Transmission electron microscopy has been used to study the relaxation of misfit strains in GaN/Al0.28Ga0.72N layers grown on (0001) sapphire. It has been observed that threading edge-type dislocations migrate laterally in the top GaN layer to form misfit dislocations. This leads to reactions with other threading dislocations, to generate either closed loops or more complex nodes, which reduce the total threading dislocation density. The proportion of edge-type dislocations compared to screw-type and mixed dislocations is reduced with increasing GaN thickness. It is proposed that the lateral migration of the dislocations is predominantly by a climb rather than a glide mechanism.