MKI67 and Down syndrome: By co-immunostaining microglia in 50 μm fetal brain sections from 7.5 to 16 gestational weeks (gw) and combining high-resolution scanning, we identified a highly proliferative microglia aggregate (0.108−2.129 mm  2 ) that expanded in Down’s Syndrome fetal brain (4.168 mm  2 ) and was located near the ganglion eminence, in which Ki67  + microglia accounted for 23.4% of total microglia compared to 6.3% in other brain regions.