Liver Anatomy
In 1897, Cantlie first described the main anatomical division
of the liver by showing that it was not divided along the line of the falciform ligament
but along a main plane (Cantlie’s line) extending from the gallbladder fossa to the
vena cava.(Fig. 1) Couinaud refined the functional anatomy of the liver and demonstrated
that the liver was divided in four sectors (formerly called segments) and eight segments.
In this nomenclature, the liver is divided by vertical and oblique planes or scissurae
defined by the three main hepatic veins and a transverse plane or transverse scissura
following a line drawn through the right and left portal branches. Thus, the four
traditional segments (right anterior, right posterior, left medial, and left lateral) have
been replaced by sectors (right posterior, right anterior, left anterior, left posterior)
(Fig. 2) and these sectors are divided into segments by the transverse scissura. (Fig. 3)
The eight segments are numbered clockwise in a frontal plane. Each segment is therefore an
independent functional unit supplied by a single portal triad.
The caudate lobe or segment I is unique because it derives its
blood from several branches of the right and left portal vein and hepatic artery. It
drains directly into the vena cava through a main caudate vein and minor hepatic veins. It
is located between the liver and the vena cava. Its three main divisions, from right to
left, are: 1/ the caudate lobe process connecting the caudate lobe to segment V and VIII;
2/ the paracaval portion between the hepatic hilus and the vena cava posterior to segment
IV; and, 3/ and the papillary process of the caudate lobe to the left of the inferior vena
cava.
Couinaud’s nomenclature provides critical information as to
the potential resection planes. Recent advances in hepatic surgery have made possible
anatomical (also called typical) resections along these planes while minimizing morbidity
and blood loss. Within the same sector, it is now possible to remove one segment while
leaving the other one in place. This nomenclature is an invaluable asset for both
radiologists and surgeons, allowing them to define the location of tumors and their
relationship with major vascular structures.
Liver Regeneration
Major resections, including up to 75% of the liver, can be
performed provided the future liver remnant is not functionally compromised. The liver
regenerates following extended right or left hepatectomies (also called
trisegmentectomies) or other major atypical resections provided two or three adjacent
segments remain. Liver regeneration is a fundamental parameter of liver response to
injury. Normally, liver cells are dormant in the G0 phase (non–proliferative) of the
cell cycle. Mitoses are noted in 1 in 10,000 hepatocytes. The exact mechanism triggering
the switch to regeneration following resection remains speculative. Hepatocyte growth
factor has been identified as the most powerful mitogenic growth factor stimulating liver
regeneration. Many other growth factors and cytokines such as epidermal growth factor,
transforming growth factor–a , interleukin–6, and tumor necrosis factor
stimulate mitogenesis. Comitogenic factors such as estrogen, glucagon, and insulin
upregulate the activity of mitogenic factors and can accelerate the process of liver
regeneration. Prolonged stimulation by some hepatotrophic factors can lead to hypertrophy
or development of neoplasms. Prolonged intake of estrogen, mainly in the form of
high–dose estrogen oral contraceptives or anabolic steroids, is associated with the
development of hepatocellular adenomas and an increased risk for the development of
hepatocellular carcinoma.
The distribution of the liver mass is maintained by a complex
control mechanism in which bile flow, portal vein, and hepatic vein flows are the main
regulators. Lobar, sectoral, or segmental atrophy may occur as a result of the
interruption of venous inflow (portal vein) or outflow (hepatic vein) or impaired biliary
excretion. The association of marked lobar atrophy with contralateral hypertrophy in
cholangiocarcinoma suggests combined ipsilateral portal and biliary obstruction. The
presence of the atrophy–hypertrophy complex should dictate further evaluation to
determine whether vascular obstruction is present.
Recently, preoperative percutaneous portal vein embolization has
been used to induce hypertrophy of the future liver remnant before surgery. This technique
is currently used in patients in which a complex extended hepatectomy is anticipated and
the future liver remnant is small or functionally compromised. The recent advent of
helical computed tomography (CT) allows accurate 3–dimensional measurements before
and after portal vein embolization preoperatively.
