Single thalamocortical axons diverge to multiple patches in neonatal auditory cortex

Thalamic afferents originating in the ventral division of the medial geniculate body (vMGB) terminate in patches within lamina III/IV of the primary auditory cortex in adult rabbits. Focal iontophoretic injections of the anterograde tracer, biocytin, were made into the vMGB of neonatal rabbits to examine the morphological organization of auditory thalamocortical (TC) afferents prior to hearing onset. TC afferents terminated in distinct patches as early as postnatal day 1 (PD-0 = day of birth), 6 days before the behavioral onset of hearing. In contrast to TC afferents in adults, the terminal arbors of neonatal vMGB axons occupied the entire depth of the cortical plate and lamina I. Serial section reconstructions revealed that single vMGB axons in neonates branched to form multiple patches within the cortical plate. Collaterals also extended to lamina I where they coursed tangentially for several millimeters. An unusual feature of the neonatal TC patches was the contribution of descending collaterals from axons coursing in lamina I. The presence of distinct patches prior to hearing onset indicates that the segregation of auditory TC axons occurs in the absence of acoustically driven activity. The extensive postnatal remodeling of TC axons, however, may indicate activity-dependent refinement of arbor size.