Understanding Tendon Fibroblast Biology and Heterogeneity

Abstract

Tendon regeneration has emerged as an area of interest due to the challenging healing process of avascular tendon tissue. During tendon healing after injury, the formation of a fibrous scar can limit tendon strength and lead to subsequent complications. The specific biological mechanisms that cause fibrosis across different cellular subtypes within the tendon and across different tendons in the body continue to remain unknown. Herein, we review the current understanding of tendon healing, fibrosis mechanisms, and future directions for treatments. We summarize recent research on the role of fibroblasts throughout tendon healing and describe the functional and cellular heterogeneity of fibroblasts and tendons. The review notes gaps in tendon fibrosis research, with a focus on characterizing distinct fibroblast subpopulations in the tendon. We highlight new techniques in the field that can be used to enhance our understanding of complex tendon pathologies such as fibrosis. Finally, we explore bioengineering tools for tendon regeneration and discuss future areas for innovation. Exploring the heterogeneity of tendon fibroblasts on the cellular level can inform therapeutic strategies for addressing tendon fibrosis and ultimately reduce its clinical burden.

Keywords: biomaterials; fibroblast; fibrosis; tendon; tenocyte.

Figure 1

Schematic of stages of tendon healing and fibrosis and the cells involved. A normal tendon is made up of multiple populations of tenocytes and a tendon stem and progenitor cells in a collagen matrix and is surrounded by the epitenon. Macrophages have also been documented to be present at baseline. After injury, tendon healing and fibrosis go through three overlapping phases: inflammatory, fibrotic/proliferative, and remodeling. In normal tendon healing, macrophages, neutrophils, and fibroblasts migrate into the site of the injury during the inflammatory phase. Then, intrinsic tenocytes and extrinsic fibroblasts begin laying down collagen III in the proliferative phase. In the remodeling phase, cellularity decreases, and collagen III is replaced with collagen I. All postnatal tendons heal with a scar, which has a compromised structure compared to normal tendons; however, some tendons go through a process of “over-healing” following the inflammatory phase (bottom row). In these fibrotic tendons, fibroblasts lay down an excess of disorganized collagen and other matrix proteins during the proliferative phase. During the remodeling phase, the increased scar is maintained by replacing collagen III with collagen I. Figure created using BioRender.com (accessed on 8 March 2024).

Figure 2

Representative image of tenocyte subpopulations from Kendal et al. [49]. (A) Uniform Manifold Approximation and Projection (UMAP) showing diseased, or tendinopathy, and healthy patient samples. Eight overall cell populations and five tenocyte populations were identified. All cell populations were present in both diseased and healthy tendon tissue. Tenocytes were defined as cells expressing COL1A1 or COL1A2. (B) Split Violin plots displaying gene expression for diseased (black) versus healthy (blue) tenocytes in all five subpopulations. Every dot represents an individual cell’s gene expression level. Figure adapted from Kendal et al. [49], an open access publication.

Figure 3

Considerations for tendon tissue engineering. Tendon tissue engineering is dependent on multiple factors and combinations of these factors: biomaterials, engineering strategies, choice of stem cells or mature cells, growth factors, and drugs. Abbreviations. PGA: polyglycolic acids, PLA: polylactic acids, PCL: polycaprolactones, PLGA: poly(lactic-co-glycolic) acids, PLCL: poly (lactil-co-captolactone) acids, ESCs: embryonic stem cells, iPSCs: induced pluripotent stem cells, AECs: amniotic epithelial stem cells, AMCs: amniotic mesenchymal stem cells, AFCs: amniotic fluid stem cells, UB-MSCs: umbilical cord mesenchymal stem cells, BMSCs: bone marrow mesenchymal stem cells, ADSCs: adipose derived mesenchymal stem cells, TPSCs: tendon progenitors stem cells, TGFβ: transforming growth factor beta, BMPs: bone morphogenetic proteins, CTGF: connective tissue growth factor, FGFs: fibroblastic growth factors, IGF-1: insulin-like growth factor-1, VEGF: vascular endothelial growth factor, PDGFs: platelet-derived growth factor, NSAIDs: non-steroidal anti-inflammatory drugs. Figure adapted from Citeroni et al. [70], an open access publication. Adaptation created using BioRender.com.