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Abstract. Circulating CD34' progenitors were separated from normal human peripheral blood on the basis of size and density by counterflow centrifugal elutriation (CCE). The CD34' cells, 0.15% of peripheral blood mononuclear cells, were heterogeneous with respect to their elutriation characteristics, mainly size and density. The least mature CD34' cells, characterized by lack of CD38 antigen, were predominantly found in the small lymphoid cell fraction. In fractions containing larger and denser cells (large lymphocytes, monocytes, and granulocytes), CD38 was increasingly expressed on the CD34+ cells, as were lineage commitment markers CDlO (B lymphoid), CD33 (myeloid), CD13 (myelomonocytic) and CD71 (erythroid) antigens. The smaller and less dense CD34+ cells expressed CD34 antigen brightly while the larger and denser CD34' cells expressed it dimly. The smaller and less dense CD34+b'gb cells failed to establish colony growth in short-term culture while the larger and denser CD34''"" cells gave rise to high counts of colony forming units-granulocyte macrophage (CFU-GM). Physical separation on the basis of size and density by CCE differentiates between two main classes of steadystate CD34' cells from normal human peripheral blood. The smaller and less dense CD34+h1gb cells correspond to the earliest progenitors that express differentiation markers poorly but CD34 antigen 'Present address Sir

The purpose of our study was to determine whether the number of human very small embryonic-like stem cells (huVSELs) would vary depending on the age of humans. HuVSELs frequency was evaluated into the steady-state (SS) peripheral blood (PB) of healthy volunteers using flow cytometry analysis. Their numbers were compared with volunteers' age. Blood samples were withdrawn from 28 volunteers (age ranging from 20 to 70 years), who were distributed among three groups of age: “young” (mean age, 27.8 years), “middle” (mean age, 49 years), and “older” (mean age, 64.2 years). Comparing the three groups, we did not observe any statistically significant difference in huVSELs numbers between them. The difference in mRNA expression for PSC markers as SSEA-4, Oct-4, Nanog, and Sox2 between the three groups of age was not statistically significant. A similar frequency of huVSELs into the SS-PB of young, middle-aged, and aged subjects may indicate that the VSELs pool persists all along the life as a reserve for tissue repair in case of minor injury and that there is a continuous efflux of these cells from the BM into the PB.