Early development of olfactory receptor cell axons.

An electron microscope study was done on development of olfactory receptor cell axons in rat fetuses 13-17 days after conception (E13-E17). Initiation of axon outgrowth was first seen on E13. On E14, small bundles of olfactory axons, accompanied by epithelial cells, grow out of the epithelium and, by so doing, breach the basal lamina. Close examination of these epithelial cells from E14 fetuses has revealed that they can be grouped into two types, one containing a ribosome-rich, dense cytoplasm, the other containing fewer free ribosomes and a more lucent cytoplasm. The first of these types is present infrequently or not at all at later times in development. The fate of the migrating cells is not known. Another notable observation was the indentation of olfactory epithelium by blood vessels during the developmental stages studied. There is very close association between processes of vascular endothelium and cells of the olfactory epithelium, so close, in fact, that a basal lamina is frequently absent. It is possible that vascular endothelium plays a role in breakdown of basal lamina, thus enabling olfactory axons to breach this barrier as they leave the epithelium.