Liver and Pancreas

Learning Objectives

Describe the organization of and roles played by the pancreas and liver in initiating and facilitating particular digestive processes
Name the exocrine cells of the pancreas and explain how they are organized
Describe the structure of the liver lobule and its component cell, the hepatocyte, and how this structure is related to the flow of blood and bile

Lab Content

Introduction

In the previous laboratory, we discussed in detail the different portions of the GI tract and their component cellular structures. This laboratory will focus on the liver and pancreas.

Pancreas

The pancreas consists of two functionally distinct parts: an exocrine part that produces digestive secretions that are discharged into the duodenum via a system of ducts, and an endocrine part consisting of the islets of Langerhans, which secrete insulin and glucagon to regulate carbohydrate metabolism. This image shows the two main functional domains of the pancreas. The endocrine pancreas that secretes insulin and glucogon is more lightly stained and its cells cluster to form the Islets of Langerhans. The rest of the image consists of the exocrine pancreas that produces several enzymes critical for digestion and adsorption of food. Note how the cells of the exocrine pancreas form small clusters or acini.

Pancreatic Acinar Cells

This H&E section of the exocrine pancreas shows several of its characteristic features. The exocrine cells show a strongly basophilic cytoplasm that represents the area occupied by the rough endoplasmic reticulum. The apical side of the cells is filled with zymogen granules that contain a variety of digestive enzymes. Intercalate ducts are also visible. The cells which line the ducts secrete bicarbonate in response to secretin that is produced in the duodenum. Bicarobonate raises the pH of the contents in the duodenum. Also visible are centroacinar cells that form the terminal lining of the intercalated ducts.

Pancreatic Acinar Cells EM

This electron micrograph shows in detail the acinar cells of the pancreas. These cells have basally located nuclei and numerous zymogen granules at their apical pole. They also have abundant endoplasmic reticulum in the basal portion of the cytoplasm. These cells will secrete their granule contents into the lumen of the duct, which will carry the enzymes out of the pancreas and to the duodenum.

Liver Organization

The liver is the largest organ of the body. Like the pancreas, it releases secretory products into the digestive tract. The liver has numerous functions:

Secretes bile into the duodenum via the common bile duct.
Serves as a highly efficient filter of portal blood from the intestine.
Participates in large-scale synthesis and discharge of plasma proteins and lipoproteins.
Detoxifies drugs and toxins.
Plays a major biosynthetic and degradative role in regulating the macromolecular composition of the blood plasma.

Portal triads convey blood into the liver and bile out of the liver. Hepatic venules surround portal triads and drain blood from the liver. In between the portal triads and hepatic venules lie hepatocytes that secrete bile and process nutrients, toxins and other material in the blood from the portal triad. The red arrows show the direction that blood flows from the portal triad to hepatic venule, whereas the yellow arrows show the flow of bile from hepatocytes to portal triad.

Portal Triad

Portal triads are composed of three major tubes. Branches of the hepatic artery carry oxygenated blood to the hepatocytes, while branches of the portal vein carry blood with nutrients from the small intestine. The bile duct carries bile products away from the hepatocytes, to the larger ducts and gall bladder.

Hepatocytes and Sinusoids

This image shows the close proximity between the blood in the sinusoids and hepatocytes. Endothelial cells line the sinusoids. These endothelial cells form a discontinuous endothelium that has wide gaps between cells and lacks a basement membrane and is therefore very permeable. Hepatocytes secrete bile into canaliculi that are defined by junctions between hepatocytes. Bile flows through theses narrow tubes toward the bile duct. Also visible is a Kupffer cell. Kupffer cells are the resident macrophages of the liver and are typically found within the lumen of the sinusoids.

Hepatocytes EM

This electron microscope shows two hepatocytes separated by their cell membranes. A junctional complex is located near the apical portion of these cells. Above this junction is the bile canaliculus into which the cells secrete bile from their apical surface. Also available in these cells are numerous mitochondria.

Gall Bladder

The gall bladder stores and concentrates bile. It has several important characteristic features that can be used to distinguish it from other organs in the GI system. These include irregularly shaped villi that are lined by abnormally tall columnar epithelial cells. The smooth muscle in the wall of the gall bladder contracts under the influence of the hormone cholecystokinin to expel the bile into the duodenum.