Digestive System: Accessory Organs of Digestion
Histology of the salivary glands
There are two major types of salivary glands: serous and mucous. Serous salivary glands secrete proteins and enzymes like amylase, which assists with carbohydrate digestion. Serous glands stain pink with H&E staining because proteins are acidophilic (stains with eosin). Mucous glands have a soapy bubble appearance, and their cytoplasm does not stain well. Mucous glands secrete watery mucus which contains the glycoprotein mucin, which is why they do not stain well with H&E staining.
The oral cavity has three major salivary glands: parotid (sides of cheek), submandibular (under jaw), and sublingual (under tongue). The parotid gland is entirely serous and produces amylase (carbohydrate digestion). The submandibular gland is seromucous; it contains both serous glands (amylase and lysozyme production) and mucous glands (mucous production). The sublingual gland is almost entirely mucus-producing.
Figure 1: Serous and mucous salivary glands with and without illustration overlay
Histology of the liver
The liver plays two major roles in the digestive system. The first role is processing of the nutrient-rich venous blood which is carried from the digestive system organs to the liver by the hepatic portal circulation. The second role is production of bile, which is stored in the gallbladder and transported by the bile duct to the duodenum. Bile is important for emulsification of lipids so they can be effectively digested.
The liver is divided into hexagon-shaped lobules, which each have a central vein (Figure 2). At the intersections between lobules are three structures called the portal triad, which contains a branch of the hepatic portal vein, a branch of the hepatic artery, and a bile duct (Figure 2 and 3). The vein and artery are hard to tell apart because they appear as a ring of simple squamous epithelium (endothelium). The bile duct, however, is easy to identify because it appears as a ring of simple cuboidal epithelium. Figure 3 illustrates hepatocytes arranged into hepatic plates around sinusoidal capillaries.
Figure 2: Liver lobules with and without illustration overlay
Figure 3: Liver portal triad with and without illustration overlay
Histology of the pancreas
As we discussed previously in the Endocrine System Chapter (link: Endocrine System), the pancreas has dual functions. The endocrine function of the pancreas: pancreatic islets of Langerhans contain cells that produce insulin (beta) and glucagon (alpha). The exocrine function of the pancreas: pancreatic acinar cells produce digestive enzymes that are transported to the duodenum.
Additionally, the pancreatic duct cells produce bicarbonate, which helps to neutralize acid from the stomach. Pancreatic secretions, including enzymes and bicarbonate, drain into the main pancreatic duct, which joins the main bile duct at the hepatopancreatic ampulla of Vater. Relaxation of the sphincter of Oddi allows drainage of bile and pancreatic secretions into the duodenum.
Figure 4: pancreas with and without illustration overlay
Chapter Illustrations By:
Georgios Kallifatidis, Ph.D.
