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During the early development of the neocortex, thalamocortical axons arrive potentially in time to instruct migrating cortical neurons in several aspects of local differentiation, such as number of layer IV neurons and efferent connectivity. Migration of layer IV neurons into the cortical plate just precedes thalamocortical invasion, suggesting that these neurons could cue or tropically direct thalamic ingrowth. To explore the interactions of layer IV neurons and their thalamocortical input, we administered a mitotic inhibitor methylazoxymethanol acetate (MAM) intraperitoneally to timed pregnant hamsters on E14 when layer IV neurons are normally being generated in striate cortex. Reduced numbers of cortical neurons overall, the absence of small-diameter granule neurons, and the absence of the zone of reduced density of callosally projecting neurons suggest that neither the depletion of layer IV cells in the ventricular zone nor thalamic afferents in the subplate or cortical plate respecify the later generated cohort of neurons (presumptive layer II/III neurons) to acquire morphological and connectional properties of layer IV. Dil injections into the dorsal lateral geniculate nucleus (LGd) of animals from embryonic (E15) and postnatal (P7) ages show that the final position of thalamic axons with respect to layer V is not affected by the absence of E14 neurons. In the normal visual cortex, geniculocortical axons have begun their arborization in their presumptive target layer in the upper cortex immediately below the undifferentiated cortical plate on P4, while in MAM animals, this process occurs 1 d later. The extent and density of arborization is much reduced in the thinner cortex of the MAM animals. We thus find no evidence for instruction of migrating neurons by thalamocortical axons to assume the layer IV phenotype; if instruction does occur, it must take place in a very restricted time window. Thalamic axons can also find their laminar position in the absence of cells of this phenotype.

Cortical Target Depletion and Ingrowth of Geniculocortical Axons: Implications for Cortical Specification