Microglial numbers attain adult levels after undergoing a rapid decrease in cell number in the third postnatal week

“Contrary to the developing brain, microglia in the healthy adult CNS have low mitotic activity (Harry and Kraft, 2012) and are characterized by a ramified morphology, with highly motile processes that constantly survey their microenvironment (Nimmerjahn et al., 2005).”

“Contrary to neurons and glia that originate from the neuroectoderm, microglial cells are derived from mesodermal tissue originating in the yolk sac, and they populate the CNS during embryogenesis (Ginhoux et al., 2010; Schulz et al., 2012).”

“However, it is not yet clear whether the increased expression of these factors plays any role in the developmental regulation of microglial numbers, although mice lacking Cx3Cr1 (the receptor for CX3CL1), have slightly reduced microglial numbers between postnatal days 8 and 28 (Paolicelli et al., 2011).”

“In the developing brain, microglia are essential for synaptic pruning, developmental neuronal apoptosis and remodeling (Paolicelli et al., 2011; Schafer et al., 2012; Tremblay et al., 2010; Wake et al., 2009); whereas in the adult CNS, microglia are involved in neuroplasticity, maintaining…”

“to pathogens, injury or pathological processes, microglia become activated, and they can proliferate andmigrate to the site of disturbance (Davalos et al., 2005; Kettenmann et al., 2011).”

“…(Paolicelli et al., 2011; Schafer et al., 2012; Tremblay et al., 2010; Wake et al., 2009); whereas in the adult CNS, microglia are involved in neuroplasticity, maintaining homeostasis and surveillance (Davalos et al., 2005; Kettenmann et al., 2011; Nimmerjahn et al., 2005; Parkhurst et al., 2013).”

“…(Paolicelli et al., 2011; Schafer et al., 2012; Tremblay et al., 2010; Wake et al., 2009); whereas in the adult CNS, microglia are involved in neuroplasticity, maintaining homeostasis and surveillance (Davalos et al., 2005; Kettenmann et al., 2011; Nimmerjahn et al., 2005; Parkhurst et al., 2013).”

“Contrary to neurons and glia that originate from the neuroectoderm, microglial cells are derived from mesodermal tissue originating in the yolk sac, and they populate the CNS during embryogenesis (Ginhoux et al., 2010; Schulz et al., 2012).”

“In the developing brain, microglia are essential for synaptic pruning, developmental neuronal apoptosis and remodeling (Paolicelli et al., 2011; Schafer et al., 2012; Tremblay et al., 2010; Wake et al., 2009); whereas in the adult CNS, microglia are involved in neuroplasticity, maintaining homeostasis and surveillance…”

“…brain, microglia are essential for synaptic pruning, developmental neuronal apoptosis and remodeling (Paolicelli et al., 2011; Schafer et al., 2012; Tremblay et al., 2010; Wake et al., 2009); whereas in the adult CNS, microglia are involved in neuroplasticity, maintaining homeostasis and…”

“Recently, Kierdorf et al. (2013) identified CD45+ c-kitlo CX3CR1− cells in the yolk sac asmicroglial precursors that mature into CD45+ c-kit− CX3CR1+ cells that proliferate and differentiate into microglia in a Pu.1- and Irf8-dependent manner.”

“In the adult CNS, microglia are long-lived and have a very slow turn-over rate (Lawson et al., 1992; Saijo and Glass, 2011), and our data also support this observation.”

“To assess the potential regulatory mechanisms controlling developmental changes in microglial number, we evaluated brain expression levels of M-Csf and Il-34, key growth factors necessary for microglial proliferation and survival (Wang et al., 2012; Wegiel et al., 1998).”

“Emerging studies suggest that microglia also play active roles in the healthy CNS (Tremblay et al., 2011).”

“Although microglia colonize the CNS during embryogenesis before the blood–brain-barrier closes, they retain high mitotic activity during the first two postnatal weeks, resulting in an increased number of these cells in the developing brain (Alliot et al., 1999; Zusso et al., 2012).”

“In the adult brain, microglia typically express CD11b and low levels of CD45 (Nikodemova and Watters, 2012), distinguishing them frommacrophages that are characterized by high CD11b and CD45 expression (Sedgwick et al., 1991).”

“Decreased Cd200 expression in the adult CNS is in linewith previous reports (Schwarz et al., 2012).”

“Mice lacking the receptor for macrophage-colony stimulating factor (CSF1R) are severely depleted of microglia (Erblich et al., 2011), suggesting that this receptor is essential for microglial development.”

“3A) as has been reported previously (Otto, 1990).”

“, 2007; Eyo and Wu, 2013), that regulate microglial proliferation and/or survival in vivo and in vitro (Boehme et al., 2000; Chitnis et al., 2007; Deckert et al., 2006; Lee et al., 2010; Liu et al., 2013, 2014; Tang et al., 2014).”

“Both M-CSF and IL-34, ligands for the CSF1R, are highly expressed in the CNS (Berezovskaya et al., 1995; Nandi et al., 2012; Wang et al., 2012).”

