Movie 1.

Interaction between macrophages and human erythroblasts in vitro. Time lapse videomicroscopy of cells obtained after 14 days of culture under Human Erythroid Massive Amplification (HEMA) conditions from peripheral blood of one healthy donor. Samples were observed by an Olympus phase-contrast microscope equipped with incubator chamber and high speed, high-resolution camera (Hamamatsu), objective lens ×20. Two hours of video recording are shown. Frames were collected every 30 seconds and movie was accelerated at 10 frames per second. The movie presents erythroblasts at various stages of maturation surrounding a macrophage (top right) recognizable for being larger than the other cells, for its motility, and for its ability to protrude its cytoplasm. During the movie, the macrophage appears to explore the space in its surroundings protruding filaments of different sizes. These filaments establish contacts with numerous erythroblasts. Some of the contacts are temporary while others persist in time. The variegation of these interactions suggests that the macrophage is engaging in multiple types of biological conversations possibly in response to specific signals emitted by each erythroblast. Kindly provided by Mario Falchi PhD, National Aids Center, Istituto Superiore di Sanità, Rome, Italy.