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 1, 2 or 4 chains via integrin 7{beta}1, 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 1{beta}1 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.