A study of so-called 'retrograde fine-grain' degeneration in the thalamus

Following lesions of the neocortex a uniform dust-like silver deposit may appear in certain thalamic nuclei stained with the Nauta or Fink-Heimer methods. Because this deposit lacks the characteristic morphology of axons undergoing Wallerian degeneration and often appears in regions of retrograde cell degeneration, it has been interpreted as a retrograde reaction7,15. The present study, however, employing reduced silver and autoradiographic techniques, suggests that this silver deposit is an anterograde rather than a retrograde phenomenon. In cats with lesions of the posterior cingulate and presubicular cortex the Fink-Heimer method showed the fine-grain silver deposit in the lateralis dorsalis (LD) and the anteroventral nucleus (AV). The 'dust' was densest in the dorsomedial zones of the anteroventral nucleus. After the survival times used (6-8 days) no retrograde perikaryal changes were seen in these nuclei. In other cats, 3 weeks after injections of radioactive proline in the posterior cingulate cortex, the distribution of the label in the LD and AV nuclei was almost identical to the distribution of the 'dust' in the Fink-Heimer sections. Since the autoradiographic method reveals anterograde transport, it appears that both methods are demonstrating corticofugal axons. Thus it is suggested that the 'fine-grain' degeneration arises from an anterograde not a retrograde reaction.