Adipocyte hypertrophy and lipid dynamics underlie mammary gland remodeling after lactation

Fibroblasts are the major mesenchymal cell type in connective tissue and deposit the collagen and elastic fibers of the extracellular matrix (ECM) 1 . Even within a single tissue fibroblasts exhibit remarkable 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, 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 (APM), which controls piloerection. The other forms the lower dermis, including the reticular fibroblasts that synthesise the bulk of the fibrillar ECM, and the pre-adipocytes 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-epithelialisation. Epidermal beta-catenin activation stimulates 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 follicles 2-4 . They also form a platform for discovering fibroblast lineages in other tissues and for examining fibroblast changes in ageing and disease.

The majority of eukaryotic cells synthesize neutral lipids and package them into cytosolic lipid droplets. In vertebrates, triacylglycerol-rich lipid droplets of adipocytes provide a major energy storage depot for the body, whereas cholesteryl ester-rich droplets of many other cells provide building materials for local membrane synthesis and repair. These lipid droplets are coated with one or more of five members of the perilipin family of proteins: adipophilin, TIP47, OXPAT/MLDP, S3-12, and perilipin. Members of this family share varying levels of sequence similarity, lipid droplet association, and functions in stabilizing lipid droplets. The most highly studied member of the family, perilipin, is the most abundant protein on the surfaces of adipocyte lipid droplets, and the major substrate for cAMP-dependent protein kinase [protein kinase A (PKA)] in lipolytically stimulated adipocytes. Perilipin serves important functions in the regulation of basal and hormonally stimulated lipolysis. Under basal conditions, perilipin restricts the access of cytosolic lipases to lipid droplets and thus promotes triacylglycerol storage. In times of energy deficit, perilipin is phosphorylated by PKA and facilitates maximal lipolysis by hormone-sensitive lipase and adipose triglyceride lipase. A model is discussed whereby perilipin serves as a dynamic scaffold to coordinate the access of enzymes to the lipid droplet in a manner that is responsive to the metabolic status of the adipocyte.

Mature adipocytes are generated through the proliferation and differentiation of precursor cells. Our prior studies identified adipocyte progenitors in white adipose tissue (WAT) as Lin−:CD29+:CD34+:Sca-1+:CD24+ (CD24+) cells that are capable of generating functional WAT 1 . Here, we employ several Cre recombinase mouse models to identify the adipocyte cellular lineage in vivo. While it has been proposed that white adipocytes are derived from endothelial 2 and hematopoietic 3, 4 lineages, we find that neither of these lineages label white adipocytes. However, platelet-derived growth factor receptor α (PdgfRα)-Cre trace labels all white adipocytes. Analysis of WAT from PdgfRα-Cre reporter mice identifies CD24+ and Lin−:CD29+:CD34+:Sca-1+:CD24− (CD24−) cells as adipocyte precursors. We show that CD24+ cells generate the CD24− population in vivo and the CD24− cells express late markers of adipogenesis. From these data we propose a model where the CD24+ adipocyte progenitors become further committed to the adipocyte lineage as CD24 expression is lost, generating CD24− preadipocytes. This characterization of the adipocyte cellular lineage will facilitate study of the mechanisms that regulate WAT formation in vivo and WAT mass expansion in obesity.