EXPRESSION OF INTEGRINS DURING LIVER ORGANOGENESIS IN HUMANS

Integrins play a pivotal role in organogenesis, by mediating the inter actions between differentiating cells and the extracellular matrix. We analyzed the expression of integrins and their ligands during human l iver organogenesis. The expression of beta 1, beta 3, and beta 4 integ rins and the distribution of several extracellular matrix proteins wer e studied by immunoperoxidase in fetal liver samples from 5 to 40 week s' gestation. Hepatoblasts expressed only the beta 1, alpha I, alpha 5 , alpha 6, and alpha 9 integrin chains. Fetal hepatocytes, emerging at the 8th week of gestation, initially retained the same combination of integrins, but presented a progressive decrease in their expression l evels. After 15 weeks' gestation, the expression levels of beta 1, alp ha 1, alpha 5, and alpha 9 reached levels comparable to those observed in the adult state. alpha 6 expression became undetectable after 30 w eeks' gestation. As compared to hepatoblasts, intrahepatic biliary epi thelial cells, differentiating at the 8th week of gestation in the duc tal plate, were characterized by the progressive loss of alpha 1, the marked induction of alpha 6, and the de novo acquisition of the beta 4 , alpha 2, and alpha 3 integrin chains. The disappearance of integrin receptors for laminin on hepatocytes was associated with the rarefacti on of laminin in the perisinusoidal matrix, whereas their induction on biliary epithelial cells was associated with laminin deposition at th e point of contact with the ductal plate. In conclusion, integrins lik ely play an important role in the differentiation of the epithelial ce ll populations of the liver.