Identification and ultrastructural characterization of small hepatocyte-like cells in birds.

Small hepatocytes (SH) have been identified in regenerative organs and have been proposed to be hepatocyte progenitor cells. Their characteristic presence in birds, and their maturation into functional and mature hepatocytes, have not yet been elucidated. We previously demonstrated the appearance of chicken SH, which express CD44, in a model of chicken hepatopathy treated with bile duct ligation (BDL). We expanded on our previous research and performed a detailed study of the ultrastructure of chicken SH. Four weeks after BDL, we observed chicken SH with high electron density cytoplasm and with colony formation. In the chicken SH, electron microscopical analysis found no formation of tight junctions and no glycogen. Ultrastructural analysis also revealed the existence of various types of chicken SH with characteristics lying between those of chicken SH with colony formation and mature hepatocytes. The analysis of immunoelectron microscopy showed CD44 expressed on the surface of the extensive SH-like cells in the hepatic lamina. These results suggest that the expression of CD44 changes according to the differentiated stage of SH in a chicken BDL model.