The Paratenon Contributes to Scleraxis-Expressing Cells during Patellar Tendon Healing

Abstract

The origin of cells that contribute to tendon healing, specifically extrinsic epitenon/paratenon cells vs. internal tendon fibroblasts, is still debated. The purpose of this study is to determine the location and phenotype of cells that contribute to healing of a central patellar tendon defect injury in the mouse. Normal adult patellar tendon consists of scleraxis-expressing (Scx) tendon fibroblasts situated among aligned collagen fibrils. The tendon body is surrounded by paratenon, which consists of a thin layer of cells that do not express Scx and collagen fibers oriented circumferentially around the tendon. At 3 days following injury, the paratenon thickens as cells within the paratenon proliferate and begin producing tenascin-C and fibromodulin. These cells migrate toward the defect site and express scleraxis and smooth muscle actin alpha by day 7. The thickened paratenon tissue eventually bridges the tendon defect by day 14. Similarly, cells within the periphery of the adjacent tendon struts express these markers and become disorganized. Cells within the defect region show increased expression of fibrillar collagens (Col1a1 and Col3a1) but decreased expression of tenogenic transcription factors (scleraxis and mohawk homeobox) and collagen assembly genes (fibromodulin and decorin). By contrast, early growth response 1 and 2 are upregulated in these tissues along with tenascin-C. These results suggest that paratenon cells, which normally do not express Scx, respond to injury by turning on Scx and assembling matrix to bridge the defect. Future studies are needed to determine the signaling pathways that drive these cells and whether they are capable of producing a functional tendon matrix. Understanding this process may guide tissue engineering strategies in the future by stimulating these cells to improve tendon repair.

Fibrin Gels Exhibit Improved Biological, Structural, and Mechanical Properties Compared with Collagen Gels in Cell-Based Tendon Tissue-Engineered Constructs

“…4 and 5G). TNC is a protein associated with tissue remodeling during development and healing, 33,50,56 and our data illustrate that the interior of fibrinbased TECs showed greater expression of this marker compared with collagen under all treatment conditions (Fig. 4).…”

“…47 PCA is a statistical method that reduces data dimensionality into independent, linear combinations (principal components [PCs]) of the original data in which each component describes a proportion of variability in the dataset. [49][50][51] The PCs that combined for the majority of variability in the data were statistically tested using analysis of variance (ANOVA) methods, and results were interpreted by evaluating which genes significantly ( p < 0.01) and strongly correlated (R > 0.6) with each of these components. 51 Due to the unbalanced design created by delaying the onset of mechanical stimulation, two statistical designs were evaluated: (1) PC scores, TEC dimensions, biomechanical parameters, and FAF of the statically cultured TECs were tested via two-way ANOVA with material and time as fixed factors ( p < 0.05); (2) to evaluate the effects of mechanical stimulation, these measures were tested via three-way ANOVA with material, stimulation, and time as fixed factors for days 10 and 14 only ( p < 0.05).…”

Effect of adipose-derived mesenchymal stromal cells on tendon healing in aging and estrogen deficiency: an in vitro co-culture model

“…During the microwound healing, both ADSC types decreased Col3a1, and only sham ADSCs influenced the Col1a1 and collagen rate. It is noteworthy that Scx expression, the early marker of tendon development, which is more predominant in tendon progenitor cells than in differentiated ones [42], was reduced in both standard culture conditions and in the microwound-healing model by both ADSC types.…”

Gdf5 progenitors give rise to fibrocartilage cells that mineralize via hedgehog signaling to form the zonal enthesis

“…The ankle was cut in the sagittal plane to investigate the Achilles tendon and the shoulder was sectioned in coronal plane to investigate the enthesis of the supraspinatus tendon. All sections were made from undecalcified joints using cryofilm 2C (Ushiku, et al 2010; Dyment, et al 2013; Dyment, et al 2014), which maintains morphology of mineralized sections. The taped sections were glued to microscope slides using chitosan adhesive and rehydrated prior to imaging.…”