Posthatching development of synapses in the neuropil of nucleus rotundus of the zebra finch: a quantitative electron microscopic study.

The development and maturation of synapses in the nucleus rotundus of the zebra finch were examined at 1, 5, 10, 20, and 100 days posthatching. Quantitative ultrastructural techniques were applied to investigate synaptic density, size of presynaptic terminals, and length of postsynaptic thickenings. During development there is a steady increases in the number of synapses and an enlargement of the presynaptic terminals. The length of the postsynaptic thickenings, however, decreases significantly during posthatching development. All three parameters reach adult values within 20 days of age. A close relationship was found between the enlargement of the presynaptic terminals and the increase in the number of contact zones up to the 20th day. The general feature of this developmental progress is in good agreement with biochemical and, to a certain extent, behavioral studies.