Monocytes Become Macrophages; They Do Not Become Microglia

This light and electron microscopic autoradiographic study of stab injuries in the spinal cord of mice evaluated the ultrastructural characteristics of cells labeled by incorporation of the thymidine analogue 125I-5-iodo-2'-deoxyuridine (I-UdR), injected one day prior to injury. I-UdR was used instead of tritiated thymidine (H-TdR) because H-TdR can be reutilized and is therefore not a suitable pulse label for long-term studies of cell migration. Using serial thick and thin sections for autoradiography 614 labeled cells were identified. Labeled cells included 545 monocytes/macrophages, 50 lymphocytes, 17 pericytes, one endothelial cell, and one arachnoid cell. No labeled cell had the morphology of microglia. We concluded that macrophages in stab injuries of the spinal cord of mice are derived from blood monocytes. Blood-derived lymphocytes are also involved in the reaction to spinal cord stab injury. Microglia are not blood-derived and are not seen as a transitional form in the differentiation of monocytes to macrophages.