A microelectronic device comprises a stack structure comprising alternating conductive structures and insulative structures arranged in tiers, the tiers individually comprising one of the conductive structures and one of the insulative structures, first support pillar structures extending through the stack structure within a first region of the microelectronic device, the first support pillar structures electrically isolated from a source structure underlying the stack structure, second support pillar structures extending through the stack structure within a second region of the microelectronic device, the second support pillar structures comprising an electrically conductive material in electrical communication with the source structure, and bridge structures extending between at least some neighboring first support pillar structures of the first support pillar structures. Related memory devices, electronic systems, and methods are also described.