Primitive glial compartments in the floor plate of mammalian embryos: distinct progenitors of adult astrocytes and microglia support the notoplate hypothesis.

Immunolocalization of lipocortin 1 (LC1) and S100-beta in the ventral midline of the neural tube of rat embryos demonstrates a distinct substructure that may relate to the roles of the floor plate in development of the ventral commissures. A midline longitudinal raphe of primitive glia (6 to 8 cells wide) expresses LC1 to form the median compartment, whereas contiguous radial glia on both sides of the raphe express S100-beta to form the paramedian bands. These compartments extend the full length of the floor plate and are remarkably uniform in width (30 to 50 microns). In addition to reviewing factors that may relate the floor plate compartments to axon guidance during genesis of the commissures, this report discusses two novel bits of evidence. First, LC1 immunoreactivity characterizes not only floor plate raphe glia, but also adult microglia (both dendritic and activated forms). Several lines of morphological evidence are presented to support the hypothesis that LC1 glia of the floor plate raphe are progenitors of adult microglia. Second, this unexpected microglial pedigree (linking the midline of the embryonic floor plate with glial phagocytes having "mesodermal" characteristics) provides evidence from adult mammals in support of the notoplate hypothesis (that is, the notochord and floor plate arise from common ancestors at the lip of the blastopore). The suggestion that the mammalian notoplate is limited to the LC1 raphe in the midline of the floor plate may be relevant to selection of fetal tissues for transplantation.