Microelectronic devices include a stack structure comprising a vertically alternating sequence of insulative structures and conductive structures arranged in tiers. A series of pillars extends through the stack structure. At least one isolation structure extends through an upper stack portion of the stack structure. The at least one isolation structure protrudes into pillars of neighboring columns of pillars of the series of pillars. Conductive contacts are in electrical communication with the pillars into which the at least one isolation structure protrudes. Related methods and electronic systems are also disclosed.