GFAP and Gliosis: Although various HRO protocols and applications have been reported, including pathology modeling, the baseline stability of HROs under control condition has not always been studied in detail (Chirco et al., 2021): Some studies have reported an absence (Völkner et al., 2022; West et al., 2022) or presence (Leong et al., 2022) of GFAP or other reactive gliosis markers in post-mitotic HROs, and induction of gliosis upon pathology and prevention by experimental manipulations (Völkner et al., 2022; West et al., 2022).