The differentiation of dendrites in the medial trapezoid nucleus of the opossum and cat has been traced from the stage of the post-migratory neuroblast in developmental series prepared with the rapid Golgi technique. The post-migratory neuroblast is an elongated cell. Its perikaryon is located initially at the outer limiting layer of the medulla. Its primitive internal process grows into the primordial medial trapezoid nucleus and gives rise to an axon. On the part of the neuroblast adjacent to the axon's origin the endings of the afferent axons beging to differentiate. The perikaryon moves to the same part of the neuroblast through the primitive internal process. Subsequently the dendrites differentiate. Dendrites and their branches form from budding growth cones. The cell body and dendritic processes of the young growing neuron are covered with transitory filopodia. Sprouting growth cones and filopodia appear at the tips and along the shafts of the elongating and enlarging dendrites. The locomotor and synthetic activities of the growth cones establish the stereotyped dendritic branching patterns of each kind of neuron. The development of the dendritic branches accompanies the elaboration of the particular type of axonal plexus that will become synaptically related. This suggests that the patterns of the dendritic trees and of the afferent axonal end-branches derive from mutual interactions of the growing dendritic and axonal branches. These interactions may be mediated by physical contacts as well as chemotactic factors. The filopodia are implicated in the formation of dendritic appendages. Filopodia could participate in membrane synthesis, locomotion, and synaptogenesis. There is an indication that the afferent axons can induce the differentiation of the post-synaptic parts of the neuroblast. The findings imply that the influence of physical and chemical factors in the differentiation of the synaptic organization of the brain depends on their temporal and spatial sequences.
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