Defining adipose tissue-derived stem cells in tissue and in culture

Adipose tissue-derived stem cells (ADSC) are routinely isolated from the stromal vascular fraction (SVF) of homogenized adipose tissue. Similar to other types of mesenchymal stem cells (MSC), ADSC remain difficult to define due to the lack of definitive cellular markers. Still, many types of MSC, including ADSC, have been shown to reside in a perivascular location, and increasing evidence shows that both MSC and ADSC may in fact be vascular stem cells (VSC). Locally, these cells differentiate into smooth muscle and endothelial cells that are assembled into newly formed blood vessels during angiogenesis and neovasculogenesis. Additionally, MSC or ADSC can also differentiate into tissue cells such as adipocytes in the adipose tissue. Systematically, MSC or ADSC are recruited to injury sites where they participate in the repair/regeneration of the injured tissue. Due to the vasculature's dynamic capacity for growth and multipotential nature for diversification, VSC in tissue are individually at various stages and on different paths of differentiation. Therefore, when isolated and put in culture, these cells are expected to be heterogeneous in marker expression, renewal capacity, and differentiation potential. Although this heterogeneity of VSC does impose difficulties and cause confusions in basic science studies, its impact on the development of VSC as a therapeutic cell source has not been as apparent, as many preclinical and clinical trials have reported favorable outcomes. With this understanding, ADSC are generally defined as CD34+CD31- although loss of CD34 expression in culture is well documented. In adipose tissue, CD34 is localized to the intima and adventitia of blood vessels but not the media where cells expressing alpha-smooth muscle actin (SMA) exist. By excluding the intima, which contains the CD34+CD31+ endothelial cells, and the media, which contains the CD34-CD31- smooth muscle cells, it leaves the adventitia as the only possible location for the CD34+ ADSC. In the capillary, CD34 and CD140b (a pericyte marker) are mutually exclusively expressed, thus suggesting that pericytes are not the CD34+ ADSC. Many other cellular markers for vascular cells, stem cells, and stem cell niche have also been investigated as possible ADSC markers. Particularly the best-known MSC marker STRO-1 has been found either expressed or not expressed in cultured ADSC. In the adipose tissue, STRO-1 appears to be expressed exclusively in the endothelium of certain but not all blood vessels, and thus not associated with the CD34+ ADSC. In conclusion, we believe that ADSC exist as CD34+CD31-CD104b-SMA- cells in the capillary and in the adventitia of larger vessels. In the capillary these cells coexist with pericytes and endothelial cells, both of which are possibly progenies of ADSC (or more precisely VSC). In the larger vessels, these ADSC or VSC exist as specialized fibroblasts (having stem cell properties) in the adventitia.