Regulation by the microenvironment Self-organization processes are heavily influenced by surrounding conditions. Cells and organs regulate ChemµEnvs in microscopic compartments , enabling precise spatiotemporal control over material assembly (Fig. ). Synthetic approaches mimic these conditions and explore beyond natural processes 233 . Nevertheless, cellular collectives tune ChemµEnvs at microscopic scales and higher complexity than synthetics, and thus achieve hierarchical structures that are difficult to mimic 187 . The formation of such biological tissues, or organogenesis, is a complex orchestration process . Previous reviews have explored the principles of organoid self-organization . We focus on how environmental conditions influence their development 236 . To introduce a distinction from chemical microenvironments (directing the self-assembly of materials), cellular microenvironments (CellµEnv) describe the conditions regulating cell self-organization 237 (Fig. ). CellµEnvs use various conditions, communication pathways, diffusing biochemicals 238 , extracellular matrix (ECM), mechanical or bioelectric cues 241 , to name a few. Stem cell niches illustrate CellµEnvs, where surrounding signals either maintain stemness or induce cell differentiation 242 . They guide embryonic development, adult homeostasis, regeneration, and more 243 . For instance aged cells can be rejuvenated when placed in a biologically "younger" environment 244 , such as blood or skeletal cells 245 . When the blood circulations of a young and an old mouse are connected through parabiosis, systemic rejuvenation of the older mouse's tissues is observed . Key factors expressed by certain cells can also reprogram a tissue's identity, notably the thickening of skin upon volar fibroblast transplants . In many cases, only a few factors are necessary and sufficient to guide cell behavior and organoid development. For instance, only four Yamanaka factors are sufficient to reprogram differentiated cells into iPSCs 248 . As a second examples, four key signaling factors guide Lgr5-positive stem cells to grow intestinal organoid , not mentioning a few more essential parameters such as the ECM , culture medium and small molecules 250 . Tissue engineering leverages this ability of complex biological systems to arise from relatively simpler external inputs. By leveraging both tissue self-organization and organ-regulated material self-assembly, self-microfabrication aims to create high-value products, such as spider silk-based body armor 251 .