Transient matrix remodeling during direct cell-fate reprogramming enables self-assembly of dense neuronal niches

Native tissues are characterized by high cellular density and continuous reciprocal interactions between cells and the extracellular matrix (ECM), yet recreating these features in vitro remains a major challenge. Current three-dimensional (3D) culture systems typically capture either dense cell-cell interactions or a defined ECM environment, but rarely both. Here, we introduce a strategy that exploits direct fibroblast-to-neuron reprogramming to decouple microenvironment formation from final cell identity. Human dermal fibroblasts were embedded in collagen microgels, where their intrinsic matrix-remodeling capacity drove rapid ECM compaction. Concurrently, the contractile fibroblasts were converted into induced neurons (iN), generating densely packed neuronal microcultures within a remodeled collagen matrix. This approach transforms a defining function of the starting cell type into a mechanism for engineering the microenvironment of the target cell type. The resulting iN-microgels combine high cell density, defined ECM composition, and tunable biophysical properties in a scalable microscale format. By coupling matrix remodeling with cell-fate conversion, our platform establishes a general framework for engineering tissue-like microenvironments and reveals how dynamic 3D extracellular contexts can be integrated into direct cell reprogramming.