PSEN1 and Alzheimer disease: To investigate and characterize dark microglia during aging, the main risk factor for AD [96], we first determined their distribution using high magnification SEM chip mapping, within the ventral CA1 stratum lacunosum-moleculare, which exhibits considerable atrophy in APP-PS1 mice and patients with AD [4, 97], and where dark microglia were previously shown to be abundant in 14-month-old C57BL/6J and APP-PS1 mice [64].