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.

Figure 1. Paratenon cells produce circumferential collagen fibers as they span the defect space.

The paratenon changes from a thin collagenous structure consisting of 1–2 cell layers of non-Scx-expressing cells (A–B) in normal PT to a thickened structure consisting of circumferential collagen fibers (SHG – blue; white arrowhead) and several layers of green ScxGFP cells (green arrowhead) at day 14 (G–H). As the paratenon cells migrate and bridge the defect space, the tissue is hypercellular and disorganized with a reduced SHG signal (G). PT: patellar tendon, L.R.: lateral retinaculum, M.R.: medial retinaculum; L.S.: lateral strut, M.S.: medial strut. Scale bars are 200 µm in overviews (A, C, E, G) and 100 µm in insets (B, D, F, H).

Figure 2. Scleraxis (Scx) and smooth muscle actin alpha (SMAA) coexpressing cells contribute to tendon healing.

Cells within the thickened paratenon and adjacent struts express both Scx (green) and SMAA (red) following injury. Cells within the paratenon do not express Scx in the normal PT, but smooth muscle cells within blood vessels in the paratenon express SMAA (B–C; red arrows). Regions of high Scx and SMAA expression are within the thickened paratenon and at the anterior and posterior surfaces of the tendon struts (G–L). Scx and SMAA coexpression extends into the interior of the struts with time as seen by the EDM histograms (M). The white arrowheads point to Scx-SMAA coexpressing cells. Error bars indicate ± SD. Scale bars are 200 µm in overviews (A, D, G, J) and 50 µm in insets (B, C, E, F, H, I, K, L).

Figure 3. Tendon matrix transitions from predominantly fibromodulin (FMOD) to mixture of FMOD and tenascin-C (TNC).

Cells within the thickened paratenon first express tenascin-C (blue) and fibromodulin (red) on day 3 (D–F) then express scleraxis (green) on days 7 and 14 (G, J). (M) Ratio of FMOD and TNC staining across the tendon width shows that the matrix transitions from close to 95% FMOD in normal tendon to a 50∶50 mixture of FMOD and TNC at day 14. (N) This relationship holds true within the defect region as well. Error bars indicate ± SD. Scale bars are 200 µm in overviews (A, D, G, J) and 50 µm in insets (B, C, E, F, H, I, K, L).

Figure 4. Proliferation occurs primarily in non-tenogenic cells outside of tendon.

The EdU labeled cells (yellow) diminish linearly with time while the Ki67 cycling cells (red) remain consistent from days 3 to 7 then reduce at day 14 (M). However, a small subpopulation of cycling cells exists within the activated regions of paratenon and adjacent struts (white arrows), which account for less than 12% of the total cycling cells (N). The plots in the lower panels show quantification of EdU and Ki67 stained nuclei (M) and the number of cycling cells that are Scx+ (N). Error bars indicate ± SD. Scale bars are 200 µm in overviews (A, D, G, J) and 50 µm in insets (B, C, E, F, H, I, K, L).

Figure 5. Gene expression of tenogenic transcription factors and fibril assembly proteins were reduced in the defects.

Scx, Mkx, Tnmd, Dcn, and Fmod were all decreased in defects compared to contralateral shams and normal PT. (A) Principal component 1 (PC1) scores (along bottom axis) for treatment groups and loadings for genes of interest (along top axis) show genes that were up-regulated (point to left) and down-regulated (point to the right) in the defects. (B) Scatterplots depicting improved correlations with normal PT over time (horizontally) and changes in expression between the defects and shams (vertically).