Development of the nerve fiber layer in the olfactory bulb of mouse embryos.

The purpose of this study was to provide an ultrastructural description of the development of the olfactory nerve fiber layer (ONL) in the olfactory bulbs of mouse embryos. The developmental age of each embryo was determined by means of the Theiler staging system. During stages 17 and 18, the olfactory axons grew through the mesenchyme toward the cerebral vesicle; they were accompanied by a group of electron-dense cells which were referred to as peripheral glial progenitor cells (GPCs) because they ensheathed the adjacent axons. In Theiler stage 19-21 embryos, the axons and the peripheral GPCs grew dorsocaudally along the external surface of the bulb primordium forming a presumptive ONL immediately superficial to its glia limitans. Through small breaks in this glia limitans, the axons, but not the peripheral GPCs, penetrated into the marginal zone of the bulb primordium. Even though by the first half of stage 21 there were only short stretches of glia limitans separating the presumptive ONL from the marginal zone, there was no intermingling of the perikarya between the two layers. A definitive ONL could be identified by the second half of Theiler stage 21, by which time the glia limitans of the bulb primordium had totally disappeared. However, up to Theiler stage 24 of development the only cells to be found in this definitive ONL were the peripheral GPCs. Although in Theiler stage 25 and 26 embryos there was an additional population of less-electron-dense GPCs within the definitive ONL, these cells were likely derived from the previous group of peripheral GPCs rather than from cells newly emigrated from the deeper layers of the developing bulb. This developmental data necessitated a reevaluation of the cell lineages of the two glial cell types that reside within the ONL of the adult olfactory bulb.