The nail organ is a specialized appendage in which several ectodermal tissues coordinately function to sustain nail growth, a process that is coupled to digit regeneration. In this study, we show that the transcription factor Sox9 is expressed in several cell populations in the mouse digit tip. We found a SOX9+ cell population in the nail bed, and genetic lineage tracing showed that this is a transient cell population differentiated from matrix nail stem cells. In the absence of Sox9, nail matrix stem cells fail to differentiate into epithelial nail-bed cells and proliferate, thus expanding distally and following the corneocyte fate, which results in outlandishly large fingernails. In addition, the tip of the underlying terminal phalanx undergoes bone regression. Sox9-lineage tracing also revealed the existence of a continuous cell supply from a Sox9-expressing population residing in the basal layers to the entire hyponychium epidermis. Furthermore, digit-tip regeneration is compromised in Sox9-knockout mice, revealing an essential role for the gene during this process. These results will contribute to understand the cellular and molecular basis of mammalian nail organ homeostasis and disease and digit-tip regeneration and will help to design new treatment strategies for patients with nail diseases or amputation.
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