ABSTRACT
The liver develops from two anlages: the hepatic diverticulum, which buds off the
ventral side of the foregut, and the septum transversum, which is the mesenchymal
plate that partially separates the embryonic thoracic and abdominal cavities. The
endodermal cells of the hepatic diverticulum invade the septum transversum, forming
sheets and cords of hepatoblasts arrayed along the sinusoidal vascular channels derived
from the vitelline veins emanating from the yolk sac. The vitelline veins fuse to
form the portal vein, which ramifies as tributaries within the liver along mesenchymal
channels termed portal tracts. Those hepatoblasts immediately adjacent to the mesenchyme
of the portal tracts differentiate into a ductal plate, a single circumferential layer
of biliary epithelial cells. Mesenchymal cells interpose between the ductal plate
and the remaining parenchymal hepatoblasts, which differentiate into hepatocytes.
By week 7 the ductal plate begins to reduplicate, forming a double layer of cells
around the portal tract. Lumena form between the two cell layers of the ductal plate,
forming peripheral biliary tubular structures. These peripheral tubules remodel and,
with continued proliferation of the mesenchyme, by the 11th week begin to become more
centrally located within portal tracts as terminal bile ducts with a circular cross-section.
The remaining ductal plate resorbs, leaving behind only tethers of bile ductules connecting
the terminal bile ducts to the parenchyma. Abutting and within the parenchyma are
the canals of Hering, ductular structures half-lined by hepatocytes and half-lined
by biliary epithelial cells. Maturation of the intrahepatic biliary tree to the mature
tubular treelike architecture occurs from the hilum of the liver outward, beginning
around the 11th week of gestation and continuing past birth for several months. The
architecture of maturation is the same regardless of gestational age or radial location
in the liver. Importantly, the immature intrahepatic biliary system maintains patency
and continuity with the extrahepatic biliary tree throughout gestation, with no evidence
of a solid phase of development. Thus, from the earliest time of hepatocellular bile
formation beginning around the 12th week, there is a patent passage to the alimentary
canal.
KEYWORDS
bile duct - development - liver - canal of Hering - ductal plate - anatomy
