Principle of spatial-optical effect at leading pseudopodium. (a) Phase-contrast image of polarized human neutrophils showing the leading pseudopodium of the cell marked “ps” and the uropod at the rear marked “U”. The granular and organelle-free zones are marked for clarity. (b) A “sideways” or orthogonal view of the cell showing how the amount of organelle-free cytoplasm, and hence the fluor gross concentration and optical path-length (PL) available, varies along the length of the cell. (c) The differential effect of light scattering by granular zone of cytoplasm and transmission in the clear zone are illustrated in this orthogonal view. (d) The predicted difference in efficiency of fluorescence excitation along the axis of the cell. Bar (a), 5 µm