Differences in the microglial and resident macrophage populations in the CNS of neonatal and weanling mice. Cells from weanling (A) and neonatal (B) mice at 6 hpi with LPS were isolated on Percoll gradients and the myeloid cell fraction was collected from the 30/70% interface and analyzed by flow cytometry. An initial gate was drawn based on forward and side scatter to exclude cellular debris and doublets. This cell fraction was further gated on only CD45+ and CD11b + cells to directly analyze the myeloid cells. The CD45+ CD11b + cells were subsequently divided into subpopulations based on their relative expression of CD11b and CD45. These included a CD45lo, CD11b + microglial population (dark blue gate) and three populations of CD45hi cells: CD11blo (light blue), CD11bint (green) and CD11bhi (fuchsia). These populations were then analyzed using CD45 and F4/80 or CD45 and Ly6C to identify resident macrophage and monocyte populations. Data are representative of 4 to 5 mice per group.