Mold filling plays a crucial role in determining the cells' structural morphology and uniformity in high-pressure foam injection molding. This process affects the formation, deformation, and dissolution of premature cells, which nucleate in the melt/gas mixture during the mold filling. We investigated the mold-filling phenomena and the cell structure development in the high-pressure foam injection molding process with mold opening. Polystyrene was used as the matrix, and carbon dioxide and nitrogen were used as physical blowing agents. We adopted an in-situ visualization technique, which enabled us to monitor and study the formation and evolution of the cells during the mold-filling and melt-packing phases. It was found that both the shape and the size of cells, which had nucleated during the mold filling process inside the melt, were changed by the packing condition selected. Statistical analyses have shown that the packing pressure and its duration were the most influential parameters affecting the size and the shape of cells that had nucleated in the melt before the mold opening. A greater packing pressure and a longer packing time resulted in a smaller cell size with a larger deformation in the melt during mold filling. A proper packing condition eventually removed all of the cells and re-dissolved them back into the melt before mold opening. The effect of the mold-filling parameters on the final cellular structure was also investigated, using carbon dioxide and nitrogen as physical blowing agents. A complete dissolution of the premature cells during the mold-filling and packing phases resulted in a more uniform and finer cellular structure after mold opening. The obtained results and the developed knowledge provide a deeper insight into the fundamental aspects of cellular structure development in foam injection molding process, which can be used to produce highly expanded, ultra-light foams with uniform cell structure.