INTERACTIONS BETWEEN GROWING THALAMOCORTICAL AFFERENT AXONS AND THE NEOCORTICAL PRIMORDIUM IN NORMAL AND REELER MUTANT MICE

The interactions between growing thalamocortical afferent axons and th e 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 o f the fixed brains of control and reeler mutant mice. In the neocortic al primordium of the control mouse, the labeled afferents running tang entially in the intermediate zone formed a dense plexus in the subplat e, the layer below the cortical plate, as early as the 16th gestationa l 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), labele d afferents grew radially up to the upper cortical plate and terminal arborizations of the afferents were evident in the forming layer IV. T n contrast, in the E16 cerebrum of the reeler mutant mouse, in which t he cortical layers are inverted, the labeled afferents traversed the n eocortical 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 superpla te and made contact with the superplate neurons. At P4 in the reeler n eocortex, 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 synaps es were formed on the somata of the subplate neurons in the control mo use, and similar immature synapses were also formed on the superplate neurons of the reeler mutant. At E16 in the control, NGF receptor immu noreactivity was expressed in the intermediate zone, subplate and lowe r cortical plate, and the mode of expression corresponded to the distr ibution of thalamocortical afferents. At the same stage of the reeler mutant, expression of NGF receptor immunoreactivity was confined to th e afferent axons that had grown through the neocortical primordium tow ards the superplate. In the control at E17, highly polysialylated NCAM (NCAM-H), a hemophilic cell adhesion molecule, was expressed in the s ubplate, marginal zone and afferent axons. In the reeler mutant at the same stage, this adhesion molecule was expressed in both the superpla te and the bundles of the afferent axons. These findings suggest that the subplate and the superplate, which are composed of neurons generat ed at the earliest stage, attract growing thalamocortical afferent axo ns specifically by a chemotropic mechanism through the expression of N GF receptor. Furthermore, growth cones of the afferent axons may make contact with the subplate or superplate neurons specifically through t he hemophilic adhesive activity of NCAM-H expressed on them. On the ba sis of such mechanisms, the subplate or the superplate could play a ro le as a tentative target for the thalamocortical afferents prior to ar rival at their final targets, i.e., the layer IV cortical neurons in t he control and the equivalent neurons in the reeler mutant.