Coexpression of CD38 and CD34 on nonmigrating and transmigrated progenitor cells. Contour plots of one representative experiment are shown. The fluorescence intensity of CD38 was greater on the transmigrated cells, while no difference was observed regarding the expression level of CD34. More primitive, CD34⁺/CD38⁻ progenitor cells are indicated by the shaded area. This particular population was virtually absent (<1%) in the transmigrated cells, compared with 1% to 5% in the nonmigrating cells. The difference was not due to selective migration of smaller cells, because the forward scatter (FSC) of the transmigrated cells tended to be even greater.