Platelet counts of the platelet-rich plasma (PRP) reported in this study indicate no true platelet enrichment compared to normal values for whole blood, therefore indicating these preparations are in fact not “platelet rich” plasma. In addition, the methods used to measure proliferation are inadequate as applied to the study. Thymidine incorporation is sufficient to measure the kinetics of cell cycle stimulation, but it is inappropriate as an endpoint assay. It only labels cells that are in S-phase of the cell cycle during thymidine incubation and does not label cells that have progressed to G2- or M-phase in the time prior to label addition.
The total number of cells at the endpoint (assuming a known seeding density and no contact inhibition during the time course) or the kinetic effects of treatment during the initial approximate 18 hours of serum stimulation rather than the final 24 hours would be better assays.
A more likely explanation for loss in treatment efficacy with subsequent applications over longer time periods is indeed contact inhibition. Once normal somatic cells are contacting other cells, they will fail to respond to growth factor cues for proliferation. For example, if the initial serum treatment causes the cells to grow to confluence within the 4-day incubation period, the next treatment will have “no measured effect” by thymidine incorporation simply because contact inhibited cells cannot be stimulated into a new cell cycle to make DNA. The observation that tenocytes still respond to their preparations (not contact inhibited), but not to commercially available fetal bovine serum (assuming contact inhibition) after 4 days to 8 days in culture is strongly suggestive that their preparations are actually deficient in platelet growth factor content and the ability to stimulate tenocyte proliferation. However since a whole blood serum control is missing in the study, this remains inconclusive.
William Parrish, PhD
Principal Scientist, Concept Development
DePuy Synthes Mitek Sports Medicine, a Johnson and Johnson Company
Disclosures: Parrish is an employee of DePuy Synthes Mitek Sports Medicine, a Johnson and Johnson Company.