The histological features of developing dendrites are analyzed in age-graded series of cats, rats, rabbits, and opossums with the Golgi techniques. Growth cones and filopodia are consistent features of growing dendrites in the sensory nuclei, motor cranial nerve nuclei, brain stem reticular formation, cerebellar cortex, midbrain tectum, thalamus, hypothalamus, striatum, olfactory bulb, hippocampus, pyriform lobe, and neopallium. Terminal dendritic growth cones occur at the tips of dendrites; preterminal growth buds occur on the dendritic shafts. Filopodia are conspicuous on the growth cones and buds, but they occur on the dendritic shafts, also. Terminal growth cones, preterminal growth buds, and filopodia associate with incipient dendritic branches. Growth cones and filopodia generally disappear when the neurons are completely differentiated. But some dendritic growth cones persist after the rest of the brain is mature and consequently may be involved in learning and other plastic changes in neural function. The analysis illustrates the following general trends in neural ontogeny, none of which are expressed without modifications and qualifications. The dendrites of the specific types of neurons differentiate in typically circumscribed periods, which occur in a fixed sequence. Within, a specific neuronal population there may be regional gradients in the degree of dendritic differentiation. Within the same region the dendrites of large cell bodies tend to differentiate before those of small cell, bodies. The dendrites of Golgi Type II neurons differentiate later than neurons with long axons from the same thalamic nucleus. In the afferent sensory systems the neurons nearer the peripheral receptors usually begin their differentiation before those nearer the cerebral cortex. The dendrites of phylogenetically older neuronal groups tend to differentiate earlier than those of more advanced or more highly specialized groups. The dendrites and the other post-synaptic surfaces of the neurons differentiate in conjunction with the particular afferent axonal end-branches that are destined to synapse with the dendrites. Dendritic differentiation may be instigated by the afferent axons, controlled by local physico-chemical conditions, and guided by contact with the afferent axonal end-branches. © 1969 Springer-Verlag.
