In solidification, in situ observation of the solid-liquid interface is an invaluable tool to analyse the microstructure formation but under terrestrial conditions, convection effects dominate in bulk samples and prevent precise characterization of microstructure dynamics. The reduced-gravity environment of Space is therefore mandatory for fluid flow elimination. To study the dynamical formation of extended two-dimensional arrays of cells and dendrites in 3D samples under diffusive growth conditions, several series of microgravity experiments of directional solidification in a model transparent alloy have been conducted onboard the International Space Station using the Directional Solidification Insert (DSI) of the Device for the Study of Critical Liquids and Crystallization (DECLIC) facility. This facility was developed by the French Space Agency (CNES) in collaboration with NASA. In such large samples, the situation of a single crystal ideally orientated with a perfectly flat interface is out of reach. Analyses revealed the presence of numerous subboundaries that competitive growth processes, even in a cellular regime. The influence of subboundary configuration on the dynamics of primary spacing selection will be here evidenced.