Rete Ridge Topography as a Determinant of Epidermal Stem Cell Identity: Implications for Skin Aging
Stem cell niches are dynamic microenvironments that regulate tissue homeostasis. Epidermal stem cells (EpiSC) preferentially localize to concave regions of epidermal rete ridges, which serve as primary niches for stem cell maintenance. EpiSC number and functional integrity decline during chronological aging. A defining feature of aged skin is epidermal atrophy, in which the prominent rete ridges present in young skin become flattened. Whether such topographical alterations influence EpiSC homeostasis and differentiation remains unclear. To address this, we generated anatomically accurate rete ridge structures using 3D bioprinting of collagen matrices as an ex vivo model and compared EpiSC cultured within concave topography to those maintained on a flat matrix resembling aged skin. Transcriptomic analysis revealed that concave niches promoted keratinocyte differentiation, marked by increased type I and II keratin gene expression and downregulation of cell cycle-associated genes. ATAC-seq identified topography-dependent chromatin accessibility changes enriched for transcription factors regulating epidermal differentiation, including upregulation of KLF4 and GRHL3 and downregulation of SOX9, HOXA1, and ETS1. Consistently, aged human skin showed reduced KLF4 and GRHL3 and increased SOX9 compared with young skin. Our findings demonstrate that concave niche topography imposes a spatially defined EpiSC microenvironment that promotes differentiation, alters cell cycle, and when perturbed, potentially contributes to the aging process. We conclude that spatial localization within rete ridge regions significantly affects epidermal progenitor stemness properties as fundamental differences in the physical microenvironment appear to influence cell fate decisions, thus, form shapes function of EpiSC.