Fetal tendon wound size modulates wound gene expression and subsequent wound phenotype

The fetal response to small tendon injury results in regenerative or scarless healing and is characterized by a markedly diminished cellular inflammatory response, lack of fibroplasia, and restoration of normal tissue architecture. We hypothesized that an increasing fetal tendon wound size would lead to increased wound inflammation and a change from regenerative to reparative healing and scar formation. We created small or large tendon wounds in early gestation fetal sheep and used histology to assess tissue architecture, immunohistochemistry to assess the cellular inflammatory response, ovine-specific gene microarrays, and real-time reverse transcription-polymerase chain reaction to measure the gene expression in response to injury. Small tendon wounds showed a regenerative healing phenotype with orderly deposition of collagen fibers while large tendon wounds showed disorderly collagen deposition consistent with scar formation. Small tendon wounds had few inflammatory cells at 7 and 28 days after injury, whereas the large wounds showed a significant inflammatory cell infiltrate at 7 days that resolved by 28 days. At 3 days, the differential expression of genes involved in the response to injury and inflammation were seen between large and small tendon wounds. By real-time polymerase chain reaction at 7 days, large tendon wounds also had significantly increased expression of interleukin-6, interleukin-8, transforming growth factor-β1, and transforming growth factor-β3, compared with the small wounds. Increasing the fetal tendon wound size results in increased proinflammatory gene expression, inflammatory cell infiltration, and a change from regenerative to reparative healing. This model allows the process of regenerative healing to be examined without the confounding variable of gestational age.