The results lack a positive control showing that HcT can actually resolve slowed axonal transport. Unless this is known from other studies, from even older mice, or from a disease model with known axonal transport slowing, the HcT result remains unvalidated and the conclusions about slowed axonal transport in aged mice unsupported. If this is known from previous work with the same tracer, this should be clearly explained. Alternatively, the authors could use a different tracer that shows slowed transport in mice of this age, in which case the HcT results would show that age affects the speed of certain types of cargo. Without either of these data, the conclusions are unsupported.
The authors state that "Takihara et al. reported that the duration, distance, and speed of mitochondrial transport in retinal ganglion neurons increased from two to four months of age, but then declined by 12-13 months, getting worse by 23-25 months (Takihara 2015). Our data presented here contrast with these findings.." However, the present results only go to >= 13 months. For this statement to be accurate, the authors would need to present data from mice aged 23-25 months. Furthermore, unless the cited study used HcT, the results could be due to methodological differences the authors have not tested.