Learning to roll before you stop and drop.

In this issue of Blood, Sperandio and colleagues report the use of a unique intravital microscopic system to characterize an ontogenic process of blood cell and yolk sac endothelial maturation that is required to display full adult-type inflammation-induced leukocyte recruitment.(1) They report that murine fetal blood neutrophil rolling, adhesion, and extravasation from inflamed yolk sac vessels is apparent late in development, but that before embryonic day (E) 15, fetal blood neutrophils display little ability to roll or adhere to inflamed vascular endothelial cells. Similar behavior was displayed when fetal blood cells were tested in vitro on immobilized recombinant adhesion molecules.