a Left, Fluorescence micrographs of immunostained cortical sections (ALDH1L1, desaturated) from P14 control and Hmgb1ΔAstro mice. Inset represent high magnifications of astrocytes segmented for main branches. Right, Quantification of morphological metrics from all astrocytes segmented for the control group (n = 149 cells from 4 animals) and for the mutant group (n = 143 cells from 4 animals). See Extended Data Fig. 7. for all cells. Data are mean with individual values (extension length, cell area) or violin plots (center dashed line indicating median). ***p < 0.01 (two-tailed unpaired t test). b Summary of procedure used to isolate and culture primary cerebral cortex astrocytes. c Fluorescence micrographs of immunostained primary astrocytes (GFAP, red; HMGB1, green; and DNA stain DAPI, blue) cultured after isolation from P14 control and Hmgb1ΔAstro mice. Absence of HMGB1 can be confirmed in Hmgb1ΔAstro astrocytes. d Left, Representative fluorescence micrographs of immunostained primary astrocytes (GFAP, red; and DNA stain DAPI, blue) cultured after isolation from P14 control and Hmgb1ΔAstro mice. White arrowheads point at thin cellular extensions. Right, Graph shows quantification of GFAP+ cell area in culture. Data are whisker boxes (min to max, center line indicating median). **p < 0.01 (two-tailed Mann–Whitney’s test). e Left, Fluorescence micrographs of immunostained primary astrocytes (GFAP, desaturated) cultured after isolation from P14 control and Hmgb1ΔAstro mice. Yellow arrowheads point at thin cellular extensions. Right, Graph shows quantification of the proportion of astrocytes (% cells) displaying thin cellular extensions. Data are whisker boxes (min to max, center line indicating median). **p < 0.01 (two-tailed Mann–Whitney’s test). All microscopy images displayed in this figure are representative of experiments repeated in at least 5 mice per group, with similar results. Source data are provided as a Source Data file.