Regeneration (Biology)

Unlike most neurons in the adult nervous system, olfactory receptor neurons (ORN),
found in the olfactory epithelium (OE), continually turnover in the adult rat. These
neurons project their axons to the olfactory bulb which is their central target. The
present study eliminated target neurons in the bulb using N-methyl-D-aspartate
(NMDA) to examine the effects of target loss on ORN survival and maturation. We
compared the effects of the NMDA lesion to bulbectomy, a permanent surgical
removal of the bulb, which simultaneously causes damage to ORN axons. We found
that unlike bulbectomy, large numbers of dying OE cells were not observed at any
time after the lesion. The number of immature neurons increased relative to the
control side, and the number of mature neurons also slightly increased with time
following NMDA lesion. Survival of ORNs does not seem to be significantly altered
in the absence of its target.

Mammals, unlike lower vertebrates, cannot normally regenerate injured central nervous system neurons. Although rat retinal ganglion cells (RGCs), following optic nerve crush, will undergo an initial period of sprouting, axon outgrowth is limited and subsequently aborted. This study examined how extensive the changes in fast transported proteins (FTPs) were during the early response to RGC damage and whether these changes were comparable to those known to occur in lower vertebrate RGCs. Changes in mRNA for several known proteins were also analyzed. It was found that, within 2 days, axotomized rat RGCs initiated a program of cell growth, involving the differential synthesis and transport of a broad range of FTPs. This response is very similar to that of lower vertebrates and indicates that rat RGCs are capable of initiating the metabolic responses necessary for regeneration to begin. This response, however, was not sustained beyond 5 days axotomy.