Joris van Dongen

14 Chapter 1 Twenty-one days postoperative, the three zones were more clearly characterized: peripheral zone showed viable adipocyteswith only a fewpseudocysts, the intermediate zonewas characterized by infiltration of inflammatory cells aswell as more pseudocysts, while the central zone was characterized by necrosis and mainly pseudocysts. Sixty days postoperative, the transplanted fat grafts were reduced in volume (60 percent of the viable cells died) with still a collagen capsule presented. The central necrotic zone now was twice as small as the peripheral zone showing an inflammatory process with fatty pseudocysts and formation of collagen. 28 However, viability evaluated by only morphology or presence of nuclei is not sufficient to distinguish between living and dead adipocytes because an adipocyte is too large to evaluate in a single histological section. Eto et al. used perilipin, a protein that covers lipid droplets, to discriminate between living (positive for perilipin) and dead adipocytes (negative for perilipin) after fat grafting in a mouse model. 24 He showed that the fate of injected adipocytes could be explained by a combination of the ‘host cell replacement theory’ and the ‘cell survival theory’. In line with the ‘cell survival theory’, Eto et al. showed that transplanted fat grafts contained three zones: peripheral zone, regeneration zone and necrotic zone (Fig. 1). In the peripheral zone, almost all adipocytes and adipose derived stromal cells (ASCs) survive (cell survival). In line with the ‘host cell replacement theory’ most adipocytes die within one day postoperative in the regeneration zone but are replaced by newly formed small adipocytes induced by surviving ASCs (cell replacement). In the necrotic zone, all adipocytes and ASCs die without regeneration and dead space will be filled with scar formation or absorbed (Fig. 1). 24 Moreover, mature adipocytes are larger and more fragile than ASCs and have therefore less resistance to hypoxia and trauma induced by harvesting, processing and injection of fat grafts. 29-31 Nowadays, many animal and human studies have been performed to study the fate of injected fat grafts with a variety of explanations and therefore no clear conclusion. 32 Although, many studies have shown the important role of processes like fibrosis, inflammation and neoangiogenesis. 32 Adipose derived stromal cells In 2001, Zuk et al. discovered that adipose tissue is a rich source of mesenchymal stem cells (MSCs) orASCs, which are easily enzymatically isolated and expanded in culture. 33 Adipose tissue consists of two large components: adipocytes (parenchyme) and stroma. 34,35 The stroma consists of a cellular part called stromal vascular fraction (SVF) held together by extracellular matrix (ECM). The SVF comprises all non-adipocyte cell types e.g. ASCs, endothelial cells, smooth muscle cells, immune cells and fibroblasts. 36,37 ASCs are located around vessels in SVF as precursor cell types e.g. pericytes and in