This article details a method for resin casting of the biliary system in mice, allowing for the visualization of the biliary architecture. The process involves injecting a colored and radiopaque resin into the common bile duct to create a detailed 3D representation of the biliary system.
The biliary system consists of the liver, gallbladder, and a network of ducts that join to form the common bile duct or C-B-D, which transports bile to the intestine.
For biliary system resin casting, take a mouse that has been perfused to remove blood from the liver and gallbladder.
Place the mouse in the supine position.
Visualize the abdominal cavity under a dissection microscope to locate the visceral surface of the liver to identify the sphincter of Oddi - a muscle layer present at the distal end of the CBD before it enters the intestine.
Make an oblique incision in the CBD next to the sphincter of Oddi.
Insert tubing into the CBD. Connect the other end of the tubing to a syringe containing a colored and radiopaque resin suspension.
Inject the resin suspension into the duct.
Resin moves from the CBD into the network of smaller biliary ducts, eventually spreading inside the liver and gallbladder. The spread can be tracked by visualizing the movement of colored resin through the terminals of small ducts.
Remove the tubing and allow the resin to polymerize.
Obtain the micro-CT images of the ducts. The resin being radiopaque does not allow X-rays to pass through it, providing a contrast to the biliary system and helping in analyzing its 3D architecture.
Before injecting resin into the biliary system of the mouse, start moving the intestine and pancreas to the left side of the mouse using a cotton swab, wetted with PBS, to expose the common bile duct and portal vein. Then, make a small transversal incision in the inferior vena cava. Flip the ventral side of the liver towards the heart to expose the visceral surface and the hilar region, and locate the common bile duct that runs from the hilar region across the pancreas, and into the intestine at the sphincter of Oddi. Use the straight forceps to clear the surrounding tissue area of approximately 5 millimeters from the common bile duct.
Then, by placing a silk suture thread under the common bile duct, tie a loose overhand knot around the common bile duct. Hold the spring scissors flat against the common bile duct to make an oblique incision to the common bile duct at the spot where the common bile duct enters the pancreas and intestine, next to the sphincter of Oddi. Next, mix 1 milliliter of the yellow resin with 50 microliters of the MV curing agent, and fill a 1 milliliter Luer syringe with the resin-curing agent mixture.
Connect the filled syringe with the tubing set #1 and press the plunger to fill the tubing till the resin-curing agent mixture drips from the tip of the tubing. Use the forceps to straighten the area around the common bile duct incision, before inserting the tubing into the opening in the common bile duct, with the longest edge of the beveled tip downwards towards the dorsal side of the bile duct. Then, tighten the silk thread knot to secure the tubing inside the common bile duct.
After injecting the resin into the common bile duct, observe the gallbladder and the individual liver lobes. Massage the liver with a PBS-wetted cotton swab to help spread the resin equally. Resin-filled bile ducts' terminal branches will be faintly visible at the liver surface. Stop injecting the resin when dots of resin appear at the surface of the liver, or when resistance is met. Then, remove the tubing by pulling out of the common bile duct, and quickly tighten the silk knot using forceps to prevent the resin from leaking out.
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