Mesenchymal Stem Cell Exosomes Promote Growth Plate Repair and Reduce Limb-Length Discrepancy in Young Rats

Background: The objective of this study was to examine the therapeutic effects of human mesenchymal stromal/stem cell (MSC) exosomes in a rat model of growth plate injury. Methods: A growth plate defect was surgically created on the distal part of the right femur of 40 female Sprague-Dawley rats. A single intra-articular injection of 100 µg of MSC exosomes in 100 µL of phosphate-buffered saline solution (PBS), or an equivalent volume of PBS alone, was administered to the right knee immediately after surgery. At 4 and 8 weeks post-treatment, limb length was measured with micro-CT, and tissue repair was assessed with histological, immunohistochemical, and histomorphometric analyses. Results: A single injection of MSC exosomes significantly increased limb length from 3.29 ± 0.07 cm at 4 weeks to 3.37 ± 0.11 cm at 8 weeks (p = 0.047). However, no improvement in limb length was observed in the PBS control group. The limb-length discrepancy between the involved limb and the contralateral limb in the exosome-treated group was significantly less than the discrepancy in the PBS-treated group at both 4 weeks (2.52% ± 1.30% versus 4.11% ± 0.93%; p = 0.006) and 8 weeks (5.27% ± 2.11% versus 8.06% ± 2.56%; p = 0.016). Consistent with the reduced limb-length discrepancy, the exosome-treated defects displayed significantly more chondrocytes (p < 0.05) and a higher area percentage with deposition of sulphated glycosaminoglycan (p < 0.05) and collagen II (p < 0.05) than PBS-treated defects at 8 weeks. However, bone bridge formation was not inhibited in either group. Conclusions: A single intra-articular injection of MSC exosomes significantly enhanced physeal repair and reduced limb-length discrepancy but did not inhibit bone-bridge formation. Clinical Relevance: This proof-of-concept study demonstrates for the first time the potential use of MSC exosomes as a minimally invasive cell-free therapeutic to promote physeal repair and reduce limb-length discrepancy following growth plate injuries.