Fig. 2. Conceptual illustrations of a microfluidic teleost gut-on-chip. (A) The top chamber contains the fish GI microbiome grown on a porous membrane covered in mucin, to allow the perfusion of metabolites into the middle chamber, which contains the gut epithelial cells grown on a second membrane. This forms the lumen. In the bottom (serosal) chamber, a layer of companion cells (e.g. fibroblasts) is grown on the other side of the membrane supporting the gut cells. (B) A representation of a microfluidic chip with the three separate chambers: the microbiome (grey channel), gut chamber (blue channel) and serosal chamber (pink channel). Each channel is connected to a syringe pump providing bespoke fluid to each chamber. The gut chamber is connected to a pressure chamber (yellow), which is designed to mimic the pressure changes associated with peristalsis. (C) A cross-section of the three chambers, with the pressure chamber designed to mimic the natural movement of the epithelium and a list of the chamber contents to the right. Based on the design by Marrero et al. (2021).
Carol Bucking, Nic R. Bury, Henrik Sundh and Chris M. Wood. Making in vitro conditions more reflective of in vivo conditions for research on the teleost gastrointestinal tract. Journal of Experimental Biology
ABSTRACT
To date, the majority of in vitro or ex vivo fish gastrointestinal research has been conducted under unrealistic conditions. In a living fish, ionic conditions, as well as levels of ammonia, pH, HCO3 − and PCO2 differ considerably between the different regions of the gastrointestinal tract. These factors also differ from those of the saline often used in gut research. Furthermore, the oxygen gradient from the serosa to the gut lumen is rarely considered: in contrast to the serosa, the lumen is a hypoxic/anoxic environment. In addition, the gut microbiome plays a significant role in gut physiology, increasing the complexity of the in vivo gut, but replicating the microbial community for in vitro studies is exceptionally difficult. However, there are ways in which we can begin to overcome these challenges. Firstly, the luminal chemistry and PO2 in each gut compartment must be carefully considered. Secondly, although microbiological culture techniques are improving, we must learn how to maintain the microbiome diversity seen in vivo. Finally, for ex vivo studies, developing mucosal (luminal) solutions that more closely mimic the in vivo conditions will better replicate physiological processes. Within the field of mammalian gut physiology, great advances in ‘gut-on-chip’ devices are providing the tools to better replicate in vivo conditions; adopting and adapting this technology may assist in fish gut research initiatives. This Commentary aims to make fish gut physiologists aware of the various issues in replicating the in vivo conditions and identifies solutions as well as those areas that require further improvement.