Neuronal migration, with special reference to developing human brain: a review
A general rule in the developing central nervous system is that cells are generated in sites different from those in which they will later reside. The intervening migrations, particularly in the human nervous system, form the subject of this review. The basic columnar organization in the early stages of development favors radial migration of cells. During later stages in primates, when young neurons migrate to the distant cerebral cortex, they follow radial glial guides across the widening intermediate zone as they pass from the juxtaventricular site of genesis to the cortical plate. Somas of later-generated cells take positions external to somas of their predecessors. The final position along the radial vector may be influenced by afferent axons. Cell relationships in the developing cerebellar cortex are essentially similar, though the key migration of granule cell neurons is in the reverse direction, from the external surface inward past Purkinje dendrites and somas. Bergmann glial fibers provide the radial guidance in this instance. The degree of dependence of developing neurons upon other cells and cell processes in their immediate environment has been clarified by study of mutant mice in which cerebral or cerebellar cortices are malformed. Other special migrations in the fetal human brain are reviewed, particularly the passage of neurons from the rhombic lip through the transient corpus pontobulbare to mainly the inferior olives and pontine gray nuclei, and from the ganglionic eminence of the cerebrum through the corpus gangliothalamicum into the pulvinar region of the thalamus. It was suggested that the special relationships involved in these various migrations are probably mediated by cell surface properties, and that such surface properties will come to be defined through analysis of reaggregation tissue cultures, experimental and natural chimeras, and by immunological definition of antigens on CNS cells at different stages of development.