Three-dimensional computer-assisted reconstruction of ductal plate in the rat embryo (Carnegie stages 19-23)

In bile duct morphogenesis it has been established that the extrahepatic bile ducts in human originate from hepatic diverticulum while intrahepatic bile ducts arise from the ductal plate (DP), a network of primitive biliary epithelium that develops in the periportal connective tissue. The aim of this work was to reconstruct in rat embryos, stages 19-23, the three-dimensional (3D) distribution of the DP by means of a computer-assisted method. Six specimens, stages 19-23, fixed, dehydrated and paraffin-embedded, were submitted to serial histological sections and stained by hematoxylin-eosin and Heidenhain techniques. The images were directly digitalized with a CCD camera. The serial views were aligned anatomically by software and the data were analyzed following segmentation and thresholding. At stage 19, the DP was not yet organized. The periportal mesoderm (M) was gaining ground with some cords of cubic cells evoking primitive ductal cells. At stage 20, a row of ductal cubic cells went around the transverse portal sinus at the junction between M and liver cells. At stage 21, the DP developed at the periphery of periportal connective tissue and appeared in direct continuity with the hepatic duct (HDu). Four evaginations emerged from the DP and were growing up in the hepatic parenchyma. At stage 23, the DP appeared as a large network in continuity with the HDu located at the periphery of periportal M and presenting several evaginations radiating in the liver parenchyma. This work in the rat embryo permits the clear visualization of the development of the junctional zone in the hepatic hilum. Three phenomena are observed: (1) proximal left and right hepatic ducts and their segmental branches are derived from DP and not from the HDu; (2) the extrahepatic biliary system is in contact with the developing hilar ducts; (3) ductal maturation begins at the hilum and proceeds centrifugally. These observations are of great relevance in explaining pathological changes appearing at the hepatic hilum of neonates: hepatic polycystic disease, intrahepatic bile duct agenesis or atresia, and cyst of the extrahepatic bile duct.