Unilateral Sciatic Nerve Crush Induces White Blood Cell Infiltration of the Contralateral Nerve

Nerve injury leads to the accumulation of white blood cells derived from the bone marrow in the lesioned nerve, but it is still unknown whether there are similar responses in unlesioned nerves. To address this question, sciatic nerves of mice expressing enhanced green fluorescent protein (EGFP) in their bone marrow were crushed unilaterally to observe the invasion of bone marrow-derived cells into the contralateral unlesioned nerve. Two days after surgery, EGFP(+) cells began to infiltrate both the damaged and undamaged nerves. These cells gradually amplified to the highest point within 14 days and slowly lowered. In ipsilateral (lesioned) and contralateral (unlesioned) nerves, the time course of infiltration of EGFP(+) cells was similar, but the magnitude was much less for the unlesioned one. Through CD68 staining, some cells were identified as macrophages. Transmission electron microscopy revealed slight demyelination and phagocytosing macrophages in the contralateral nerve. The data showed that infiltration by white blood cells is a response to nerve injury, even in uninjured nerves.