Cerebral Cortex 1992; 2:336-352
© Oxford University Press 1992
research-article |
Emergence of Connectivity in the Embryonic Rat Parietal Cortex
Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology Cambridge, Massachusetts
Correspondence should be addressed to R. S. Erzurumlu, E25 634, MIT, Cambridge, MA 02139
In order to understand how cortical circuitry is put together, we examined the emergence of corticofugal projection cells and the arrival of subcortical afferents in the presumptive parietal cortex of the embryonic rat cerebrum. Afferent and efferent projections were selectively labeled by applications of the lipophilic tracers Dil and DiA in aldehyde-fixed brains of 12–1 8-d-old rat embryos (E12–E18; gestation: 21 d). On E12 and E13, the neocortical anlage consists of a ventricular zone and a preplate, with no extracortical connections. By E14, just prior to the appearance of the cortical plate, poly morphic cells located in the vontrolateral proplate of the telencephalic vesicle send out the first group of corticofugal axons toward the ganglionic eminence. Shortly thereafter, the cortical plate emerges as a dense band of radially oriented cells that also contribute to the corticofugal projection. By E15, axons of the early cortical projection cells cascade through the striatal pri mordium, the future site of the internal capsule. At the time of cortical plate formation and initial corticofugal axon outgrowth, ascending corticopetal axon systems have not yet arrived in the neocortex.
Double-labeling experiments in which one dye is placed in the neocortex and the other in the ipsilateral dorsal thalamus reveal that cortical efferents encounter the first ascending wave of thalamofugal axons at the level of the striatum. Collectively, these two axonal systems bridge the neocortex and the diencephalon. Upon their arrival in the neocortex on E16, thalamic axons follow a ventrolateral to dorsomodial course within the intermediate zone.
Thalamic axons are the first subcortical afferent system to arrive in the neocortex. Other ascending afferont systems arising from the midbrain tegmentum enter the neocortex after Eli. Comparison of thalamocortical and tegmontocortical projections in two halves of the same brain and across various embryonic ages clearly reveals that the two projection systems differ in their trajec tories as well as in their time of arrival. Present observations challenge the view that the precocious arrival of subcortical axons provides the impetus for cortical maturation, and suggest that cortical plate differentiation and the initial organization of corticofugal projection patterns occur independent of ascending pathways.
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