Accessory Organs of the Digestive System Dr Buthaina

Accessory Organs of the Digestive System Dr. Buthaina Ibrahem Al-ezzi Zahraa Ahmed

The accessory organs of the digestive system are located outside of the digestive tube. Excretory glands from the salivary glands open into the oral cavity. The liver, gallbladder, and pancreas are also accessory organs of the digestive tract that deliver their secretory products to the small intestine by excretory ducts. The common bile duct from the liver and the main pancreatic duct from the pancreas join in the duodenal loop to form a single duct common to both organs. This duct then penetrates the duodenal wall and enters the lumen of the small intestine. The gallbladder joins the common bile duct via the cystic duct. Thus, bile from the gallbladder and digestive enzymes from the pancreas enter the duodenum via a common duct.

LIVER : GENERAL FEATURES : Liver is situated mainly in the right hypochondrium, below the right dome of diaphragm in the abdomen. It is irrigated by two types of blood vessels, namely, portal vein (70%) and hepatic artery (30%). Liver performs the following exocrine and endocrine functions and is involved in: – Synthesis and secretion of bile (exocrine function) for fat digestion, – Excretion of bilirubin into the bile, which is a toxic chemical formed in the body during degradation of worn out erythrocytes by the phagocytic cells (Kupffer’s cells) of liver, – Synthesis and secretion of plasma proteins like albumin, prothrombin and fibrinogen (endocrine function), – Storage of glucose as glycogen, – Detoxification of various drugs and harmful substances like alcohol, – Hemopoiesis in fetuses, – Clearing the blood of cellular debris and particulate material by the phagocytic function of the Kupffer’s cells.

Liver Lobule : A classical liver lobule is hexagonal (polygonal) in shape and forms the structural unit of liver. It has a vein at the center, called the central vein. human liver lobules are not completely demarcated by connective tissue septa. So, it is very difficult to precisely identify the limit of the lobule. The portal tract contains connective tissue derived from Glisson’s capsule, containing three structures, namely, a portal venule, hepatic arteriole and a small hepatic ductule. As these three structures are always found in the portal tract, the portal tracts are often referred to as portal triad. The main structural and functional components of the liver are the hepatocytes, which are arranged in one cell thick plates radiating from the central vein towards the periphery of the lobule. The irregular spaces between the hepatic plates are occupied by liver sinusoids which are lined by discontinuous fenestrated endothelial cells. Some of the endothelial cells are modified to become phagocytic cells called Kupffer’s cells which phagocytose worn out RBCs. These cells form a part of the mononuclear phagocytic system.

Portal Triad

PANCREAS : GENERAL FEATURES : Pancreas is an exocrine digestive gland as well as an endocrine gland. It extends from the concavity of the duodenum on the right to the spleen on the left in the posterior abdominal wall retroperitoneally. 1 - Exocrine Pancreas : The exocrine part of pancreas is formed by serous acini arranged into many lobules, the lobules are separated by interlobular septa of connective tissue which carry neurovascular structures and ducts. Each serous acinus is made of pyramidal serous cells surrounding a small lumen. These cells are darkly stained at the base and lightly at the apex and contain zymogen granules. There are no myoepithelial cells. Instead, there are myofibroblast like cells called pancreatic stellate cells found encircling the base of the acinus in the periacinar connective tissue. Some of the acini exhibit pale staining centroacinar cells within the lumen, they are cuboidal in shape and represent the intra-acinar part of the intercalated duct, which instead of arising from the periphery of the acinus, has invaginated into the acinus and starts from inside it.

Intercalated Duct

2 -Endocrine Pancreas : The endocrine part of pancreas is formed by islets of Langerhans, which appear as pale staining spherical bodies among the serous acini, they are more in the tail region of pancreas. There about 1 to 2 million islets in the pancreas. They are made of branching cords of endocrine cells of the following types, supported by reticular fibres and a rich network of capillaries, : 1. Alpha (α) cells : Form 20% of the total population they are large cells with eosinophilic granules and found mainly at the periphery of islet, they Secrete glucagon, that increases glucose level in the blood. 2. Beta (β) cells : Form 70% of the population they are small cells with basophilic granules found mostly in the center, they Secrete insulin which decrease glucose level in the blood. 3. Delta (δ) cells : Form 5% of the population, Secrete somatostatin, which inhibits secretion of GH, glucagon and insulin. They also decrease pancreatic exocrine secretion and bile secretion. 4. F cells/PP Cells : Secrete pancreatic polypeptides which inhibit pancreatic secretion.

GALL BLADDER : Gall bladder has from inner to outer, the following coats: 1. Mucosa : - It includes the lining epithelium of simple columnar variety and the supporting lamina propria rich in elastic fibres and blood vessels. - As this epithelium is involved in absorption of water, it is provided with microvilli which give a brush border appearance to the epithelium under light microscope. - Mucosa is thrown into small folds when the bladder is empty. - Muscularis mucosa and submucosa are absent. (Muscularis mucosa and Muscularis externa fuse. ) 2. Fibromuscular layer : This layer is composed of circularly arranged smooth muscle fibres intermixed with connective tissue. 3. Serosa/Adventitia : The fundus and lower surface of body of gall bladder is covered by peritoneum (serosa), whereas the upper surface is attached to the fossa for gall bladder by means of connective tissue (adventitia). So depending on the area selected for sectioning, the outer coat is made of either serosa or adventitia.

It is characterized by (i) presence of mucosal folds lined by simple tall columnar epithelium; (ii) presence of fibromuscular layer; (iii) absence of muscularis mucosa and submucosa.

References : 1 - Textbook of Histology A Practical guide - Gunasegaran, J. P. 2 nd ed 2010 2 - Di. Fiore’s Atlas of Histology with Functional Correlations, 11 th ed 2008

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