Straddling the Line Between and Investigations.

Tissue engineering research fundamentally relies on experiments to advance knowledge, utilizing various models for research on both humans and animals. With scientific progress, experimental models have become increasingly complex over time. This complexity sometimes blurs the distinction between categories, making terminology less consistent. In biomedical research, three overarching terms are commonly used to characterize experimental environments: , , and . While translates from Latin as "in glass," referring historically to experimental conditions in a test tube or petri dish, experiments occur within a living organism's natural environment. Conversely, originates from living tissue outside its host environment while striving to maintain conditions as close to the host surroundings as possible. In the tissue engineering and regenerative medicine (TE&RM) community, there needs to be more clarity between and terminology, with historical definitions sometimes disregarded and new terms often introduced without rigorous scientific justification. At this juncture, the question arises of when to refer to experiments as or or whether the terms may be used synonymously in some instances. Moreover, what criteria must experiments meet to be legitimately defined as such? This perspective is intended to address questions that would assist the TE&RM community in better understanding the differences between and models. Impact Statement In the tissue engineering & regenerative medicine literature, the terms "in vitro" and "ex vivo" are often used interchangeably to describe experiments. This interchangeable usage can lead to a compromised interpretation of research results and, consequently, misleading scientific conclusions and teachings. This perspective aims to provide clarity on the various definitions of experimental designs. It also highlights the issue of using terms with inconsistent meanings that have origins dating back to the distant past. It's important to note that scientific definitions constantly evolve, and there is a scientifically rooted responsibility to evaluate and rethink the current state of affairs critically.