Adipose-derived mesenchymal stromal/stem cells: An update on their phenotype in vivo and in vitro

Adipose tissue is a rich, ubiquitous and easily acces-sible source for multipotent stromal/stem cells and has, therefore, several advantages compared to other sourc-es of mesenchymal stromal/stem cells. Several studies have tried to identify the origin of the stromal/stem cell population within adipose tissue in situ. This is a complicated attempt because no marker has currently been described which unambiguously identifies native adipose-derived stromal/stem cells(ASCs). Isolated and cultured ASCs are a non-uniform preparation consisting of several subsets of stem and precursor cells. Cultured ASCs are characterized by their expression of a panel of markers(and the absence of others), whereas their in vitro phenotype is dynamic. Some markers were ex-pressed de novo during culture, the expression of some markers is lost. For a long time, CD34 expression was solely used to characterize haematopoietic stem and progenitor cells, but now it has become evident that it is also a potential marker to identify an ASC subpopula-tion in situ and after a short culture time. Nevertheless, long-term cultured ASCs do not express CD34, perhaps due to the artificial environment. This review gives an update of the recently published data on the origin and phenotype of ASCs both in vivo and in vitro. In addition, the composition of ASCs(or their subpopula-tions) seems to vary between different laboratories andpreparations. This heterogeneity of ASC preparationsmay result from different reasons. One of the main problems in comparing results from different laborato-ries is the lack of a standardized isolation and culture protocol for ASCs. Since many aspects of ASCs, suchas the differential potential or the current use in clinical trials, are fully described in other recent reviews, this review further updates the more basic research issues concerning ASCs' subpopulations, heterogeneity andculture standardization.