Initial development of a small subclass of rat olfactory receptor neurons characterized by antigenicity to HSP 70

We have described a subclass of rat olfactory receptor neurons (ORNs) that constitutively shows immunoreactivity with a monoclonal antibody (2A4) directed to the 70-kDa heat shock protein. These ORNs are scattered nonuniformly in olfactory epithelium (OE) Zones II-IV and project to just 2-3 glomeruli at consistent locations in the ventrolateral and ventromedial olfactory bulb (OB) via consistent pathways. To examine early neurogenesis of this subpopulation, paraffin sections from embryonic day 14 (E14) to postnatal day 63 (P63) rats were examined using immunoperoxidase techniques. Results show: (i) a few faintly reactive 2A4(+) ORNs first appear between P7 and P10. Their numbers (and immunoreactivity (IR) intensity) increase to adult levels by P21, reach a peak density approximately twice that of adults by P49, and then decline to adult values by P56. (ii) tritiated thymidine [3H]TdR autoradiographic birthdating studies show that the vast majority of 2A4(+) ORNs present at P21, when adult 2A4(+) ORN densities are first observed, were 'born' postnatally, between P3 and P10. (iii) The initial 2A4(+) ORN OE zonal distribution is the same as in adults. (iv) Through P21 2A4(+) ORN cell bodies are situated quite apically within the OE, but then assume more basal locations as well. (v) In the OB, glomeruli showing 2A4(+) axons appear in some animals as early as P14 and in all animals by P21. Initial location of the (+) glomeruli is similar to that of adults, despite extensive growth and development postnatally. The postnatal neurogenesis of the 2A4(+) ORNs, in contrast to the very early (E13) initial appearance of ORN subclasses characterized on the basis of their putative olfactory receptor mRNAs, indicates that different ORN subclasses may vary in the time of their initial neurogenesis.