Single thalamocortical axons diverge to multiple patches in neonatal auditory cortex

doi:10.1016/0165-3806(94)90077-9

Developmental Brain Research

Volume 81, Issue 1, 12 August 1994, Pages 135-142

https://doi.org/10.1016/0165-3806(94)90077-9 Get rights and content

Abstract

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.

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Here we examined quantitatively axonal projections originating from the ventral posteromedial thalamic nucleus (VPM) to layer I of the primary somatosensory cortex (SI) by extracellular and intracellular injections of biocytin as an anterograde tracer. Following the extracellular injections, two types of VPM afferents with different arborization patterns in SI were observed. The type I extended vertically, forming dense plexus in layers IV and VI, and projected collaterals to layer I. The type II rarely branched in SI, converged in the plexus formed by the type I, and projected no collaterals to the supragranular layers. The labeled fibers in layer I derived from the first type ran parallel to the brain surface, and their mean length was 339.7 ± 87.5 μm. Intracellular injection into VPM neurons bearing both types of afferent demonstrated the full axonal arborization in both the reticular thalamic nucleus (Rt) and SI. The total length of the axon of a neuron bearing the type I was 86,968.8 μm, and the length of axonal collaterals in layer I of SI was 433.1 μm. The total axonal length of a neuron bearing the type II was very small. The present study is the first to demonstrate substantial projections from VPM to layer I of SI, and provide quantitative data on the entire extent of the axonal arborization of thalamocortical projections from single VPM neurons.
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Short communicationSingle thalamocortical axons diverge to multiple patches in neonatal auditory cortex

Abstract

Trends Neurosci.

Dev. Brain Res.

J. Neurosci. Methods

Neuron

Brain Res.

Dev. Brain Res.

Brain Res.

Brain Res.

Brain Res.

Exp. Neurol.

Brain Res.

Brain Res.

Brain Res.

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J. Neurosci.

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Soc. Neurosci. Abstr.

Postnatal development of thalamocortical afferent ‘patches’ in auditory neocortex

Soc. Neurosci. Abstr.

Thalamic axons confer a blueprint of the sensory periphery onto the developing rat somatosensory cortex

Dev. Brain Res.

A comparative study of the development of hearing and vision in various species commonly used in experiments

Acta. Otolaryngol. (Stockh.)

Pathfinding and target selection by developing geniculocortical axons

J. Neurosci.

Morphology and intracortical projections of functionally characterised neurones in the cat visual cortex

Nature

Tonotopic organization and dendrite orientation in primary auditory cortex of the rabbit

Soc. Neurosci. Abstr.

Laminar and columnar distribution of geniculocortical fibers in the macaque monkey

J. Comp. Neurol.

Functional architecture of macaque monkey visual cortex

Projection patterns of individual X- and Y-cell axon from the lateral geniculate nucleus to cortical area 17 in the cat

J. Comp. Neurol.

Terminal arbors of axons projecting to the somatosensory cortex of the adult rat. I. The normal morphology of specific thalamocortical afferents

J. Neurosci.

Short communication
Single thalamocortical axons diverge to multiple patches in neonatal auditory cortex