The extrahepatic bile duct is the primary tissue initially affected by the cholangiopathy
biliary atresia. Biliary atresia affects neonates exclusively and current animal models
suggest that the developing bile duct is uniquely susceptible to damage. In this study,
we aimed to define the anatomical and functional differences between the neonatal
and adult mouse extrahepatic bile ducts. We studied mouse passaged cholangiocytes,
mouse BALB/c neonatal and adult primary cholangiocytes and isolated extrahepatic bile
ducts, and a collagen reporter mouse. Methods included transmission electron microscopy,
lectin staining, immunostaining, rhodamine uptake assays, bile acid toxicity assays,
and in vitro modeling of the matrix. The cholangiocyte monolayer of the neonatal extrahepatic
bile duct was immature, lacking the uniform apical glycocalyx and mature cell-cell
junctions typical of adult cholangiocytes. Functional studies showed that the glycocalyx
protected against bile acid injury and that neonatal cholangiocyte monolayers were
more permeable than adult monolayers. In adult ducts, the submucosal space was filled
with collagen I, elastin, hyaluronic acid, and proteoglycans. In contrast, the neonatal
submucosa had little collagen I and elastin, although both increased rapidly after
birth. In vitro modeling of the matrix suggested that the composition of the neonatal
submucosa relative to the adult submucosa led to increased diffusion of bile. A Col-GFP
reporter mouse showed that cells in the neonatal but not adult submucosa were actively
producing collagen. We identified four key differences between the neonatal and adult
extrahepatic bile duct. We showed that these features may have functional implications,
suggesting the neonatal extrahepatic bile ducts are particularly susceptible to injury
and fibrosis. Biliary atresia is a disease that affects newborns and is characterized
by extrahepatic bile duct injury and obstruction with resulting liver injury. We identify
four key differences between the epithelial and submucosal layers of the neonatal
and adult extrahepatic bile duct and show that these may render the neonatal duct
particularly susceptible to injury.