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Significance Many studies have investigated how cells can move toward the source of a chemoattractant, but relatively little is known about mechanisms that can cause cells to move away from a location. Here, we show that small aggregates of the social amoebaDictyostelium discoideumcan show dispersal behavior during which cells are moving away from the aggregate. Using a combination of experiments and modeling, we show that this dispersal can arise due to a competition between the diffusible chemoattractant and the enzyme that degrades it, and that the localized degradation of the chemoattractant may be a mechanism for morphogenesis. 

The thermal contraction of aqueous cryoprotectant solutions on cooling to cryogenic temperatures is of practical importance in protein cryocrystallography and in biological cryopreservation. In the former case, differential contraction on cooling of protein molecules and their lattice relative to that of the internal and surrounding solvent may lead to crystal damage and the degradation of crystal diffraction properties. Here, the amorphous phase densities of aqueous solutions of glycerol and ethylene glycol at T = 77 K have been determined. Densities with accuracies of <0.5% to concentrations as low as 30%( w / v ) were determined by rapidly cooling drops with volumes as small as 70 pl, assessing their optical clarity and measuring their buoyancy in liquid nitrogen–argon solutions. The use of these densities in contraction matching of internal solvent to the available solvent spaces is complicated by several factors, most notably the exclusion of cryoprotectants from protein hydration shells and the expected deviation of the contraction behavior of hydration water from bulk water. The present methods and results will assist in developing rational approaches to cryoprotection and an understanding of solvent behavior in protein crystals.