What is the anatomy and embryology of the liver?

Anatomy and Embryology of the Liver

Embryological Development

The liver develops from the ventral foregut endoderm as a result of ductal plate malformation, where the ductal plate disconnects from the biliary tree and progresses into cystic structures. 1 The ductal plate is a double cylinder of hepatoblasts encircling portal vein branches that provides the scaffold for bile duct development. This embryological origin explains the development of various hepatic structures and cystic diseases of the liver.

Key embryological points:

• The liver originates from the ventral foregut endoderm
• Ductal plate malformation is responsible for cystic liver diseases
• Epithelial cells in hepatic cysts retain secretory function, producing fluid that creates positive luminal pressure

Functional Anatomy of the Liver

Segmental Anatomy

The liver is divided into functionally independent segments based on the Couinaud classification system, which divides the liver into eight segments 2:

• Each segment has its own vascular inflow, biliary drainage, and venous outflow
• Segments are numbered in a counterclockwise manner when viewed from the inferior surface
• Segment I (caudate lobe) is anatomically and embryologically independent of the right and left liver 3

The Couinaud classification is a simplification of actual liver anatomy, as the real number of second-order portal branches averages around 20 4. This has led to the development of the "1-2-20 concept" which better reflects the actual anatomical liver segments rather than idealized ones.

Vascular Supply

The liver has a unique dual blood supply:

1. Portal System:

   • The portal vein provides 75% of blood flow
   • Divides into right and left branches
   • The right section is perfused by the right portal branch
   • The left section is perfused by the left portal branch 5

2. Arterial System:

   • The hepatic artery provides 25% of blood flow
   • Critical for bile duct blood supply (derived entirely from the hepatic artery)
   • Arteriovenous shunting can lead to bile duct ischemia 1

3. Venous Drainage:

   • The right section drains via independent hepatic veins
   • Most of the left section drains via the main hepatic vein (formed by confluence of middle and left hepatic veins)
   • Parts of the right medial lobe and sometimes the papillary process of the caudate lobe drain directly into the inferior vena cava 5
   • The caudate lobe represents the only part of the liver in direct contact with the vena cava 3

Lobar and Sectional Anatomy

The liver can be described at three levels of complexity 4:

1. Conventional level (Couinaud's 8 segments): Used for describing location of focal hepatic lesions
2. Surgical level: Accounts for actual branching of portal pedicles and hepatic veins
3. Academic level: Addresses the contradictions between anatomical literature, radiological imaging, and surgical practice

Traditional division includes:

• Right lobe (segments V to VIII)
• Left lobe (segments I to IV)
• The caudate lobe (segment I) is considered a separate anatomical segment 1

Clinical Significance of Liver Anatomy

Surgical Applications

Understanding liver segmentation is fundamental for:

• Anatomical resections: Allows for preservation of vascular structures 2
• Liver transplantation: Facilitates partial and split-liver transplantation 1
• Living donor transplantation: Right lobe (segments V to VIII) is typically used for adult recipients, while segments II and III (left lateral lobe) are used for pediatric recipients 1

The graft-to-recipient weight ratio should be at least 0.8% for successful transplantation (e.g., for an 80 kg patient, a minimum graft weight of 640g is needed) 1.

Imaging Techniques for Liver Segmentation

Multiple imaging modalities can be used to identify liver segments 2, 6:

• Ultrasonography: Uses hepatic veins and portal branches as references
• Computed tomography: Allows for three-dimensional reconstructions
• Magnetic resonance imaging: Offers better contrast for vascular structures

Anatomical Variations

Significant anatomical variations exist in liver segmentation:

• Segmental volumes can vary up to 24% from classically described volumes
• Segments 4a, 7, and 8 tend to be larger than expected
• Segments 3 and 6 tend to be smaller than expected 2
• Portal connections between the quadrate and left medial lobe are frequently observed 5

Pathological Considerations

Understanding liver anatomy is crucial for recognizing and managing various liver conditions:

• Cystic liver diseases: Result from ductal plate malformation during embryological development 1
• Vascular disorders: Hereditary hemorrhagic telangiectasia can lead to arteriovenous, portovenous, and arterioportal shunting 1
• Biliary disorders: Caroli disease involves dilatation of intrahepatic bile ducts, while Caroli syndrome combines small bile duct dilatation and congenital hepatic fibrosis 1

Clinical Applications

The detailed understanding of liver anatomy has facilitated major advances in:

• Liver transplantation: Including split-liver and living donor procedures 1
• Hepatic resections: Anatomical resections based on vascular territories 2
• Interventional procedures: Such as portal vein embolization and transjugular intrahepatic portosystemic shunts 6
• Diagnostic imaging: Accurate localization of focal liver lesions 1

Understanding the complexities of liver anatomy is essential for all physicians involved in the diagnosis and treatment of liver diseases, as it enables precise surgical planning and improved patient outcomes.