Figure 1.

Possible influx of CCR2⁺ monocytes in perinatal brains and neonatal injury. (A) The classical view of “fountains of microglia” near the choroid plexus (CPx) and as amoeboid microglial cells (AMC) beneath the corpus callosum in the 25th gestational week (GW) of human fetus and their correlates in E17 mouse embryos (left is modified from Kershman, 1939 ³³). (B-E) Brains of E17 CCR2^RFP/+; CX3CR1^GFP/+ (R/G) embryos contained clusters of GFP⁺, RFP⁺, and RFP⁺/GFP⁺ double-positive cells in the CPx between the two primordial hippocampi (Hip). Shown are typical images in three embryos; arrows were added to compare the GFP- and/or RFP- expression. (F-I) Time-lapsed live imaging of E17 R/G mouse brain slice showed disappearance of RFP fluorescence in RFP⁺/GFP⁺ cells within 2 hours in the CPx (arrow). (J-M) The subcortical AMCs in P2 R/G mice were CX3CR1^GFP+, but CCR2^RFP-, and often co-labeled with anti-EdU (5-ethynyl-2'-deoxyuridine, arrows in M) that was injected one hour earlier. (N-U) The P10 R/G mouse brains contained numerous CX3CR1^GFP+ microglia, but only scant CCR2^RFP+ cells, when they were uninjured (N-O) or received an intraperitoneal injection of 0.3 mg/kg LPS 23 h earlier (P). (Q-U) Both pure-HI and LPS/HI injuries induced CCR2^RFP+ cell infiltration in the ipsilateral cerebral cortex 24 h later (R-T), but not in the contralateral hemisphere (Q-S). Many amoeboid RFP⁺/GFP⁺ cells appeared in the ipsilateral cerebral cortex at 72 h after LPS/HI injury (arrows in U). CC: corpus callosum; Ctx: cortex; EC: external capsule; GE: ganglionic eminence; LV: lateral ventricle; St: striatum. N = 3 for each time point. Scale bar: 50 μm.