Gradient perception by neutrophil leucocytes

The mechanisms by which neutrophil leucocytes show oriented movement towards attractant sources are still unclear. It has been suggested that they use either a temporal or a spatial sensing system to gradients. Temporal gradient sensing requires that a moving cell perceives changes in concentration with time and requires some sort of memory; spatial sensing requires a cell to compare receptor occupancy over its length and coordinate this into a directed response. The terms were first applied in describing the behaviour of bacteria, whose mode of swimming alters with changing concentrations of attractant, resulting in random changes in direction. Bacteria adapt to changes in the concentration of attractant: the response, a change in the mode of swimming, is transient and returns to a baseline in the continued presence of the attractant (Hazelbauer, 1980). Thus, adaptive responses in bacteria allow great sensitivity to changes in concentration rather than to absolute concentration. Although neutrophils are very different from bacteria, it has nevertheless been argued that they use similar mechanisms to read gradients of attractant. Zigmond (1974) suggested that neutrophils did not use a temporal mechanism because stationary cells oriented towards a gradient source before they moved. Gerisch & Keller (1981) showed that attractant delivered by micropipette stimulated local extension of-a pseudopod and suggested that both spatial and temporal sensing might be occurring, because the latter requires only part of a cell to move in order to sample changes of concentration with time. In such experiments cells are not exposed to stable spatial gradients but to developing gradients in which a stationary cell experiences rapid changes in concentration with time (Vicker et al. 1986). If neutrophils do use a temporal sensing mechanism to read gradients, they can maintain sensitivity to changes in concentration only by adapting to the absolute concentration, as bacteria do. Neutrophils, unlike bacteria, move only when stimulated by an attractant (Zigmond & Sullivan, 1979; Shields &