Hepatic stem cells: from inside and outside the liver?

Abstract

The liver is normally proliferatively quiescent, but hepatocyte loss through partial hepatectomy, uncomplicated by virus infection or inflammation, invokes a rapid regenerative response from all cell types in the liver to perfectly restore liver mass. Moreover, hepatocyte transplants in animals have shown that a certain proportion of hepatocytes in foetal and adult liver can clonally expand, suggesting that hepatoblasts/hepatocytes are themselves the functional stem cells of the liver. More severe liver injury can activate a potential stem cell compartment located within the intrahepatic biliary tree, giving rise to cords of bipotential transit amplifying cells (oval cells), that can ultimately differentiate into hepatocytes and biliary epithelial cells. A third population of stem cells with hepatic potential resides in the bone marrow; these haematopoietic stem cells may contribute to the albeit low renewal rate of hepatocytes, but can make a more significant contribution to regeneration under a very strong positive selection pressure. In such instances, cell fusion rather than transdifferentiation appears to be the underlying mechanism by which the haematopoietic genome becomes reprogrammed.

Figure 1

Current understanding of the origin and inter‐relationships of the cells involved in liver development and regeneration. Foetal liver contains bipotential hepatoblasts capable of differentiating into hepatocytes and cholangiocytes. These cells are capable of self‐renewal after loss, but when hepatocyte renewal is compromised, bipotential oval cells are activated from the canal of Hering cells (potential stem cell niche) to take over the burden of regenerative growth. The bone marrow also harbours cells with liver potential, but the factors determining their durable ingress are poorly understood: in the Fah null mouse, fusion between bone marrow cells and deficient hepatocytes occurs. Evidence that bone marrow can transdifferentiate into biliary cells is weak.

Figure 2

(a) Partial hepatectomy (PH) in a rat involves resection of the left lateral (LL) and median (M) lobes comprising two‐thirds of the liver mass and the pre‐operative mass is restored within 10 days. (b) Twenty‐four hours after PH, many hepatocytes are in DNA synthesis as indicated by bromodeoxyuridine labelling, but note the relative absence of labelling around the hepatic vein (HV) at this time, although within 48 h after PH almost all hepatocytes will have traversed the cell cycle at least once.

Figure 3

Oval cell behaviour in the rat liver treated by the AAF/PH protocol. (a) AFP expression (IHC staining) is typically observed in the migrating oval cells; note absence of staining in the inter‐lobular duct in the portal tract (PT). (b) The oval cell response can be visualized by CK8 immunostaining, with cords of cells emanating from the portal tract (PT). (c) At later times the cords of oval cells differentiate into small hepatocytes (SH) but with a notable lack of CYP immunoexpression (brown staining). Note occasional residual oval cell ductules expressing CK19 (purple staining).

Figure 4

The canals of Hering (green) extend from the portal areas into the proximate third of the hepatic lobule in the human liver (see Theise et al. 1999) and major hepatocyte damage activates the lining cells to divide and probably differentiate into hepatocytes.

Figure 5

The Fah null mouse can be rescued by wild‐type bone marrow. Even in the severely damaged Fah null liver, low levels of bone marrow engraftment are found (perhaps one for every million indigenous hepatocytes), but following prolonged application of a positive selection pressure for the engrafted cells, achieved by NTBC withdrawal, clonal expansion of the bone marrow cells that have fused with host hepatocytes occurs – leading to the correction of a potentially fatal metabolic liver disease (see text for further details).

Figure 6

Tracking bone marrow by gender mismatch bone marrow transplantation. Male to female transplantation has traditionally been used for detecting transdifferentiation by detection of the Y chromosome. However, cell fusion may be missed as not all chromosomes might be visible in the tissue section. Female to male transplantation is more suitable for detecting fusion events; marked bone marrow‐derived parenchymal cells with a Y chromosome have definitely been formed by fusion with host cells.