3- Liver

Liver Overview

  • The liver is the second largest organ and the heaviest gland in the body, weighing approximately 1-1.5 kg.

  • It is located in the abdominal cavity beneath the diaphragm, specifically in the right upper quadrant.

  • The blood supply to the liver mainly comes from the portal vein (70-80%) with a smaller portion (20-30%) supplied by the hepatic artery.

  • All absorbed materials from the intestine pass to the liver via the portal vein, except for lipids, which pass through lymphatic vessels.

  • The liver acts as both an exocrine gland (by secreting bile through the bile duct) and an endocrine gland (by releasing secretions directly into the bloodstream).

Histological Structure

  • The liver is enveloped by a connective tissue capsule known as Glisson's capsule and visceral peritoneum, with the exception of the bare area.

  • At the porta hepatis, the connective tissue capsule thickens and sends branching septa into the liver, providing structural support.

  • Blood vessels, nerves, and bile ducts accompany these septa into the liver's interior.

  • The liver lobes are divided into numerous tiny lobules, which constitute the functional unit of the liver known as the hepatic lobule.

Classical Hepatic Lobule

  • The classical hepatic lobule appears as a polygonal mass under a light microscope, made up of radially arranged hepatic cells (hepatocytes) resembling bricks in a wall.

  • These cellular plates extend from the periphery of the lobule towards the center where the central vein is located.

  • Hepatic plates are separated by vascular channels called hepatic sinusoids.

Portal Area

  • Each portal area consists of connective tissue containing:

    1. A venule (a branch of the portal vein)

    2. An arteriole (a branch of the hepatic artery)

    3. Bile ductules and lymphatic vessels.

  • Each lobule contains 3 to 6 portal areas.

Hepatocytes

  • Hepatocytes are the principal structural unit of the liver, arranged in plates directed from the lobule's periphery to its center.

  • These plates are only one cell thick.

Histological Characteristics of Hepatocytes

  • In histological sections stained with hematoxylin and eosin (H&E), hepatocytes appear eosinophilic due to the abundant presence of mitochondria and endoplasmic reticulum.

  • Hepatocytes contain one or two nuclei with corresponding nucleoli, as well as organelles like the Golgi complex, lysosomes, and free ribosomes.

Importance of Hepatocyte Surfaces

  • Hepatocytes have three significant surfaces crucial for transferring substances: A. Sinusoidal Surface:

    • Comprising 70% of the total hepatocyte surface, it is located adjacent to sinusoids separated by the space of Disse.

    • Covered by short microvilli, this surface is essential for material transfer between sinusoids and hepatocytes. B. Canalicular Surface:

    • Accounting for 15% of hepatocyte surface area, this surface helps drain bile from hepatocytes into canaliculi.

    • Lined with irregular microvilli, the cytoplasm near canaliculi is rich in actin filaments that regulate the canaliculi's diameter and bile flow rate. C. Intercellular Surface:

    • The surface connecting adjacent hepatocytes, contributing to 15% of the hepatocyte surface area; adjacent cells are held together by tight junctions.