Spatial gradients of blood vessel fragments and cell-to-vessel distance frequencies. Mean areal concentrations of CD34⁺ vessels in active bone marrow as a function of distance into the marrow cavities of (A) ribs, (B) L₁ vertebrae, and (C) iliac crest, and frequency distribution of distances separating pairs of hematopoietic cells and blood vessels within the marrow cavities of (D) ribs, (E) L₁ vertebrae, and (F) iliac crest. Dashed line represents an exponential or Lorentzian fit to the data with R² values shown. Also shown is the population mean average concentration (horizontal solid) ± 1σ (horizontal dashed).