Lineage Identity and Location within the Dermis Determine the Function of Papillary and Reticular Fibroblasts in Human Skin

Abstract

Human skin dermis is composed 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 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 FAPCD90 cells display increased proliferative potential, express PDPN and NTN1, and cannot be differentiated into adipocytes, FAPCD90 fibroblasts express high levels of ACTA2, MGP, PPARγ, and CD36 and readily undergo adipogenic differentiation, a hallmark of reticular fibroblasts. Flow cytometric analysis of fibroblasts isolated from superficial and lower layers of human dermis showed that FAPCD90 cells are enriched in the papillary dermis. Altogether, functional analysis and expression profiling confirms that FAPCD90 cells represent papillary fibroblasts, whereas FAPCD90 fibroblasts derive from the reticular lineage. 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.

Deciphering the functional heterogeneity of skin fibroblasts using single‐cell RNA sequencing

“…Recent work by Tabib et al already suggested a higher heterogeneity of dermal FBs than previously expected . Although our study revealed some similarities with their work, we found major differences between our and their analyses.…”

“…As FBs have been described to show phenotypic heterogeneity, gene expression, and ability to synthesize ECM, we next wanted to compare the transcriptomes of every identified FB cluster (Data will be made available upon request). Interestingly, this comparison did not reveal the classical discrimination of papillary and reticular FBs, since markers for these FB subsets, such as netrin 1 ( NTN1 ), podoplanin ( PDPN ), and matrix Gla protein ( MGP ), were detected in all FB clusters (Figure S5). We also screened our data set for genes which have recently been shown to identify major FB subsets in the skin, such as THY1 , FAP , secreted frizzled‐related protein 2 ( SFRP2 ), and flavin containing monooxygenase 1 ( FMO1 ) .…”

“…Interestingly, this comparison did not reveal the classical discrimination of papillary and reticular FBs, since markers for these FB subsets, such as netrin 1 ( NTN1 ), podoplanin ( PDPN ), and matrix Gla protein ( MGP ), were detected in all FB clusters (Figure S5). We also screened our data set for genes which have recently been shown to identify major FB subsets in the skin, such as THY1 , FAP , secreted frizzled‐related protein 2 ( SFRP2 ), and flavin containing monooxygenase 1 ( FMO1 ) . Remarkably, THY1 (Figure A), FAP (Figure B), and FMO1 (Figure C) showed only marginal expression in our dataset (15,4% THY1 + , 16,3% FAP + , and 3% FMO1 + FB).…”

“…While DPP4 was previously found in mouse papillary dermis during embryonic and early postnatal development, in adult mice DPP4 expression was mainly located in the reticular dermis . In contrast, Korosec et al have recently demonstrated that DPP4 + FBs in human skin are mainly located in the papillary dermis, but also detectable at lower levels in the reticular dermis . In addition, Tabib et al show DPP4 expression in FBs of both dermal layers in human skin, and suggest a novel major FB population characterized by co‐expression of DPP4 and secreted frizzled related protein 2 ( SFRP2 ), a soluble modulator of Wnt signaling .…”

“…Recently, other strategies have been employed to decipher the roles of papillary and reticular FBs. For instance, Korosec et al have used a dermatome to mechanically separate papillary and reticular dermis and characterized the major FB population in each dermal layer by the expression levels of Thy‐1 cell surface antigen (THY1) and fibroblast activation protein‐alpha (FAP) . Although the so defined populations show all characteristics of either papillary or reticular FBs, their appearance is not strictly limited to the respective skin layer.…”