Morphological and quantitative evaluation of olfactory bulb development in Xenopus after olfactory placode...

We found previously that the number of olfactory axons is correlated with the number of mitral/tufted cells (output neurons of the olfactory bulb) during normal larval development. To examine the significance of this quantitative relationship, we evaluated the effects of transplanting an extra olfactory placode on the development of the larval olfactory bulb. We found that the transplanted tissue retained the normal, pseudostratified, columnar appearance and had the same cell types as normal olfactory epithelium, and the olfactory bulbs had the same laminar organization as control bulbs. With gross examination of the olfactory bulb, the side innervated by the transplant appeared slightly larger than the contralateral side in animals analyzed at a young larval stage (stage 50) and in 2 of the 9 animals examined at late larval stages (57/58). Tissue sections and area measurements, however, revealed that the volume of the olfactory bulbs in animals with a transplant was not significantly different from control values. Our quantitative analysis also showed that in stage-50 animals with a transplant, the total number of olfactory axons (in nerves from the transplanted and host olfactory organs) appeared to be greater than in control animals, but not to a statistically significant level. The number of mitral/tufted cells was not different from controls. In animals examined at stage 57/58, there was no difference from controls in either the total number of olfactory axons, total number of mitral/tufted cells, or convergence ratio of olfactory axons onto mitral/tufted cells. Thus, in the late-stage larvae, the quantitative relationship between olfactory axons and mitral/tufted cells was not altered by the experimental manipulation. These results suggest that the olfactory bulb can regulate the number of afferent fibers.