Lineage identity and location within the dermis determine the function of papillary and reticular fibroblasts in human skin.

Human skin dermis is comprised of the superficial papillary dermis and the reticular dermis in the lower layers, which can easily be distinguished histologically. In vitro analyses of fibroblasts from explant cultures from superficial and lower dermal layers suggest that human skin comprises at least two fibroblast lineages with distinct morphology, expression profiles and functions. However, while for mouse skin cell surface markers have been identified allowing the isolation of pure populations of one lineage or the other via fluorescence-activated cell sorting (FACS), this has not been achieved for human skin fibroblasts. We have now discovered two cell surface markers that discriminate between papillary and reticular fibroblasts. While FAP+/CD90- cells display increased proliferative potential, express PDPN and NTN1, and cannot be differentiated into adipocytes, FAP-/CD90+ fibroblasts express high levels of ACTA2, MGP, PPARγ and CD36, and readily undergo adipogenic differentiation, a hallmark of reticular fibroblasts. Importantly, flow cytometric analysis of fibroblasts isolated from superficial and lower layers of human dermis revealed that FAP+/CD90- cells are enriched in the papillary dermis. Altogether, functional analysis and expression profiling confirms that FAP+/CD90- cells represent papillary fibroblasts, while FAP-/CD90+ fibroblasts derive from the reticular lineage. Interestingly, although papillary and reticular fibroblasts are enriched in the upper or lower dermis, respectively, they are not spatially restricted, and the microenvironment seems to affect their function.