The development of the intrahepatic bile ducts in man was studied using an immunohistochemical technique on 56 liver specimens ranging in age from 6 weeks of gestation to 8 months after birth. On paraffin sections, two monoclonal anticytokeratin antibodies (CAM 5.2 and KL-1) that normally stain both hepatocytes and bile duct cells and two polyclonal anticytokeratin antisera that in normal adult liver stain bile ducts only were applied. For immunohistochemical staining of cryostat sections (only available from 14 weeks of gestation on), four monoclonal antibodies specifically directed against individual cytokeratin polypeptides 7, 8, 18 and 19 were used. Adult human hepatocytes contain cytokeratin 8 and 18 whereas bile duct cells also express cytokeratin 7 and 19 in addition to cytokeratin 8 and 18. On paraffin sections, primitive hepatocytes were stained with monoclonal antibodies CAM 5.2 and KL-1 from 6 weeks of gestation on. On cryostat sections, they were positive with monoclonal antibodies anticytokeratin 8 and 18 from the earliest time point examined (14 weeks). From 9 weeks of gestation on, portal vein branches were surrounded by a layer of cells showing a stronger positive reaction with monoclonal antibodies CAM 5.2 and KL-1 on paraffin sections and with monoclonal antibodies anti-cytokeratin 8 and 18 on cryostat sections (only available from 14 weeks on).
This layer, referred to as the ductal plate, first appeared around large portal vein branches close to the hilum and subsequently around more peripheral branches. The duct plates became duplicated over variably long segments of their perimeter, lumina appeared and tubular structures were formed. The latter gradually became incorporated in the connective tissue surrounding the portal vein, resulting in the appearance of individualized bile ducts. Ductal plate cells were stained by both polyclonal anticytokeratin antisera on paraffin sections. On cryostat sections (available from 14 weeks of gestation on), they were immunoreactive for cytokeratin 19 but negative with the monoclonal antibody directed against cytokeratin 7 until 20 weeks of gestation.
From then on, weakly positive staining for cytokeratin 7 was detected, but staining intensity subsequently increased and reached the level observed in adult liver at 1 month after birth. At birth, the smallest branches of the portal vein were still surrounded by a discontinuous ductal plate. We conclude that intrahepatic bile duct cells develop from hepatocytes around branches of the portal vein. During the phenotypic shift toward bile duct type cells, hepatocytes first display increased reactivity for cytokeratin 8 and 18 and express cytokeratin 19, followed only later on by cytokeratin 7. The intrahepatic bile duct system is still immature at birth.