Objective: To identify portal segmentation and a portal fissure in the caudate lobe of the human liver in relation to the hepatic venous system and the external notch at the caudal edge of the caudate lobe.
Summary Background Data: Although the anatomy of the caudate lobe has been studied, the detailed anatomy has not yet been clarified; this is necessary to develop safe procedures for caudate lobe resection.
Methods: A total of 88 formalin-fixed human livers were dissected to visualize the portal vein and hepatic vein systems of the caudate lobe in relation to the external notch.
Results: The patterns of portal branching were classified into two types. In 58 livers (67.4%), the territories of the first-order portal branches were clearly divided into two areas (the Spiegel lobe and the paracaval portion). In the remaining 28 livers (32.6%), the territories of the second-order portal branches were clearly divided into two areas. These two areas were distinctly separated by an internal plane, which was coincident with the external notch. The caudate lobe had a systematized hepatic venous system that consisted of one (87.5%) or two (11.4%) proper hepatic veins and plural accessory hepatic veins. The proper hepatic veins laid along the internal plane between these two portal areas.
Conclusions: The caudate lobe exhibited distinct portal segmentation with a portal fissure that was indicated internally by the proper hepatic vein and externally by the notch at the caudal edge of the caudate lobe.
The caudate lobe has been studied from various aspects due to its unique anatomical location in the liver and its characteristic configuration, which resembles a joining pin of a fan. 1-3 Recently, isolated resection of the caudate lobe 4-6 and caudate lobe resection combined with major hepatectomy procedures to treat hepatocellular carcinomas and hepatic hilar bile duct cancer have increased in number, 7-10 expanding our knowledge of the caudate lobe. 11-14
The caudate lobe was regarded as an independent segment and was classified by Couinaud 3 as segment 1 of the liver, which consisted of eight segments. Kumon 15 first divided the caudate lobe into three parts: the Spiegel lobe, which was equivalent to Couinaud's segment 1 and the conventional caudate lobe, the paracaval portion, and the caudate processus. Couinaud 16 as well as Healey and Schory 1 later subdivided the caudate lobe into two subsegments, named the left (segment 1l) and right dorsal segments (segment 1r). Couinaud's segment 1r is equal to Kumon's paracaval portion, and Couinaud 17 called it segment IX, which encircled the side of the inferior vena cava. In Kumon's and Couinaud's divisions of the caudate lobe, no detailed description of the boundaries among them was provided.
The ligamentum venosum lies to the left of the caudate lobe and enters the inferior vena cava. It has been used as a conventional index of the median border between the Spiegel lobe (segment 1) and the paracaval portion (segment IX). 4,5 However, it was identified as a vestigial organ of the ductus venosus in the embryo, and it is unclear whether it is correlated with the anatomical boundary of the caudate lobe. 14,16
While performing hepatectomies, we noticed a notch at the caudal edge of the caudate lobe in approximately half of the patients. This external notch may be a vestige of the portal segmentation of the caudate lobe, as demonstrated in animal livers. The livers of animals such as rats and mice are clearly lobed, and the caudate lobe of these animals has two distinctly independent lobes (anterior and posterior) and a paracaval portion. 18 If the structure of the liver is embryologically similar between these animals and humans, the caudate lobe of the human liver may demonstrate portal segmentation with a portal fissure, and the external notch may represent the portal fissure.
In previous studies, the anatomy of the caudate lobe has been studied mainly in terms of the distribution of the portal branches, 19 although the hepatic venous system is considered an essential component of the anatomical structure. Therefore, this study was performed to clarify the interrelations among portal segmentation, the hepatic venous system, and the external notch of the caudate lobe.