Observations on the development of certain ascending inputs to the thalamus in rats. I. Postnatal development

We have studied the postnatal development of the major ascending afferents to the thalamus in postnatal rats using tetramethylbenzidine histochemistry following wheat germ agglutinin-conjugated horseradish peroxidase injections into either the dorsal column nuclei, the deep cerebellar nuclei, or the inferior colliculus. By the day of birth, the efferents from each of these regions have already entered, and arborized extensively within, their appropriate thalamic relay nuclei. However, the overall distribution of each of these ascending afferent systems differs dramatically from that seen in mature rats. In neonatal rats, a substantial proportion of the ascending axons extend beyond the thalamus and often enter the internal capsule, some bypassing the thalamus altogether. In addition, some of the axons which enter and arborize within the thalamus extend beyond their appropriate terminal field into adjoining thalamic nuclei. Retrograde tracing experiments utilizing Fast blue indicate that the cells of origin of these overshooting axons are distributed similarly to the cells of origin of the definitive thalamic afferents. These early erroneous projections are all subsequently eliminated and the characteristically restricted adult distribution of each afferent system is evident by P30. These results indicate that developmental overgrowths and targeting errors of thalamic afferent fibers are not unique to the visual system (where they have been documented previously), but may be a general feature in the development of these pathways.