Fibroblasts constitute the major mesenchymal cell type in the connective tissue and their functions are remarkably diverse: here, by characterising lineages of mouse skin fibroblasts, it is shown that distinct subpopulations contribute to skin development and repair during injury. Fibroblasts are unremarkable looking cells found in most tissues in the body, where they are mainly concerned with making the collagen that supports other cell types. The cells all look much the same yet are functionally diverse, prompting the question, is there just one cell type responding differently to different stimuli, or do individual cells specialize? A transplantation and lineage tracing study in mice now shows that skin connective tissue arises from two distinct fibroblast lineages that also contribute differentially to skin development and repair after injury. One cell type forms the lower dermis and the other the upper dermis. The latter lineage is required for hair follicle production. In wounded adult skin, the initial wave of dermal repair is mediated by the 'lower' lineage, which may explain the absence of hair follicles in newly closed wounds. The authors develop a comprehensive lineage tree for all fibroblast-derived cell types in mouse dermis, including smooth muscle cells and adipocytes. Fibroblasts are the major mesenchymal cell type in connective tissue and deposit the collagen and elastic fibres of the extracellular matrix (ECM)1. Even within a single tissue, fibroblasts exhibit considerable functional diversity, but it is not known whether this reflects the existence of a differentiation hierarchy or is a response to different environmental factors. Here we show, using transplantation assays and lineage tracing in mice, that the fibroblasts of skin connective tissue arise from two distinct lineages. One forms the upper dermis, including the dermal papilla that regulates hair growth and the arrector pili muscle, which controls piloerection. The other forms the lower dermis, including the reticular fibroblasts that synthesize the bulk of the fibrillar ECM, and the preadipocytes and adipocytes of the hypodermis. The upper lineage is required for hair follicle formation. In wounded adult skin, the initial wave of dermal repair is mediated by the lower lineage and upper dermal fibroblasts are recruited only during re-epithelialization. Epidermal β-catenin activation stimulates the expansion of the upper dermal lineage, rendering wounds permissive for hair follicle formation. Our findings explain why wounding is linked to formation of ECM-rich scar tissue that lacks hair follicles2,3,4. They also form a platform for discovering fibroblast lineages in other tissues and for examining fibroblast changes in ageing and disease.

Microglia, the mononuclear phagocytes of the central nervous system (CNS), are important for the maintenance of CNS homeostasis, but also critically contribute to CNS pathology. Here we demonstrate that the nuclear factor kappa B (NF-κB) regulatory protein A20 is crucial in regulating microglia activation during CNS homeostasis and pathology. In mice, deletion of A20 in microglia increases microglial cell number and affects microglial regulation of neuronal synaptic function. Administration of a sublethal dose of lipopolysaccharide induces massive microglia activation, neuroinflammation, and lethality in mice with microglia-confined A20 deficiency. Microglia A20 deficiency also exacerbates multiple sclerosis (MS)-like disease, due to hyperactivation of the Nlrp3 inflammasome leading to enhanced interleukin-1β secretion and CNS inflammation. Finally, we confirm a Nlrp3 inflammasome signature and IL-1β expression in brain and cerebrospinal fluid from MS patients. Collectively, these data reveal a critical role for A20 in the control of microglia activation and neuroinflammation.