The "missing body problem": cellular microenvironment When trying to grow a tissue outside its native environment, a basic question is: what is lacking for its survival? In a nutshell, the rest of the body is missing. In practice, only a finite number of cues are sufficient to guide a tissue's development and maintenance. For instance, the self-renewal and proliferation of many stem cells is regulated by their niche 292 , which can be replicated in vitro . Characterizing a new tissue's CellµEnv is challenging due to its complexity and potential differences from established systems 294 . Defining these dynamic environments involves the understanding of signals from surrounding cells 295 , diffusing molecules gradients 296,297 , ECM 298 , mechanical 299 and bioelectric cues 300 . To enable self-microfabrication, addressing the "missing body problem" by maintaining functional tissues ex vivo is a crucial initial proof of concept that the organ may eventually produce material autonomously, which has been achieved with spider silk explants.