MORPHOGENESIS OF THE BILIARY TRACT
In the human embryo the first anlage of the bile ducts and liver is the hepatic diverticulum from the proximal gastrointestinal tract, as described in Chapter 413. The caudal part of this bud, known as the pars cystica, grows in length and forms the gallbladder, cystic duct, and common bile duct. At about the eighth week of gestation, the hepatic precursor cells that lie adjacent to the hilar portal vein vessels form a sleeve-like double layer of cells that extends toward the periphery along the smaller intrahepatic portal vein branches. These hepatoblasts strongly express biliary-specific cytokeratins and can be considered biliary precursor cells, which then form a continuous single-layered ring around the portal mesenchyme, known as the ductal plate. Beginning at 12 weeks of gestation and extending into the postnatal period, the ductal plate undergoes progressive remodeling. As new ductules form, they are incorporated into the periportal mesenchyme that surrounds the portal vein branches. Thus, during successive periods of fetal life, ductal plate remodeling leads to the formation of the intrahepatic biliary tree. The largest ducts are formed first, followed by segmental, interlobular, and, finally, the smallest bile ductules. Arrest or derangement in remodeling leads to the persistence of primitive bile duct configurations termed ductal plate malformations.
CONGENITAL GALLBLADDER ABNORMALITIES
A variety of structural abnormalities of the gallbladder have been described. Congenital absence of the gallbladder occurs in 1 of 7500 to 10,000 people. Failed development of the pars cystica is the likely etiology. As an isolated abnormality, this is often of little clinical significance, although symptoms of abdominal pain, nausea, and fatty food intolerance may develop because calculi form in the ductal system. In these patients, gallbladder agenesis is frequently misinterpreted as cholecystitis with cystic duct obstruction. In addition to extrahepatic biliary atresia, which may accompany agenesis of the gallbladder, other associations with an absent gallbladder include imperforate anus, malrotation, genitourinary anomalies, anencephaly, bicuspid aortic valve, and cerebral aneurysms. Hypoplasia of the gallbladder has also been described in association with neonatal diabetes, hypoplastic pancreas, and intestinal atresia. There is also an association with trisomy 18. The incidence of double gallbladder is 0.1 to 0.75 per 1000. The 2 cystic ducts may converge into a single duct, forming a Y-shaped structure. The accessory gallbladder may lie under the left lobe of the liver, draining into the left hepatic duct. A “floating gallbladder” is an anatomic variant observed in up to 5% of individuals. The floating gallbladder lacks a peritoneal coat or supporting membrane, making the pendulous gallbladder susceptible to torsion. This presents clinically as acute, severe right upper quadrant pain with nausea and vomiting. Often, symptoms follow rapid movements that generate centrifugal forces causing gallbladder torsion and volvulus. Symptoms are consistent with an acute cholecystitis requiring operative intervention.
DUCTAL PLATE MALFORMATIONS
CONGENITAL HEPATIC FIBROSIS