Joris van Dongen

165 The power of fat and ASCs for scar treatment: a review THERAPEUTIC MODE OF ACTION OF ADSC ADSC: stem or stromal cells? Because of their ability to differentiate into different cell types, ADSC are sometimes referred to as adipose stem cells. However, a true stem cell has the potential to differentiate into other cell types, while maintaining a stable population of stem cells by the process of self-renewal 55 with indefinite proliferation capability due to telomerase activity 56 . Embryonic stem cells are an example of such pluripotent stem cells: they can undergo an infinite number of cell divisions and can differentiate into all cell types of the three germ layers during embryonic development 57 . ADSC, on the other hand, are a type of adult stem cell that have no telomerase activity and therefore have a limited capacity of proliferation 58 . ADSC can only differentiate into a limited number of cell types, which makes them multipotent progenitor cells. Hence, in the case of ADSC, the authors prefer to speak of adipose-derived stromal cells instead of adipose-derived stem cells. Isolation ADSC can be isolated either from intact adipose tissue or from lipoaspirates. The adipose tissue or lipoaspirate is subjected to enzymatic digestion using proteases such as collagenase, dispase or trypsin 10,59-61 . After digestion, the Stromal Vascular Fraction (SVF) that contains ADSC as well as several other cell types, is separated from the mature adipocytes by differential or density gradient centrifugation 10,59-61 . For cell culture, the SVF is then seeded into cell culture dishes. Only ADSC adhere to the tissue culture plastic, whereas other, non-adherent cell types such as erythrocytes, endothelial cells and immune cells, are removed by washing 60 . Then, the remaining ADSC are culture-expanded or cryopreserved until further use. ADSC in vivo versus in vitro Adipose tissue contains two major components: SVF and adipocytes. SVF is a heterogeneous mix of cells of eleven main subpopulations based on CD-surface marker expression: seven adipose derived populations (CD45-) and four blood derived populations (CD45 pos ) 61 . Three important subpopulations of CD45 min cells are pericytes ( in vivo : (CD34 pos )/CD34 min /CD146 pos /CD31 min ), supra adventitial cells ( in vivo : CD34 pos /CD146 min /CD31 min ) and ADSC ( in vivo : CD34 pos /CD90 pos /CD31 min /CD105 low ) in a very low number 61-64 . Pericytes and supra-adventitial cells are both identified as precursor cells of ADSC, but there remains controversy 22,23,62,63 .