhite colour when fresh and a cream colour when fixed with formaldehyde. Like the liver, it is a gland with dual functions, endocrine and exocrine, but in this case each function is exerted by distinct and clearly separated cells in the pancreatic parenchyma. The exocrine pancreas secretes pancreatic juice, which contains the enzymes necessary for the digestion of carbohydrates, fats and proteins at the intestinal level (pancreatic amylase, pancreatic lipase, ribonuclease, DNase and the proenzymes trypsinogen, chymotrypsinogen, procarboxypeptidase, and elastase). The endocrine pancreas produces several hormones that regulate carbohydrate metabolism. The pancreas is thus a parenchymatous organ, so it is surrounded by a thin capsule of irregular fibrous connective tissue that projects fibrous septa inwards, which determines the division of the pancreatic parenchyma into lobes and, these in turn, into lobules. The interlobar and interlobular stroma is not only the architectural support of the pancreatic parenchyma, but also supports the innervation and irrigation of the gland, as well as the excretory canalicular system of the exocrine pancreas. The exocrine pancreas is a compound tubulo-acinar gland, and between the acini and ducts are located islets of cells with endocrine function, which are called the endocrine pancreas or pancreatic islets or islets of Langerhans (Figure 4). II.1. Exocrine pancreas It represents most of the organ and is made up of serous acini surrounded by reticulin fibres. The acini have epithelial cells of pyramidal morphology with the typical morphology of exocrine secreting cells. These cells are called glandular or acinous cells (Figure 4) with their base located on the basement membrane and the narrowest part directed towards the lumen of the acinus. The acinus is composed of several acinic cells and adopts a morphology with a tendency to sphericity. Blood capillaries and nerve endings are located between the exocrine acini, as well as some fibroblastic cells. Acinous cells can be easily identified in histological sections, since with usual stains they have a high basophilia in their basal area, while the apical one is usually acidophilic. The nucleus is spherical and is located towards the geometric centre of the cell. Ultrastructurally, they present the characteristics of exocrine glandular cells of the serous type: abundance of rough endoplasmic reticulum arranged in parallel cisterns that they occupy practically the whole basal pole and this is what gives it the strong basophilia. At the apical pole they contain a large quantity of zymogen granules (pancreatic enzymes). During periods of fasting, cells are usually packed with zymogen granules. Generally, zymogen granules present a finely granular structure and, depending on the maturation of the granule, show more or less electron density. Excretory duct Exocrine P. Endocrine P. Serous acinus A B C Figure 4.- Diagrams of the histological structure of the pancreas (A) and the exocrine pancreas (B) and electron micrograph of a serous cell (C). Inside the pancreatic acini, the formation of excretory ducts begins, which have an intra-acinar portion called intercalated ducts and are formed by squamous acinous cells or centroacinar cells. Ultrastructurally, these cells do not have secretory granules and have scarce RER. The intercalated ducts merge to form the intralobular ducts which converge to form the interlobular ducts. The interlobar ducts discharge their contents into the main pancreatic duct that joins the common bile duct before opening into the duodenum at the level of the ampulla of Vater. II.2. Endocrine pancreas The endocrine pancreas is located between the exocrine pancreatic acini as islands or islets of endocrine cells that discharge their secretion into a highly developed internal capillary network. That is why this portion of the pancreas is also called pancreatic islets or islets of Langerhans. The islets are separated from the exocrine tissue by a thin layer of reticulin fibres. Within the islet there is little loose connective tissue associated with capillaries. The blood capillaries are of the fenestrated type. Likewise, they are innervated by myelinated and unmyelinated fibres of the autonomic nervous system, which are located on the periphery of the blood vessels. The cells are arranged forming irregular anastomosed cords and are of five types: A or alpha (α) cells, which secrete glucagon; B or beta (β) cells, which produce insulin; D or delta (δ) cells, which secrete somatostatin; F cells, which produce the pancreatic polypeptide; and C cells, which do not have secretory granules. The different endocrine cell types are not identifiable with usual laboratory techniques since they all show a very similar morphology but with immunohistochemical techniques they can be precisely identified.