Biliary Tract: What Are They? Structure, Clinical Significance and Bioengineering of Bile Ducts

A bile duct or line is any of a series of long, tube-like structures that carry bile, and is present in most vertebrates.

Bile, necessary for the digestion of food, is secreted by the liver into passages that transport bile into the hepatic duct, which joins the cystic duct (transporting bile to and from the gallbladder) to form the bile duct common , which opens to the intestine.

The biliary tree is the entire network of ducts of various sizes that branch out through the liver.

The route is as follows: bile canaliculi → Hering channels → interlobular bile ducts → intrahepatic bile ducts → left and right hepatic ducts fuse to form → common hepatic duct leaves the liver and joins → cystic duct (of the gallbladder) forming → common bile duct → joins with the pancreatic duct → forming the ampulla of Vater → enters the duodenum.

Structure of the bile duct

The bile duct is green like the gallbladder, due to bile stains.

Innervation:

The inflation of a balloon in the bile duct causes, through the vagus nerve, the activation of the brainstem and the insular cortex, the prefrontal cortex and the somatosensory cortex.

Clinical significance of the bile ducts

Blocking:

The blockage of the bile duct by gallstones, the healing of a lesion or cancer prevents bile from being transported to the intestine and the active ingredient of bile ( bilirubin ) accumulates in the blood.

This condition causes jaundice , where the skin and eyes turn yellow due to bilirubin in the blood. This condition also causes severe itching of the bilirubin deposited in the tissues.

In certain types of jaundice, the urine will be noticeably darker, and the stools will be much paler than normal. This is caused by the bilirubin that goes into the bloodstream and is filtered into the urine by the kidneys, instead of being lost in the stool through the ampulla of Vater.

Jaundice:

Jaundice is commonly caused by conditions such as pancreatic cancer, which causes blockage of the bile duct that passes through the cancerous portion of the pancreas, cholangiocarcinoma, cancer of the bile ducts.

Also blockage by a stone in patients with gallstones; and of scars after a lesion in the bile duct during removal of the gallbladder.

Sewer system:

Biliary drainage is done with a tube or catheter (called biliary drainage, biliary stent or biliary catheter) by a surgeon or, usually, an interventional radiologist.

It can be used to relieve a blockage in the bile duct, either permanently or as a temporary solution before definitive treatment, such as surgery. Drainage can be placed percutaneously through the liver, and the procedure is called percutaneous transhepatic bile drainage.

This can also be done as part of a percutaneous transhepatic cholangiography, then a form of interventional radiology.

Bile drainage can also be used to take bile samples for diagnosis or monitoring of diseases, as well as to provide a route of administration for medical substances.

In babies with biliary atresia, the hepatoportoenterostomy is an alternative method to provide biliary drainage.

Cholangiocarcinoma:

Cholangiocarcinoma or bile duct cancer is a form of cancer that is composed of mutated epithelial cells (or cells that show features of epithelial differentiation) that originate in the bile ducts.

It is considered that cholangiocarcinoma is an incurable and rapidly lethal cancer unless both the primary tumor and any metastases can be completely removed by surgery.

There is no potentially curative treatment, except surgery, but most people have advanced disease at the time of presentation and are inoperable at the time of diagnosis.

Bioengineering: regenerated bile ducts

The development of a protocol to isolate and expand the cell population that lines the bile ducts has allowed the in vitro generation of bioengineered ducts. These can replace the native bile ducts when they are transplanted into mice.

Bile, which facilitates the absorption of lipids in the small intestine, is highly toxic to the liver cells where it is produced. Therefore, this fluid must be drained from the organ through the intra and extrahepatic bile ducts, which are respectively inside and outside the liver.

Damage to the bile ducts can prevent adequate drainage, causing often fatal liver diseases. A possible treatment is the implantation of artificial bile ducts with bioengineering.

The inner lining of the bile ducts consists of cells called cholangiocytes. The first step to generate bile ducts in vitro is, therefore, to obtain extrahepatic cholangiocytes.

The protocols have been successfully developed to generate intrahepatic cholangiocytes from human pluripotent stem cells, which have the potential to give rise to any type of cell in the body.

But intrahepatic and extrahepatic cholangiocytes have different origins and functions, and these protocols can not be used to generate the last cells. An alternative method is to isolate extrahepatic cholangiocytes from the gallbladder (in which bile is stored) by enzymatic digestion processes.

However, isolated cells have limited potential to proliferate when grown in two-dimensional layers, making it difficult to produce enough cells to form a bile duct.

Sampaziotis et al. demonstrated that extrahepatic cholangiocytes could be mechanically isolated by brushing or scraping cells from the interior of human extrahepatic bile ducts.

This method is minimally invasive, in contrast to the isolation of the gallbladder, which requires surgery. In addition, it allows easy access to human cells, often a considerable limitation for therapies.