Callosal neurons in the cingulate cortical plate and subplate of human fetuses

Given the scarcity of data on the development of the cerebral cortex and its connections in man, four brains of human fetuses at 25, 26, 30, and 32 weeks postovulation were used to investigate the following: 1) the radial distribution of callosal neurons in the cingulate cortex at the immediate postmigratory period; 2) the existence of callosally projecting neurons in the cortical subplate; and 3) the dendritic morphology of developing callosal neurons. The carbocyanine dye (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) (DiI) was used as a fluorescent postmortem tracer for the identification and morphological description of callosal neurons, 4-6 months after the insertion of DiI crystals at the callosal midplane. Sixty-one completely labeled neurons were selected for microscopical analysis, drawn by use of a camera lucida and photographed. The main findings were the following: 1) the human cingulate cortex at 25-32 weeks postovulation contains callosally projecting neurons both in the cortical plate and in the subplate; 2) callosal cells in the plate are mostly spiny pyramids with somata distributed uniformly throughout the depth of the plate, irrespective of rostrocaudal position. They have well-differentiated basal dendrites and apical dendrites that consistently ramify within layer 1; 3) subplate callosal cells are smooth neurons of diverse dendritic morphology, distributed widely throughout the subplate depth. They were classified into four cell types according to the dendritic morphology: radially oriented, horizontally oriented, multipolars, and inverted pyramids. These findings extend to the human brain some of the evidence obtained in animals concerning the development of the cerebral cortex, especially those that are relevant to the formation of a transitory circuitry in the subplate.