Published online before print June 28, 2007

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Copyright © 2007 American Society for Investigative Pathology
American Journal of Pathology, doi:10.2353/ajpath.2007.061044

Accepted for publication April 20, 2007.

Article

Population Control of Resident and Immigrant Microglia by Mitosis and Apoptosis

Martin Wirenfeldt^*@, Lasse Dissing-Olesen^*, Alicia Anne Babcock^*, Marianne Nielsen, Michael Meldgaard^*, Jens Zimmer, Iñigo Azcoitia, Robert Graham Quinton Leslie, Frederik Dagnaes-Hansen^¶, and Bente Finsen^*

From the Medical Biotechnology Center* and the Departments of Anatomy and Neurobiology and Immunology and Microbiology, Institute of Medical Biology, University of Southern Denmark, Odense, Denmark; the Department of Medical Microbiology and Immunology,^¶ University of Aarhus, Aarhus, Denmark; and the Departamento de Biología Celular, Facultad de Biología, Universidad Complutense, de Madrid, Madrid, Spain

^@ To whom correspondence should be addressed. E-mail: mwirenfeldt{at}gmail.com.

	Abstract

Microglial population expansion occurs in response to neural damage via processes that involve mitosis and immigration of bone marrow-derived cells. However, little is known of the mechanisms that regulate clearance of reactive microglia, when microgliosis diminishes days to weeks later. We have investigated the mechanisms of microglial population control in a well-defined model of reactive microgliosis in the mouse dentate gyrus after perforant pathway axonal lesion. Unbiased stereological methods and flow cytometry demonstrate significant lesion-induced increases in microglial numbers. Reactive microglia often occurred in clusters, some having recently incorporated bromodeoxyuridine, showing that proliferation had occurred. Annexin V labeling and staining for activated caspase-3 and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling showed that apoptotic mechanisms participate in dissolution of the microglial response. Using bone marrow chimeric mice, we found that the lesion-induced proliferative capacity of resident microglia superseded that of immigrant microglia, whereas lesion-induced kinetics of apoptosis were comparable. Microglial numbers and responses were severely reduced in bone marrow chimeric mice. These results broaden our understanding of the microglial response to neural damage by demonstrating that simultaneously occurring mitosis and apoptosis regulate expansion and reduction of both resident and immigrant microglial cell populations.

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