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Liver Histology

Notes

Liver Histology

Hepatic Portal Triad

  • The liver drains bile via the common hepatic duct
  • Bile enters the cystic duct for storage in the gallbladder (and is later secreted via the common bile duct into the duodenum)
  • Blood is supplied to the liver by two vessels:
    • The proper hepatic artery carries high oxygen blood
    • The hepatic portal vein carries mixed oxygen blood from the digestive tract
  • Collectively, these vessels and the common hepatic duct are referred to as the portal triad.
  • Filtered blood exits the liver via hepatic veins that drain to the inferior vena cava.

Pause to consider the blood arriving at the liver via the hepatic portal vein: coming from the digestive tract, it is rich in nutrients, vitamins, minerals, and, when ingested, drugs and toxins. Recall that the liver is responsible for their metabolism, detoxification, and storage.

Classical Lobule Model

Historically, hepatic tissue is modeled as hexagon-shaped lobules surrounding central venules (aka, centralobular venules). The area of the lobule is bound by the peripheral portal triads.

  • The liver tissue comprises rows, aka, plates, of hepatocytes radiating from the central venule
  • Between the plates, sinusoids provide passage for blood and bile.
  • Portal triads, aka, portal tracts, lie at the periphery, with the smaller vessels connecting them.
  • Hepatocytes are epithelial cells with round nuclei; they are often binucleate, and, in some histological samples, glycogen granules are visible in the cytoplasm. The hepatocytes directly encircling the central venule are the limiting plate hepatocytes.
  • Bile canaliculi facilitate bile passage between adjacent hepatocytes.
  • Sinusoids run between plates of hepatocytes; are lined by fenestrated endothelial cells, which allow molecules, including bile, to pass from the tissue to the lumen;
  • Kupffer cells, which are macrophages that remove expired red blood cells and other debris, are also present in sinusoid lining.
  • Reticular fibers also line the sinusoids and form a supporting network for the liver tissues.
  • Bile flows towards the peripheral portal triad, enter hepatic ducts and exit the liver.
  • Arterial and venous blood flows towards the central venule; Filtered, deoxygenated blood drains to the hepatic veins and to the inferior vena cava to re-enter general circulation.

Portal Triad Model

  • The area of the portal lobule, which comprises all hepatocytes that drain into a single bile duct, is a triangle bound peripherally by the central venules, with the portal triad at the center.

This is the exact opposite arrangement as the classical model, which places the portal triads at the periphery and the central venule centrally.

  • Because "central" is not accurate in this model, some authors refer to the "central venules" as the "terminal hepatic venule." However, for consistency and context, we'll continue to refer to it as the central venule.

The portal lobule model is physiologically useful when we study the exocrine functions of the liver, because it highlights the pathway of bile secretion.

Acinus Model

  • Focuses on the direction of oxygenated blood from the hepatic artery branches.
  • The area of the acinus lobule is determined by the afferent vascular supply.

This model is of physiological significance because we can infer that the hepatocytes closest to the portal triad (zone I) receive relatively more oxygen and solutes than those closer to the central vein (zone III). Consequently, enzymes are preferentially expressed according to the specific microenvironments.

Images:

Histology (Mark Braun, MD, & Indiana University: http://medsci.indiana.edu/c602web/602/c602web/toc.htm; http://www.indiana.edu/~anat215/virtualscope2/start.htm)