Stereological estimation of volume, total neuron number and neuronal nuclear area of chick brainstem auditory nuclei and hippocampus following prenatal patterned and unpatterned sound stimulation

E-mail address: slodato@mgh.harvard.edu (S. Lodato). The activity and function of the mammalian cerebral cortex rely on the integration of an extraordinary diversity of excitatory projection neurons and inhibitory interneurons into balanced local circuitry. The developmental events governing the proper interaction between excitatory projection neurons and inhibitory interneurons are poorly understood. Here, we report that different subtypes of projection neurons uniquely and differentially determine the laminar distribution of cortical interneurons into cortical layers. We find that in Fezf2−/− cortex, the exclusive absence of subcerebral projection neurons and their replacement by callosal projection neurons cause distinctly abnormal lamination of interneurons. This results in physiological imbalance of excitation due to altered GABAergic inhibition. In addition, experimental generation of either corticofugal neurons or callosal neurons below the cortex is sufficient to recruit cortical interneurons to these ectopic locations. Strikingly, the identity of the projection neurons generated, rather than strictly their birth date, determines the specific types of interneurons recruited. These data demonstrate that in the neocortex individual populations of projection neurons cell-extrinsically control the laminar fate of interneurons and the assembly of local inhibitory circuitry.