A study of neurogenesis in the forebrain of opossum pouch young

Summary Morphological changes of growing axons and dendrites are traced in rapid Golgi preparations of the olfactory bulb and the striatum and the parietal, cingulate, and hippocampal cortex. In age-graded series of opossum pouch young changes in the form and position of the differentiating neurons are related to the genesis of the neuronal architecture of each region. In the olfactory bulb there is a specific sequence of changes, starting with the formation of the olfactory glomeruli and the apical dendrites of the mitral cells and ending with the formation of the granule cell processes and the lateral dendrites of the mitral cells. In the ganglionic eminence, the external processes of neuroblasts form axons as they pass from the matrix zone into the presumptive striatal neuropil. Subsequently the perikarya move through the external processes from the matrix zone to the origins of the axons, the external processes disappear, and the dendrites of the striatal neurons start to form. The region of neuronal differentiation is defined by the intersection of the primitive processes of the neuroblasts with the growth cones of specific afferent axons. In the cerebral cortex the neuroblasts resemble primitive epithelial cells (spongioblasts). Their perikarya are in the matrix zone; their primitive processes extend between the internal and external limiting membranes. When afferent growth cones appear in the mantle zone of the parietal area, the perikarya of the pyramidal neuroblasts move to the external limiting layer through their primitive processes. Next these processes disappear; then axons and dendrites differentiate. The order of these events differs in other cortical areas. There is no sign of free, ameboid migration of the neuroblasts. The classical problems of neurogenesis are redefined in the light of these findings.Supported by U.S. Public Health Service Research Grant NS 06115 and GRS Grant 5 S01 FR 05381-08 to Harvard University.With the technical aid of R. R. Morest and P. E. Palmer.