Adipose tissue represents an abundant and accessible source of multipotent adult stem cells and is used by many investigators for tissue engineering applications; however, not all laboratories use cells at equivalent stages of isolation and passage. We have compared the immunophenotype of freshly isolated human adipose tissue-derived stromal vascular fraction (SVF) cells relative to serial-passaged adipose-derived stem cells (ASCs). The initial SVF cells contained colony-forming unit fibroblasts at a frequency of 1:32. Colony-forming unit adipocytes and osteoblasts were present in the SVF cells at comparable frequencies (1:28 and 1:16, respectively). The immunophenotype of the adipose-derived cells based on flow cytometry changed progressively with adherence and passage. Stromal cell-associated markers (CD13, CD29, CD44, CD63, CD73, CD90, CD166) were initially low on SVF cells and increased significantly with successive passages. The stem cell-associated marker CD34 was at peak levels in the SVF cells and/or early-passage ASCs and remained present, although at reduced levels, throughout the culture period. Aldehyde dehydrogenase and the multidrug-resistance transport protein (ABCG2), both of which have been used to identify and characterize hematopoietic stem cells, are expressed by SVF cells and ASCs at detectable levels. Endothelial cell-associated markers (CD31, CD144 or VE-cadherin, vascular endothelial growth factor receptor 2, von Willebrand factor) were expressed on SVF cells and did not change significantly with serial passage. Thus, the adherence to plastic and subsequent expansion of human adipose-derived cells in fetal bovine serum-supplemented medium selects for a relatively homogeneous cell population, enriching for cells expressing a stromal immunophenotype, compared with the heterogeneity of the crude SVF. STEM CELLS 2006;24:376 -385

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The relationship between bone and fat formation within the bone marrow microenvironment is complex and remains an area of active investigation. Classical in vitro and in vivo studies strongly support an inverse relationship between the commitment of bone marrow-derived mesenchymal stem cells or stromal cells to the adipocyte and osteoblast lineage pathways. In this review, we focus on the recent literature exploring the mechanisms underlying these differentiation events and discuss their implications relevant to osteoporosis and regenerative medicine.