Basement membrane components define the microenvironment of aggregated fibroblasts in the skin and support their aggregation in vitro

The dermal papilla (DP), a specialized fibroblast aggregate in mammalian skin, plays a pivotal role in hair follicle development and regeneration through epithelial‒mesenchymal interactions. While its aggregated configuration is critical for its function, the mechanism maintaining this organization has remained unclear. Here, we show that human DP cells are embedded in a basement membrane (BM)-like extracellular matrix (ECM) rather than in conventional interstitial fibrillar ECM, such as collagen I. This BM-like ECM occupies the intercellular space as a diffuse, mesh-like structure, with minimal cell‒cell adhesion. These fibroblasts interact with specific laminin isoforms containing α1, α2 or α4 chains via integrin α7β1, resulting in weak adhesiveness. In vitro , DP spheroids remained aggregated in a BM extract-based matrix, Matrigel, but dispersed in collagen I gel in an α1β1 integrin-dependent manner. Our findings suggest that the BM components within DP aggregates mediate cell‒ECM interactions and maintain the cohesive DP structure in the absence of strong cell‒cell contacts. These results reveal an unconventional role for the BM as an adhesive microenvironment that sustains fibroblast aggregation and offer a new perspective on mesenchymal tissue organization and in vitro culture substrates for DP cells.