Lately, there has been a focus on the role of exosomes and microvesicles. These are secreted by all cells and play a role in cell to cell communication. There is a theory that these play a role in the symptomatic improvement in conditions like knee osteoarthritis after injection with mesenchymal stem cells. The thought is that the stem cells release these exosomes and microvesicles which then act as signals to the existing cells and tissue to stimulate changes that result in improvement of symptoms. Below is a reference from a recent study discussing the roles of exosomes and their therapeutic potential.
Ma ZJ, Yang JJ, Lu YB, Liu ZY, Wang XX. Mesenchymal stem cell-derived exosomes: Toward cell-free therapeutic strategies in regenerative medicine. World J Stem Cells. 2020;12(8):814-840. doi:10.4252/wjsc.v12.i8.814