Spatial gradients of CD34+ and CD117+ hematopoietic stem and progenitor cells in bone marrow. Mean areal concentrations of hematopoietic cells in active bone marrow as a function of distance into the marrow cavities of (A) ribs (CD34+), (B) L1 vertebrae (CD34+), (C) iliac crest (CD34+), (D) ribs (CD117+), (E) L1 vertebrae (CD117+), and (F) iliac crest (CD117+). Data points indicate specimen-averaged mean values binned every 50 μm, whereas the dashed line represents a logarithmic or linear fit to the data with R2 values shown. Also shown is the population mean average concentration (horizontal solid) ± 1σ (horizontal dashed).