Leukocyte migration into the brain. (A) Schematic to illustrate the expansion of the secondary lesion. The contusion (black) is necrotic immediately after controlled cortical impact (CCI), and neuronal cell death occurs in the penumbra (gray) throughout the first 24 h. (B) Quantification of CD45-positive cells (that is, leukocytes) per hemisphere. The black columns indicate the numbers of leukocytes in the contralateral hemisphere 24 h after CCI. Compared to the contralateral hemisphere, the number of leukocytes in the ipsilateral hemisphere (traumatic brain injury (TBI), gray columns) increased significantly over time, reaching the maximum value at 24 h post-CCI. (C) Time course of secondary lesion expansion (measured as necrotic tissue) and leukocyte accumulation in the tissue. The secondary growth of the cortical lesion (open circles) shows a shift to the left as compared to the time course of leukocyte invasion (closed circles). Accordingly, these data suggest that tissue dies first and leukocytes accumulate in the contusion thereafter.