Glial progenitor cells of the nerve fiber layer of the olfactory bulb: effect of astrocyte growth media.

There are two morphologically distinct types of glial cells (i.e., ensheathing cells and astrocytes) in the nerve fiber layer (NFL) of the adult mammalian olfactory bulb. Ensheathing cells provide ensheathment for olfactory axons, whereas astrocytes occupy the interfascicular spaces of the olfactory NFL. During embryonic development, however, only one type of glial cell is found in this layer of the olfactory bulb, namely, the ensheathing cell. Even though ensheathing cells take up residence within the CNS, they are actually derived from the olfactory placode. Far less is known about the developmental origin of interfascicular astrocytes, which arise either from the glial progenitor cells that give rise to ensheathing cells or from astrocyte precursor cells that migrate into the NFL from deeper layers of the bulb primordium. In the present study, enriched populations of ensheathing cells were grown in vitro in media known to promote the growth and differentiation of astrocytes to determine whether ensheathing cell progenitors could differentiate into astrocytes. These media failed to induce the appearance of astrocytes in the ensheathing cell cultures. It was concluded that the astrocytes of the NFL most likely arise from progenitor cells that migrate into this layer from deeper parts of the developing bulb.