“CD11b and CD45 are cell surface proteins important for immune cell function, and expression levels of thesemarkers can be used to distinguish different subsets of immune cells including microglia (CD11b/CD45) and macrophages (CD11b/CD45) (Penninger et al., 2001; Schafer et al., 2012; Wakselman et al., 2008).”

“genes in postnatalmicroglia, although they did not acquire the typicalM1 phenotype (Crain et al., 2013).”

“Indeed, many CNS disorders are characterized by a several fold in microglial cell numbers (Ladeby et al., 2005; Nikodemova et al., 2014).”

“CD11b and CD45 are cell surface proteins important for immune cell function, and expression levels of thesemarkers can be used to distinguish different subsets of immune cells including microglia (CD11b/CD45) and macrophages (CD11b/CD45) (Penninger et al., 2001; Schafer et al., 2012; Wakselman et al., 2008).”

“The total number ofmicroglia in brain parenchyma increases during the first two postnatal weeks as has been previously reported (Dalmau et al., 2003; Harry and Kraft, 2012), reaching a peak around 14 days of age.”

“We found that in the adult brain, only a small percentage of CD11b cells (around 4%) are in the G2/M phase as assayed by DNA content, and less than 2%expressKi67, a nuclear protein associated with proliferation, consistent with previous reports using autoradiography (Imamoto and Leblond, 1978).”

“, 2007; Eyo and Wu, 2013), that regulate microglial proliferation and/or survival in vivo and in vitro (Boehme et al., 2000; Chitnis et al., 2007; Deckert et al., 2006; Lee et al., 2010; Liu et al., 2013, 2014; Tang et al., 2014).”

“, 2007; Eyo and Wu, 2013), that regulate microglial proliferation and/or survival in vivo and in vitro (Boehme et al., 2000; Chitnis et al., 2007; Deckert et al., 2006; Lee et al., 2010; Liu et al., 2013, 2014; Tang et al., 2014).”

“The total number ofmicroglia in brain parenchyma increases during the first two postnatal weeks as has been previously reported (Dalmau et al., 2003; Harry and Kraft, 2012), reaching a peak around 14 days of age.”

“Although microglia colonize the CNS during embryogenesis before the blood–brain-barrier closes, they retain high mitotic activity during the first two postnatal weeks, resulting in an increased number of these cells in the developing brain (Alliot et al., 1999; Zusso et al., 2012).”

“Because the majority of synaptic pruning, neuronal circuit refinement and myelination occurs during the first three postnatal weeks of mouse brain development (Hashimoto and Kano, 2013; Verma et al., 2005), microglia play a vital role in all of these processes, and the reduction in microglial number coincides with CNS…”

“, 2007; Eyo and Wu, 2013), that regulate microglial proliferation and/or survival in vivo and in vitro (Boehme et al., 2000; Chitnis et al., 2007; Deckert et al., 2006; Lee et al., 2010; Liu et al., 2013, 2014; Tang et al., 2014).”

“, 2007; Eyo and Wu, 2013), that regulate microglial proliferation and/or survival in vivo and in vitro (Boehme et al., 2000; Chitnis et al., 2007; Deckert et al., 2006; Lee et al., 2010; Liu et al., 2013, 2014; Tang et al., 2014).”

“5 times more microglia in the adult CNS (P42) compared to littermate controls (De et al., 2014); (Fig.”

“Because the majority of synaptic pruning, neuronal circuit refinement and myelination occurs during the first three postnatal weeks of mouse brain development (Hashimoto and Kano, 2013; Verma et al., 2005), microglia play a vital role in all of these processes, and the reduction in microglial number coincides with CNS…”

“Indeed, many CNS disorders are characterized by a several fold in microglial cell numbers (Ladeby et al., 2005; Nikodemova et al., 2014).”

“M-CSF-deficient op/op mice and IL-34-deficient mice have significantly decreased numbers of microglia in the CNS (Kondo and Duncan, 2009;Wang et al., 2012).”

“RNA isolation and qRT-PCR were performed as we described in detail previously (Crain et al., 2009; Nikodemova and Watters, 2011).”

“All appropriate compensation and fluorescence minus one (FMO) controls were performed and utilized in the analysis, as previously described (Smith et al., 2014).”

“7) and glycerol (1:1), as previously described (Jensen et al., 2010; Lykidis et al., 2007), until further analysis.”

“…invasive methods have revealed microglia to be highly dynamic in their interaction with the microenvironment, responding to inflammatory signals (Nimmerjahn et al., 2005; Davalos et al., 2005) and interacting with neuronal circuits at the synaptic level (Tremblay et al., 2010;Wake et al., 2009).”

“Interestingly the decline in microglial numbers after the third postnatal week was not mitigated by upregulating brain Csf-1 levels (Nikodemova et al. 2015), indicating the existence of alternative microglial responses to unknown developmental signals that drive the apoptosis of a population of microglial cells during…”