title: Microglial numbers attain adult levels after undergoing a rapid decrease in cell number in the third postnatal week.
abstract: During postnatal development, microglia, CNS resident innate immune cells, are essential for synaptic pruning, neuronal apoptosis and remodeling. During this period microglia undergo morphological and phenotypic transformations; however, little is known about how microglial number and density is regulated during postnatal CNS development. We found that after an initial increase during the first 14 postnatal days, microglial numbers in mouse brain began declining in the third postnatal week and were reduced by 50% by 6weeks of age; these "adult" levels were maintained until at least 9months of age. Microglial CD11b levels increased, whereas CD45 and ER-MP58 declined between P10 and adulthood, consistent with a maturing microglial phenotype. Our data indicate that both increased microglial apoptosis and a decreased proliferative capacity contribute to the developmental reduction in microglial numbers. We found no correlation between developmental reductions in microglial numbers and brain mRNA levels of Cd200, Cx3Cl1, M-Csf or Il-34. We tested the ability of M-Csf-overexpression, a key growth factor promoting microglial proliferation and survival, to prevent microglial loss in the third postnatal week. Mice overexpressing M-Csf in astrocytes had higher numbers of microglia at all ages tested. However, the developmental decline in microglial numbers still occurred, suggesting that chronically elevated M-CSF is unable to overcome the developmental decrease in microglial numbers. Whereas the identity of the factor(s) regulating microglial number and density during development remains to be determined, it is likely that microglia respond to a "maturation" signal since the reduction in microglial numbers coincides with CNS maturation.
authors.affiliation: Department of Comparative Biosciences, University of Wisconsin-Madison, United States.
authors.forename: Maria
authors.initials: M
authors.lastname: Nikodemova
authors.affiliation: Department of Comparative Biosciences, University of Wisconsin-Madison, United States.
authors.forename: Rebecca S
authors.initials: RS
authors.lastname: Kimyon
authors.affiliation: School of Pharmacy and Molecular and Cellular Pharmacology Graduate Program, University of Wisconsin-Madison, United States.
authors.forename: Ishani
authors.initials: I
authors.lastname: De
authors.affiliation: Department of Comparative Biosciences, University of Wisconsin-Madison, United States.
authors.forename: Alissa L
authors.initials: AL
authors.lastname: Small
authors.affiliation: School of Pharmacy and Molecular and Cellular Pharmacology Graduate Program, University of Wisconsin-Madison, United States.
authors.forename: Lara S
authors.initials: LS
authors.lastname: Collier
authors.affiliation: Department of Comparative Biosciences, University of Wisconsin-Madison, United States. Electronic address: jjwatters@wisc.edu.
authors.forename: Jyoti J
authors.initials: JJ
authors.lastname: Watters
mesh_terms: D000375:Aging / Q000502:physiology*; D000818:Animals; D000831:Animals, Newborn; D017209:Apoptosis / Q000502:physiology*; D001921:Brain / Q000166:cytology* / Q000254:growth & development*; D039481:CD11b Antigen / Q000378:metabolism; D049109:Cell Proliferation; D016207:Cytokines / Q000235:genetics / Q000378:metabolism*; D005260:Female; D005434:Flow Cytometry; D008297:Male; D051379:Mice; D008813:Mice, Inbred ICR; D017628:Microglia / Q000502:physiology*
chemical_list: D039481:CD11b Antigen; D016207:Cytokines
keywords: Apoptosis; Development; Flow cytometry; Gene expression; M-CSF; Proliferation