Interactions between growing thalamocortical afferent axons and the neocortical primordium in normal and reeler mutant mice

Abstract

The interactions between growing thalamocortical afferent axons and the neocortical primordium were examined during neocortical development of the mouse cerebrum, by labeling the afferents with the carbocyanine fluorescent dye, DiI, which was introduced into the dorsal thalamus of the fixed brains of control and reeler mutant mice. In the neocortical primordium of the control mouse, the labeled afferents running tangentially in the intermediate zone formed a dense plexus in the subplate, the layer below the cortical plate, as early as the 16th gestational day (E16). Small numbers of the afferents invaded the lower cortical plate at E16 and increasing numbers of labeled growing axons extended into the cortical plate at E17. At the 4th postnatal day (P4), labeled afferents grew radially up to the upper cortical plate and terminal arborizations of the afferents were evident in the forming layer IV. In contrast, in the E16 cerebrum of the reeler mutant mouse, in which the cortical layers are inverted, the labeled afferents traversed the neocortical primordium directly towards the superplate, the superficial layer above the cortical plate and the equivalent of the subplate in the control mouse. Thick bundles of labeled axons reached the superplate and made contact with the superplate neurons. At P4 in the reeler neocortex, the afferent axons that had reached the superplate began to change their direction of growth and turned towards the deeper layer. Electron-microscopic observations at E16 revealed that immature synapses were formed on the somata of the subplate neurons in the control mouse, and similar immature synapses were also formed on the superplate neurons of the reeler mutant. At E16 in the control, NGF receptor immunoreactivity was expressed in the intermediate zone, subplate and lower cortical plate, and the mode of expression corresponded to the distribution of thalamocortical afferents. At the same stage of the reeler mutant, expression of NGF receptor immunoreactivity was confined to the afferent axons that had grown through the neocortical primordium towards the superplate. In the control at E17, highly polysialylated NCAM (NCAM-H), a homophilic cell adhesion molecule, was expressed in the subplate, marginal zone and afferent axons. In the reeler mutant at the same stage, this adhesion molecule was expressed in both the superplate and the bundles of the afferent axons. These findings suggest that the subplate and the superplate, which are composed of neurons generated at the earliest stage, attract growing thalamocortical afferent axons specifically by a chemotropic mechanism through the expression of NGF receptor. Furthermore, growth cones of the afferent axons may make contact with the subplate or superplate neurons specifically through the homophilic adhesive activity of NCAM-H expressed on them. On the basis of such mechanisms, the subplate or the superplate could play a role as a tentative target for the thalamocortical afferents prior to arrival at their final targets, i.e., the layer IV cortical neurons in the control and the equivalent neurons in the reeler mutant.