Optimized Number and Type of Cells for Pre-Enrichment of Adipose Tissue Grafts for Maximum Fat Retention

Sir: An interesting article entitled “Cell-Enriched Fat Grafting Improves Graft Retention in a Porcine Model: A Dose-Response Study of Adipose-Derived Stem Cells versus Stromal Vascular Fraction,” by Rasmussen et al., was recently published in Plastic and Reconstructive Surgery.1 In this article, the authors described pre-enrichment of fat grafts with different concentrations (2.5 × 106 to 20 × 106) of culture-expanded adipose stem cells and also with stromal vascular fraction of adipose tissue in a 1:1 concentration. The pre-enriched fat grafts (30 ml) were injected in a minipig. The use of different adipose stem cell concentrations, direct comparison of adipose stem cells with stromal vascular fraction, assessment with magnetic resonance imaging, and use of animals (minipigs) that also served as their own controls are venerable for performing such studies and to answer some current hot questions, such as “what is the optimized number and type of cells for maximized fat retention?” Fat grafting is not a new technique. It has been used successfully for decades by plastic surgeons for aesthetic and reconstructive purposes. It was later realized by surgeons that the beneficial effects of fat grafting are actually due to regenerative cells in adipose tissue. These regenerative cells exert their positive effects by differentiating into tissue-specific cells and by their paracrine mechanism. During adipose tissue processing, these regenerative cells can be obtained at two stages: (1) as stromal vascular fraction, the pellet obtained after digestion with collagenase followed by centrifugation, and (2) as adipose stem cells, which is a pure population of mesenchymal stem cells obtained by further culturing of stromal vascular fraction. In their study, Rasmussen et al.1 noted more fat retention in cell-pre-enriched groups as compared with nonenriched grafts. Although there was no significant difference among cell-enriched groups, the authors found higher graft retention (41 percent) in the 10 × 106 group. Interestingly, doubling this number does not seem to enhance more graft retention. To correctly find the optimized number of cells, more groups should be added, instead of directly doubling this (10 × 106) number. Previously, my colleagues and I used 1 × 106 cells/ml for fat pre-enrichment and found significant fat retention in patients with contour deformities of the face.2,3 In addition, there may be more graft retention if a model with some disease or deformity is used. Without an internal signal in the form of an injury or inflammation, there is more chance of graft absorption. Most importantly, the authors did not find a difference in fat graft retention when grafts were enriched with either adipose stem cells or stromal vascular fraction. This finding is very important, because if similar results can be obtained using stromal vascular fraction instead of culture-expanded adipose stem cells, it will make the overall procedure of fat grafting less expensive (no culturing of cells is required) and reduce patient discomfort (a single liposuction procedure is required instead of two). It is, however, not clear from the study how stromal vascular fraction–enriched fat grafts can have similar retention as seen with pre-enriched adipose stem cells. The beneficial effects of using adipose stem cells or stromal vascular fraction are due to regenerative cells. However, stromal vascular fraction only contains approximately 3 percent of these regenerative cells in it. If this effect is due to paracrine factors in stromal vascular fraction and not due to the (3 percent) adipose stem cells in stromal vascular fraction, then another group excluding these adipose stem cells from stromal vascular fraction must be used to compare with control and adipose stem cell–enriched fat grafts. Adipose stem cells are attached to the plastic surface within hours of stromal vascular fraction culturing and thus can easily be excluded. Such comparison is crucial to correctly define the optimized cell type or cell fraction for pre-enriched grafts. In addition to this discrepancy, the authors harvested adipose tissue from the neck region for isolation of stromal vascular fraction, whereas it was harvested overall from the dorsal area, which raises questions for this comparison. Furthermore, first medium replacement on day 6 (it is usually after 24 hours) and subculturing at 100 percent (it is usually at 70 percent to 90 percent confluence) may also have compromised culture purity and cell viability, respectively. DISCLOSURE The author has no financial conflicts of interest to disclose.