Transient cells of the developing mammalian telencephalon are peptide-immunoreactive neurons

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Abstract
In the development of the mammalian telencephalon, the genesis of neurons destined for the various layers of the cerebral cortex is preceded by the generation of a population of cells that comes to reside in the subplate and marginal zones1 (see ref. 2 for nomenclature). In the cat, these cells are present in large numbers during development, when their location is correlated with the arrival and accumulation of ingrowing axonal systems3–6 and with synapses7–12. However, as the brain matures, the cells disappear and the white matter and layer 1 of the adult emerge1,13,14. Their disappearance occurs in concert with the invasion of the cortical plate by the axonal systems and with the elimination of the synapses from the subplate1,4,7,9,12. Here we report that the subplate cells have properties typical of mature neurons. They have the ultra-structural appearance of neurons and receive synaptic contacts. They also have long projections and are immunoreactive for MAP2 (microtubule associated protein 2). Further, subpopulations are immunoreactive for one of several neuropeptides. These observations suggest that during the fetal and early postnatal development of the mammalian telencephalon the subplate cells function as neurons in synaptic circuitry that disappears by adulthood.