1C) and reactive astrocytes (heavily stained by antibodies against GFAP;Fig. Chx10-positive V2a neurons are lost and there is no reduction in the number of V1-derived Renshaw cells. Using two complementary methods, we show that this represents an exacerbation of a normal, underlying developmental pattern: The extent of each populations decrease inPcdh-mutants is precisely commensurate both with the extent of its loss during normal embryogenesis and with the extent of its increase inBax/mice, in which apoptosis is usually genetically blocked. Interneuron apoptosis begins during the first wave of synaptogenesisis in the spinal cord, occuring first among ventral populations (primarily between E14 and E17), and only later among dorsal populations (primarily after P0). Utilizing a new, conditionalPcdh-mutant allele, we show that this -Pcdhs can promote survival non-cell autonomously: mutant neurons can survive if they are surrounded by normal neurons, while normal neurons can undergo apoptosis if they are surrounded by mutant neurons. Keywords:ventral horn, spinal cord, apoptosis, synapse formation, interneurons, programmed cell death == INTRODUCTION == In the embryonic central nervous system (CNS), there is a vast overproduction of neurons, with 50% or more undergoing apoptosis during the developmental period (Oppenheim, 1991;Nijhawan et al., 2000;Buss et al., 2006). Apoptosis has been shown to occur in proliferative zones, where it may act as a selection mechanism to remove aberrant progenitors (Blaschke et al., 1996;Blaschke et al., 1998;Kuida et al., 1996;Kuida et al., 1998), as well as in cell types that are needed only during a restricted developmental period, such as subplate neurons in the cerebral cortex and cells of the roof and floor plates in the spinal cord (Allendoerfer and Shatz, 1994;Buss and Oppenheim, 2004;Homma et al., 1994;Buss et al., 2006). The best characterized role of apoptosis in the developing nervous system, however, occurs during synaptogenesis, where competition for synaptic activity and/or trophic factors leads to the loss of extra neurons and the size matching of afferent and efferent populations (Buss et al., 2006;Lowrie and Lawson, 2000;Mennerick and Zorumski, 2000;Oppenheim, 1991). The demonstration of developmental apoptosis has been clearest in experimentally accessible, clearly identifiable MYO7A neuronal populations with defined patterns of connectivity in which the target cells can also be examined. Less clear has been the extent to which developmental apoptosis occurs in populations that are intermixed and Donepezil that project widely or diffusely within a given region of the CNS (Lowrie and Lawson, 2000). Spinal interneurons make up > 95% of the spinal cord (Hochman, 2007) and can Donepezil be grouped Donepezil into at least 13 cardinal populations (9 dorsal and 4 ventral), which are derived from unique progenitor domains and which differ in terms of neurotransmitter phenotype, somal location, axonal projection pattern, and expression of recognized transcription factor markers (Goulding et al., 2002;Goulding and Pfaff, 2005;Helms and Donepezil Johnson, 2003;Lewis, 2006) (seeFig. 2A). Dorsal interneurons are involved primarily in the processing and relaying of sensory information from your trunk and limbs, while the main function of ventral interneurons is usually to coordinate motor output via modulation of motor neurons. Spinal interneuron populations receive inputs from diverse sources, including dorsal root ganglia (DRG) sensory afferents, descending axons from the brain, interneurons within the same or different spinal cord segments, and, in the case of the ventral Renshaw cells, motor neurons. These diffuse patterns of connectivity make it more difficult to conceptualize the role of developmental cell death, and studies examining whether spinal interneurons, like sensory and Donepezil motor neurons, undergo a period of naturally occurring apoptosis have produced conflicting results. == Physique 2. Loss of molecularly-defined spinal interneuron populations inPcdh-null mutant mice. == A) Schematic of spinal cord domains and the molecular markers used in this study to identify discrete interneuron populations. (BH) Hemicords from control andPcdh-del/delP0 mice immunostained using antibodies against the indicated markers and counterstained with DAPI. At this age, dorsal interneuron populations are not reduced in thePcdh-del/delspinal cord (B, G). In contrast,.