Microphysiologic 2D and 3D aquatic intestinal in vitro models: current advancements and future applications

The intestinal tract maintains animal health directly (barrier integrity, nutrient absorption, etc.) and indirectly (microenvironment for commensal organisms). Due to its close interactions with the internal and external environment, the intestinal tract is the site of interactions between resident stromal cells, commensal and pathogenic organisms, and immune cells. As a result of its complex functions, physiology, and structural morphology, in vivo studies have been the most reliable method for understanding intestinal biology, until recently. The development of microphysiological and 3D cell culture based in vitro models are powerful tools to help address questions of drug and heavy metal cytotoxicity, barrier function, effects of nutritional supplements on local immune responses, and pathogenic immunomodulatory effects, while also helping to minimize live animal use (3Rs). While these physiologically-relevant in vitro systems have historically been used in mammalian species, fish researchers have begun to adopt these models to study fish intestinal physiology, pathogen defenses, and mucosal immunity, though to a more limited extent. Compared to the thousands of fish species and over 900 fish cell lines, only 10 fish intestinal cell lines exist. Here, we discuss the various intestinal in vitro models, their current and future applications for studying fish mucosal immunity and overall health, as well as proposed future directions for studying intestinal pathogens and diseases in aquatic invertebrate species of economic and environmental importance.