by Aaradhana Natarajan ’20

Adult mesenchymal stem cells (MSCs) are effective in osteoarithritic repair of bone cartilage. Currently, the efficacy of stem cell treatments has been ascribed to the exosomes released by the MSCs. Dr. Zhang and researchers at the National University of Singapore recently investigated the ability of MSC exosomes to treat osteochondral defects.
Researchers created osteochondral defects on the trochlear grooves on the distal femurs of rats to test for exosome-induced cartilage growth. One group of rats were given injections of exosomes extracted from human embryonic stem cells, while the control group was injected with a phosphate buffered saline solution (PBS). After six and twelve weeks, the femurs were resected and their appearances was analyzed by observers using the modified O’Driscoll histologic scoring system for human cartilage repair.
At twelve weeks, the femurs treated with the exosomes demonstrated regular neotissue filling and cartilage integration while those injected with the PBS had incomplete and irregular neotissue filling. Staining indicated that 83% of exosome-treated tissue formed smooth hyaline cartilage comparable to native cartilage, while 50% of the PBS-treated femurs showed signs of degeneration. At this time, the O’Driscoll score gap had increased from that of six weeks, with the exosome-treated group score improving from 22.5 to 23.5 as the PBS-treated group score fell from 14.0 to 12.7.
The study may have been limited by the MSCs cellular capacities for self-renewal, proliferation and differentiation— issues that are exacerbated with donor age. As such, novel research should study how the exosome activity of cells varies with donor age. While further testing is needed to make exosome therapies safe and accessible for humans, they have the potential improve treatment accessibility.
References:

1. S. Zhang, Exosomes derived from human embryonic mesenchymal stem cells promote osteochondral regeneration. Osteoarithritis and Cartilage 24, 2135–2140 (2016). Doi: 10.1016/j.joca.2016.06.022.
2. Image retrieved from: https://upload.wikimedia.org/wikipedia/commons/1/12/Bone_cross-section.svg