adhesion and ''homing'' during the inflammatory process, The sialomucin CD34 is expressed on human and mouse and in stem/progenitor cell localization/adhesion in the bone hematopoietic stem cells and is used as an important marker marrow. 1 CD34 is not only expressed in hematopoietic stem/ for isolating the hematopoietic stem/progenitor cells. The progenitor cells, but also in small-vessel endothelial cells, involvement of hepatic stem cells in liver regeneration under and embryonic fibroblast. 1 Human, 3,4 mouse, 5-7 and canine 8 certain conditions in adult rats is now well supported. The homologues of CD34 have been reported, and extensive studobjective of the present research was to explore the idea that ies have shown a high degree of homology at the amino acid CD34 might also be expressed on hepatic stem cell progeny.level, especially in the intracellular domain (ú90%) among Polymerase chain reaction (PCR)-based cloning of rat CD34 these species, but not in the extracellular domain (Ç45%), was partially accomplished. During the hepatic stem cell actisuggesting a functional importance of the intracellular part. vation (2-acetylaminofluorene/partial hepatectomy [AAF/PH]In light of these observations, we decided to examine the model), the CD34 transcripts were increased and reached the expression of CD34 in the rat and to test the possible involvepeak level between 9 and 12 days after partial hepatectomy ment of this gene and its product in the activation and expanwhen the progenitor cells (e.g., oval cells, early hepatocytes in sion of hepatic stem cells 9 (and references therein) during basophilic foci, and intestinal type of cells) are most abundant.liver regeneration. Both in situ hybridization and immunohistochemistry, usingIt is well known that mature hepatocytes proliferate after anti-mouse CD34 antibody, which recognizes the cytoplasmic partial hepatectomy (PH), resulting in regeneration of the domain, clearly showed the expression of CD34 on oval cells liver mass. However, if PH is performed when the hepatoas well as on endothelial cells of large hepatic vessels. In cytes are mitoinhibited and/or functionally compromised, the addition, bile ductular epithelial (BDE) cells both in the AAF/ bile duct-like cells, commonly referred to as oval cells, ex-PH model and in normal liver expressed CD34, suggesting a pand instead of mature hepatocytes and regenerate the liver. close relationship between BDE cells and the hepatic stem-The oval cells are thought to be the progeny of the normally cell compartment. Taken together, the data indicate that dormant hepatic stem-cell compartment. 9 We have pre-CD34 would, similar to its role in the hematopoietic system, viously shown by using an experimental model that combe an important probe for characterizing the cellular biology bines PH and the administration of a noncarcinogenic dose of the hepatic stem-cell compartment. (HEPATOLOGY 1997; of 2-acetylaminofluorene (AAF/PH model) 10 that oval cells 26:720-727. ) are capable of differentiating into hepatocytes. In th...