Wound healing, fibroblast heterogeneity, and fibrosis

Abstract

Fibroblasts are highly dynamic cells that play a central role in tissue repair and fibrosis. However, the mechanisms by which they contribute to both physiologic and pathologic states of extracellular matrix deposition and remodeling are just starting to be understood. In this review article, we discuss the current state of knowledge in fibroblast biology and heterogeneity, with a primary focus on the role of fibroblasts in skin wound repair. We also consider emerging techniques in the field, which enable an increasingly nuanced and contextualized understanding of these complex systems, and evaluate limitations of existing methodologies and knowledge. Collectively, this review spotlights a diverse body of research examining an often-overlooked cell type-the fibroblast-and its critical functions in wound repair and beyond.

Keywords: fibroblast heterogeneity; fibroblasts; fibrosis; regeneration; wound healing.

Figure 1:. Differences in the architecture of mouse versus human skin.

A) Depiction of mouse skin structure, which includes a panniculus carnosus (PC) muscle layer under the hypodermis. B) Depiction of human skin structure. Human skin differs from mouse skin in that it is thicker overall, has sparser hair follicles (HF), and contains eccrine sweat glands (ESG) which are not found in mice outside the skin of the paws. For both mouse and human skin schematics, various mesenchymal subpopulations and characteristic markers are shown, including microanatomically defined fibroblast lineages (papillary, reticular, hypodermal fibroblasts), functionally defined fibroblast lineages (Engrailed-1 lineage-positive and lineage-negative fibroblasts), HF-associated fibroblasts (dermal sheath, papilla, arrector pili), and pericytes. SG, sebaceous gland; DWAT, dermal white adipose tissue; APM, arrector pili muscle.

Figure 2:. Fibroblast heterogeneity in mouse skin wound healing.

Left: Heterogeneous responses to wounding for the three microanatomically-defined lineages of fibroblasts (papillary, reticular, hypodermal). In small wounds, reticular (middle) and hypodermal (bottom) lineages dominate, giving rise to scarring myofibroblasts. In larger wounds (e.g., WIHN), papillary fibroblasts (top) preferentially interact with epidermal elements to drive sparse adnexal regeneration, and a subset of reticular layer-derived myofibroblasts regenerates adipocytes. Pericytes (bottom middle) and possibly circulating hematopoietic cells (“fibrocytes”, top middle) may also generate scarring myofibroblasts. Right: Heterogeneity in wound healing responses by Engrailed-1 (En-1) lineage-positive (green, EPF) and -negative (red, ENF) fibroblasts. EPFs are responsible for the vast majority of dorsal skin scarring in response to wounding, while ENFs do not contribute to fibrosis; a subset of ENFs reactivates En-1 to contribute postnatally-derived scarring EPFs. Proposed sources of scarring EPFs include the dermis (all layers), adipose tissue, circulating cells, and fascia. How these different lineages of fibroblasts (papillary, reticular, adipose, EPFs/ENFs, etc.) overlap and/or give rise to one another remains unclear.

Figure 3:. Levels of molecular regulation of fibroblast behavior.

Fibroblast behavior depends on the complex interplay of many factors, including these cells’ epigenomic, transcriptomic, and proteomic profiles. Modern technologies are able to interrogate fibroblasts at each of these molecular levels to better understand fibroblast heterogeneity and functioning.

Figure 4:. Mechanisms of mechanotransduction signaling.

Schematic depicting the diverse mechanisms and signaling pathways/cascades by which fibroblasts and other mechanosensitive cell types may communicate changes in their mechanical environment to molecular phenotypic changes. Multiple levels of evidence exist to show that fibroblasts activate pro-fibrotic machinery in response to mechanical stress, and that fibroblast mechanical signaling can be targeted to mitigate fibrosis.