Visual Anatomy 視覚解剖学

　肝小葉（かんしょうよう、英：hepatic lobule ）

肝小葉は肝実質と血管は小さな肝臓の多角形の組織学的概念的単位で、肝静脈の１つの終末枝である中心静脈の周囲に存在する肝細胞集団からなり、周辺部には門脈枝、肝動脈、胆管がある。これら肝構造単位はどの導通路を構造単位の中心と考えるかによって、肝小葉であったり門脈小葉であったりする。
強靱な肝臓被膜から肝臓内へ、海綿状の繊細な結合組織が入っていく。これを足場としてこの中には、血管と胆管が走っており、結合組織の網目の中に肝上皮が横たわっている。肝臓の表面には数mm大の多角形の肝小葉がみえる。
肝小葉は多角形の表面をもつ（西洋）ナシの形をしており、大きさは断面で１～１．５mm、長さは約２ｍｍである。これらの血管は肝小葉まわりで血管網を作るために枝を出す。この血管網から小葉へ枝が入っていき、これらの枝は門脈の血液を洞様血管へ導きいれる（小葉の周辺部は中心部より酸素の供給がよい）。基底部の（「ナシの柄」に相当する）からは中心静脈が出ていく。しかし、各小葉間の境界は、ルーペを使ってもそれほど明瞭ではない。これは小葉間を埋める結合組織すなわちグリソン鞘が人間ではあまり発達していないためである。ブタでは非常に明瞭。)

以下は「Wikipedia」の解説文となる。

The lobules of liver, or hepatic lobules, are small divisions of the liver defined at the microscopic (histological) scale. The hepatic lobule is a building block of the liver tissue, consisting of a portal triad, hepatocytes arranged in linear cords between a capillary network, and a central vein.
Lobules are different from the lobes of the liver: they are the smaller divisions of the lobes. The two-dimensional microarchitecture of the liver can be viewed from different perspectives:[1]

Name	Shape	Model
classical lobule[2]	hexagonal; divided into concentric centrilobular, midzonal, periportal parts	anatomical
portal lobule[3]	triangular; centered on a portal triad	bile secretion
acinus [4]	elliptical or diamond-shaped; divided into zone I (periportal), zone II (transition zone), and zone III (pericentral)	blood flow and metabolic

The term "hepatic lobule", without qualification, typically refers to the classical lobule.

Structure[edit]

The hepatic lobule can be described in terms of metabolic "zones", describing the hepatic acinus (terminal acinus). Each zone is centered on the line connecting two portal triads and extends outwards to the two adjacent central veins. The periportal zone I is nearest to the entering vascular supply and receives the most oxygenated blood, making it least sensitive to ischemic injury while making it very susceptible to viral hepatitis. Conversely, the centrilobular zone III has the poorest oxygenation, and will be most affected during a time of ischemia.[5]
Portal triad[edit]
A portal triad (also known as portal canal, portal field[citation needed], portal area[citation needed], or portal tract[citation needed]) is a distinctive arrangement within lobules. It consists of the following five structures:[6]

proper hepatic artery, an arteriole branch of the hepatic artery that supplies oxygen
hepatic portal vein, a venule branch of the portal vein, with blood rich in nutrients but low in oxygen
one or two small bile ductules of cuboidal epithelium, branches of the bile conducting system.
lymphatic vessels
branch of the vagus nerve

The misnomer "portal triad" traditionally has included only the first three structures, and was named before lymphatic vessels were discovered in the structure.[7][8] It can refer both to the largest branch of each of these vessels running inside the hepatoduodenal ligament, and to the smaller branches of these vessels inside the liver.
In the smaller portal triads, the four vessels lie in a network of connective tissue and are surrounded on all sides by hepatocytes. The ring of hepatocytes abutting the connective tissue of the triad is called the periportal limiting plate.
Periportal space[edit]
The periportal space (Latin: spatium periportale), or periportal space of Mall,[9] is a space between the stroma of the portal canal and the outermost hepatocytes in the hepatic lobule, and is thought to be one of the sites where lymph originates in the liver.[10]
Fluid (residual blood plasma) that is not taken up by hepatocytes drains into the periportal space, and is taken up by the lymphatic vessels that accompany the other portal triad constituents.

Function

Zones differ by function:

zone I hepatocytes are specialized for oxidative liver functions such as gluconeogenesis, β-oxidation of fatty acids and cholesterol synthesis
zone III cells are more important for glycolysis, lipogenesis and cytochrome P-450-based drug detoxification.[11] This specialization is reflected histologically; the detoxifying zone III cells have the highest concentration of CYP2E1 and thus are most sensitive to NAPQI production in acetaminophen toxicity.[12]