To study the dynamical formation of 3D arrays of cells and dendrites under diffusive growth conditions, in situ monitoring of directional solidification experiments on a transparent model alloy was carried out under low gravity in the Directional Solidification Insert of the Device for the Study of Critical Liquids and Crystallization (DECLIC-DSI) on-board the International Space Station. Microgravity environment provides the conditions to get homogeneous patterns over the whole interface corresponding to homogeneous values of control parameters. Sequences of the microstructure formation and primary spacing evolution are presented. These analyses reveal two main parameters that strongly affect the mechanisms of spacing selection. First, the critical influence of the macroscopic interfacial curvature that induces pattern gliding is highlighted. Secondly, small crystal misorientations found between the principal crystal axes and the thermal gradient axis also induce drift of specific characteristics which are presented and analyzed in terms of spacing selection and sub-boundary dynamics.