Figure 2: Schematic illustration of putative sources of macrophages after spinal cord injury.

Pro-inflammatory splenic macrophages comprise the major source of the initial wave of macrophage influx. A second wave of anti-inflammatory macrophages could be from either the bone marrow or from a self-renewing source at the injury site. The spinal cord injury site is comprised of a fibrotic core comprised of non-neural cells such as fibroblasts and macrophages surrounded by neural cells such as astrocytes and microglia. Bone marrow chimera studies (Figure 1) demonstrate the presence of Cx3cr1^lo macrophages in the fibrotic region, and Cx3cr1^hi macrophages in both the fibrotic and surrounding neural tissue. These two types of macrophages could correspond to the pro-inflammatory and anti-inflammatory subtypes from the spleen and bone marrow, respectively.