The ontogeny of dendrite bundles in rabbit visual cortex.

Time schedule and mechanisms of the appearance of dendrite bundles in laminae IV and II/III of the visual cortex have been investigated in the rabbit from the first appearance of the cortical plate during fetal development up to adult stages. Sections cut either perpendicularly or tangentially to the cortical surface were used for light- and electron-microscopic analysis. Dendrite bundles appear during the late fetal period and the first postnatal days in a biphasic process. The first step takes place during late fetal development. Due to migration of neuroblasts along radial processes of glial cells, column-like compartments of neuropil are formed in laminae II-IV which contain the apical processes of the nerve cells situated in deeper layers. They represent the units of origin for the later dendrite bundles. The second step is initiated immediately after birth when axons arrive and small oblique and horizontal dendrites start to sprout from perikarya as well as from the apical dendrites. These new cell processes proliferate and grow rapidly. However, during this growth process groups of apical dendrites of pyramidal cells remain together, and they later form the bundles. In the beginning of this process many apical dendrites are connected with each other by punctae adhaerentiae. The basic pattern of dendrite bundles is present before the eyes open and before the majority of spines and synaptic contacts are formed.