This study demonstrates that arteries and microvessels in scarred central zones are a common finding in patients with NASH and can be seen in this location as early as stage 1b (though interestingly not in the two cases of stage 1c included in this study). Isolated arteries in the hepatic lobule can be seen in liver with normal architecture and are generally believed to derive from hepatic arteries in portal zones (19
). The finding of these centrizonal arteries in early stage disease also supports the idea that at least some of these arteries have appeared in or near the centrizonal region even before the development of bridging fibrosis from the portal area to the central zone. However, the increased prevalence of these arteries with increased stage of fibrosis suggests that they may branch and/or grow as part of remodeling of the vascular architecture in these scarred zones as fibrosis progresses in NASH. This process may be similar to what is known to occur as part of the formation of collateral vascular channels in the scarring of cirrhosis (20
). Vascular reorganization may represent a compensatory or reparative mechanism of the liver in response to injury or disease-induced alterations in hepatocyte metabolism or hemodynamics (8
). Studies focusing on local stromal growth factors and chemokine variables elicited by the shift of hepatic stellate cells to a myofibroblast phenotype, and an associated change in matrix collagen content, suggest that molecular/cellular alterations could affect or promote growth of blood vessels in cirrhosis (25
). In particular, microvessel formation, which is highlighted by CD34 immunolabeling in this case, is thought to represent evidence of angiogenesis (28
). Because hepatic stellate cells are the main fibrogenic cells in liver injury and are situated within the lobule, it would follow that they could significantly impact changes taking place in diseases where extracellular matrix deposition occurs in a perisinusoidal fashion, such as in NASH. Vasoactive substances such as endothelin-1 are found to be up-regulated in cirrhotic livers and may participate in the development of liver cirrhosis by also exerting an effect on hepatic stellate cells (11
), which is in keeping with the idea that local factors may influence changes in the vascular network during development of hepatic fibrosis.
Although a causal link between obesity and insulin resistance with hepatic microvasculature perfusion deficits has been established (2
), and various mediators have been implicated, precise cellular and molecular interactions remain to be fully determined. Attention has specifically
been given to hepatic pro-angiogenic cytokines produced in response to hypoxic stimuli, particularly vascular endothelial growth factor (VEGF), and their effects on sinusoidal endothelial cells, hepatocytes, and hepatic stellate cells. These studies indicate that expression of VEGF (via regulation by hypoxia-inducible factor-1α, or HIF-1α), angiopoietin-1, and nitric oxide synthases are upregulated when these cells are subjected to low levels of oxygen (1
). In response to worsening relative hypoxia in zone 3 as fibrosis progresses, stimulation of gene expression for VEGF and, thus, microvessel formation may increase, hence providing some basis for the theory that neovascularization of the central zone could be secondary to a hypoxia-related phenomenon.
This study also demonstrated that ductular reaction is a common finding in the centrizonal scar, and is also more prevalent in the later stages of fibrosis. Of note, ductular reaction in the periportal zone in patients with NASH has been associated with increased grade of NASH activity and portal inflammation, increased degree of hepatocytic replicative arrest, and is also correlated with progressive periportal fibrosis (22
). These results were comparable to findings in chronic hepatitis C, where a similar association with periportal ductular reaction and fibrosis has been demonstrated (5
). However, in the current cases, the centrizonal ductular reaction was obviously occurring some distance away from progenitor/stem cells typically thought to be the cell of origin of ductular reaction located in the periportal zone (13
). Thus, the unusual location may suggest that the change is a metaplastic response of hepatocytes to chronic injury and/or chronic ischemia, resulting in transformation to a more ductular phenotype rather than a progenitor cell reaction. Regardless of the cellular origin of the ductular reaction, these ductular structures could still possibly play an exacerbating role in the progression of fibrosis as well. The role and nature of these ductular structures could prove to be of interest in future studies.
The finding of ductular reaction in the centrizonal scar potentially compounds the risk of diagnostic error, as the presence of a centrizonal artery, particularly in samples with ductular reaction in the same scarred central zone, could potentially mimic the anatomy of a portal area, leading to the erroneous conclusion that the centrizonal region was a portal tract, and thus result in failure to diagnose NASH by a pathologist who may not be familiar with this finding. Also, even if NASH were correctly diagnosed, this finding could lead to overstaging (i.e. stage 1 to stage 2) or mis-staging (i.e. stage 1a/1b to 1c). The determination that the scar zone is not a remnant of a portal zone can be aided by careful histologic examination to determine whether the ductular structure and artery are embedded in the correct collagenous/elastic tissue background of a normal portal zone (26
), as the normal portal zone has a more organized and uniform collagen and elastic fiber network than scar zones (10
). In addition, the placements of the aberrant artery and a ductule in relationship to the adjacent hepatocytes are typically not in the correct anatomical configuration of a portal zone, in that the ductular structure or artery may lie too far apart with no visible portal vein candidate, or they may lie immediately adjacent to or among hepatocytes (instead of the two structures remaining separated from the “limiting plate” of the parenchyma by the types of collagen and elastic fibers seen in portal zones).
In summary, arteries and microvessels in the centrizonal region are a common finding in patients with NASH, especially in later stages of fibrosis. These arteries may arise in part through a process of microvessel formation related to local ischemia. The aberrant centrizonal artery needs to be recognized by pathologists as a feature of NASH so that the central zone is not mistaken for a portal zone and a correct diagnosis (and stage) of NASH can be rendered.