Monoaminergic afferents to the neocortex: a developmental histofluorescence study in normal and Reeler mouse embryos

The patterns of distribution of monoaminergic (MA) afferents during early histogenesis of the neocortex of normal and Reeler mice are studied by histofluorescence microscopy. Fluorescing fibers appear rostrally in the cortex of both genotypes on the 14th embryonic day (E14), which is within 24 h of the development of the cortical plate. They are distributed to all regions of the cortex by the time of birth. Although the patterns of intracortical deployment differ in the two genotypes, the fibers appear to have homologous target structures. These are: (1) the polymorphic cells of the subplate in the depths of the normal and in the superplate near the surface of the mutant cortex; and (2) the zones of consolidation of apical dendrites of pyramidal cells: the external plexiform zone of normal and a series of intracortical plexiform planes in the mutant cortex. By contrast, the axons of this system do not branch significantly among the compactly ordered somata of pyramidal cells within the cortical plate of either genotype.