Latin for “in glass”; refers to experiments or procedures done outside of a living organism, typically in a lab environment. Initial experiments are usually carried out in vitro as conditions are easier to control and the system is much simpler. For example, drugs are often tested on cell lines in culture before they are tested in animal models or humans. The complexity of in vitro models has grown greatly in recent years, partially driven by high attrition rates of pharmaceuticals. Some complex in vitro models are now extremely physiologically sophisticated and can mimic high level tissue or organ function, such as multicellular spheroids, organoids, organs-on-a-chip, 3D bioprinted models, and 4D cell cultureCell culture refers to the growth of any type of cell (ie, plant, microbial, or animal) under contro.