Telocytes and interstitial cells of Cajal in the biliary system

Abstract A novel type of interstitial tissue cells in the biliary tree termed telocytes (TCs), formerly known as interstitial Cajal‐like cells (ICLCs), exhibits very particular features which unequivocally distinguish these cells from interstitial cells of Cajal (ICCs) and other interstitial cell types. Current research substantiates the existence of TCs and ICCs in the biliary system (gallbladder, extrahepatic bile duct, cystic duct, common bile duct and sphincter of Oddi). Here, we review the distribution, morphology and ultrastructure of TCs and ICCs in the biliary tree, with emphasis on their presumptive roles in physiological and pathophysiological processes.

and with a moniliform aspect. 23 From then on, this novel cell type became known as the TC. 22 In this review, we want to summarize current researches on ICCs and TCs in the biliary system and hence help to understand their underlying effect in health and disease.  24 Early work substantiated that ICCs or TCs express CD117/Kit protein as a specific maker. 25 Thus, Sun et al used immunohistochemistry employing a polyclonal antibody raised against CD117/Kit to confirm that c-Kit was expressed in the gallbladder tissues of CD1 mice. 26,27 Spindle-shaped ICCs with thin and long processes starting from the two poles appearing as a network-like structure were found to be distributed in all the layers of the gallbladder wall. At the ultrastructural level, ICCs in the murine  26 These cells are adjacent to the smooth muscle cells and nerve endings. 27 In addition, Ahmadi et al 28 showed that ICCs are present in the human extrahepatic bile ducts, where they are more densely aggregated than those present in the gallbladder. These cells are most prominent within the muscle layers of the extrahepatic bile ducts where they are organized into loosely arranged laminae running parallel to circular muscle fibres. However, ICCs are invisible in intrahepatic bile ducts. The authors further suggested that the distribution and density of ICCs in the common bile duct are similar to those in the ampullary region, where they are distributed within the circular muscle, running mainly parallel to the circular direction, and are typically clustered in groups forming a loose network. However, the authors did not describe the distribution of ICCs in the longitudinal muscle layer of the common bile duct. 28 There is now overwhelming evidence to support canonical ICCs as the origin of pacemaker potentials in the gastrointestinal tract, playing an important role in contractile activity. The gallbladder smooth muscle, which resembles the gastrointestinal smooth muscle, also exhibits spontaneous rhythmic electrical activity. Therefore, it has been proposed that in certain specific regions of the biliary system, these ICCs are likely involved in modulating the excitability of smooth muscle and tissue motility. This hypothesis was subsequently confirmed by researches conducted in China and abroad. In 2012, Pan et al 29 explored the changes in gallbladder smooth muscle contraction and ICCs in the gallbladder using common bile duct ligation (CBDL) model in guinea pigs, and they found that both the contraction response of the gallbladder strips and the number of c-Kit-positive cells were reduced. The phenomena could be improved after relieving CBD obstruction. In recent years, our laboratory has dedicated effort to studying the correlation between ICCs and gallbladder motility in acute cholecystitis, too. For example, Huang et al 30 reported that the expression levels of c-Kit and SCF mRNA and protein were markedly decreased after CBDL. Accordingly, they proposed that the loss of ICCs may be attributed to repression of the SCF/c-kit pathway during acute cholecystitis, which possibly participate in the development of decreased gallbladder motility. Moreover, as early as 2014, our colleague Zhang 31 performed a similar experiment and observed that after ligation of the CBD, the gallbladder ICCs became swollen, had a dilated endoplasmic reticulum and a reduced number of cell processes. With prolonged time after CBDL, the injury to the gallbladder ICCs was further aggravated. At 48 hour after CBDL, the cytoplasm of the neutrophils was full of granules, and the neutrophils were in close contact with the ICCs bodies as well as their processes. Of particular note, the ultrastructural impairment of ICCs was slightly alleviated, and the cells moved closer to each other or to the gallbladder smooth muscle cells after using antipolymorphonuclear antibody (anti-PNM). In addition, some authors recommended that ICCs are likely associated with choledochal cysts (CC). For instance, Duan 32 suggested an involvement of the ICCs, which showed a significantly decreased density in the CC containing sparse ICCs remnants so that their network disappeared.

| ICCS IN THE BILIARY SYSTEM
This finding was compatible with reports by Zhao 33 and Osman 34 who proposed that reduction in the ICC population may contribute to the development of CC, and could help to disclose the abnormal motility of the common bile duct. However, CC represents an extremely rare congenital disorder that primarily manifests in the paediatric stage, characterized by intra-and/or extrahepatic biliary dilatation, although the exact aetiology remains incompletely understood.

| TCS AND BILIARY SYSTEMIC DISEASE
Since the discovery of TCs in the biliary tree, increasing evidence suggests a correlation between these cells and biliary systemic motility as highlighted above. Accordingly, their potential role in the pathogenesis of some biliary systemic disorders has now begun to attract substantial attention.

| TCs and cholelithiasis
Cholelithiasis is one of the most common diseases in gastroenterol-  54 Therefore, these observations support that the higher concentration of x-6 PUFA and the lower amounts of GCA and TCA result in TCs loss and consequent gallstone formation by elevating the lithogenicity index. 54 Moreover, some authors suggested that damage to TCs could be related to the blocking of the c-kit/SCF signal pathway consequent to a high cholesterol level 40 or chronic inflammatory reaction in the gallbladder wall. 46 In conclusion, an increased bile lithogenicity index or other factors cause the deficiency of TCs, which may further induce the functional impairment of gallbladder motility, particularly given the importance of TCs in gallbladder rhythmic electrical activity.

| TCs and cholecystitis
In our most recent study, 56 we explored the effects of neutrophils on TCs in the gallbladder and examined the possibility that TCs take part in gallbladder hypomotility during acute acalculous cholecystitis induced by CBDL. The result revealed damage to the ultrastructure of TCs in both co-culture 2 group (TCs co-culture with neutrophils isolated from 24-hours CBDL group) and co-culture 3 group (TCs co-culture with neutrophils isolated from 48-hours CBDL group). In addition, the co-culture groups showed a significantly higher number of apoptotic TCs than the group of culture alone, and co-culture 2/3 group was higher than co-culture 1 group (TCs co-culture with neutrophils isolated from the sham-operated group). What's more, the protein and mRNA levels of both SCF and c-kit decreased in all three co-culture groups, and the lowest expression level was detected in co-culture 3 group. All above results suggested that the decreased gallbladder motility in acute cholecystitis is due to the effects of neutrophils on the development and function of gall bladder TCs via depression of SCF/c-kit expression. But the specific mechanisms underlying this process remain to be further illuminated.