Ultrastructural Evidence for Synaptic Interactions between Thalamocortical Axons and Subplate Neurons

Abstract

Thalamic axons are known to accumulate in the subplate for a protracted period prior to invading the cortical plate and contacting their ultimate targets, the neurons of layer 4. We have examined the synaptic contacts made by visual and somatosensory thalamic axons during the transition period in which axons begin to leave the subplate and invade the cortical plate in the ferret. We first determined when geniculocortical axons leave the subplate and begin to grow into layer 4 of the visual cortex by injecting 1,1'-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine (Dil) into the lateral geniculate nucleus (LGN). By birth most LGN axons are still confined to the subplate. Over the next 10 days LGN axons grow into layer 4, but many axons retain axonal branches within the subplate. To establish whether thalamic axons make synaptic contacts within the subplate, the anterograde tracer PHA-L was injected into thalamic nuclei of neonatal ferrets between postnatal day 3 and 12 to label thalamic axons at the electron microscope level. The analysis of the PHA-L injections confirmed the Dil data regarding the timing of ingrowth of thalamic axons into the cortical plate. At the electron microscope level, PHA-L-labelled axons were found to form synaptic contacts in the subplate. The thalamic axon terminals were presynaptic primarily to dendritic shafts and dendritic spines. Between postnatal days 12 and 20 labelled synapses were also observed within layer 4 of the cortex. The ultrastructural appearance of the synapses did not differ significantly in the subplate and cortical plate, with regard to type of postsynaptic profiles, length of postsynaptic density or presynaptic terminal size. These observations provide direct evidence that thalamocortical axons make synaptic contacts with subplate neurons, the only cell type within the subplate possessing mature dendrites and dendritic spines; they also suggest that functional interactions between thalamic axons and subplate neurons could play a role in the establishment of appropriate thalamocortical connections.

The Stop Signal Revised: Immature Cerebellar Granule Neurons in the External Germinal Layer Arrest Pontine Mossy Fiber Growth

“…1) (Mason and Gregory, 1984) and send thin filopodial processes, "combination fibers," to both migratory granule cells and Purkinje cell bodies (Ramon y Cajal, 1911;Mason and Gregory, 1984;Liesi et al, 2003). Whether these temporary contacts are fully synaptic in nature and whether, as in the subplate, they are necessary for the normal maturation of the circuitry (Herrmann et al, 1994;Kanold et al, 2003) is currently under study (A. Kalinovsky, C. A. Mason, and P. Scheiffele, unpublished observations).…”

“…They may be involved in EGL development, as shown for pioneer olfactory axons that induce proliferation of olfactory bulb progenitors (Gong and Shipley, 1995), or reach up to the borders of the EGL to initiate contact with migratory granule cells (Liesi et al, 2003). Some models of afferent development invoke immature, transient cells, such in the subplate (Herrmann et al, 1994), to serve as temporary way stations for growing axons before they can synapse on their targets. Both migratory granule cells and Purkinje cells could play such a role in cerebellar development.…”

Molecular Organization of the Ferret Visual Thalamus

“…LGN relay neurons send their axons to layer 4 in the primary visual cortex after birth and make synapses with layer 4 neurons at P12-P20 in the ferret (Herrmann et al, 1994). Because X cells and Y cells project to distinct sublayers in layer 4 LGN were stained with either anti-Islet1 antibody, ferret enkephalinase probe, or cadherin7 probe.…”

Alteration of Visual Input Results in a Coordinated Reorganization of Multiple Visual Cortex Maps

“…Although enucleation performed during this age range may not affect the initial establishment of ocular dominance columns, our earlier manipulations performed at P0 might. Indeed, there are important developmental milestones between these stages, including the formation of cytoarchitectonic eye-specific layers in the LGN and the ingrowth of thalamocortical axons to layer IV of cortex (Linden et al, 1981;Johnson and Casagrande, 1993;Herrmann et al, 1994).…”

“…Performing neonatal monocular enucleation may allow one to test these predictions experimentally, because this manipulation has significant potential (as shown by anatomical methods) to alter or remove the ocular dominance map (Rakic, 1981;Guillery et al, 1985), particularly in a species such as the ferret whose visual system is very immature at this developmental stage (Johnson and Casagrande, 1993;Herrmann et al, 1994). Furthermore, monocular enucleation leaves cortical cell responses to other stimulus parameters relatively intact, although it is unknown how it affects the two-dimensional maps of these parameters (Fregnac et al, 1981;Shook et al, 1985;Bisti and Trimarchi, 1993;Bisti et al, 1995).…”