DEVELOPMENT OF CORTICOTECTAL SYNAPTIC TERMINALS IN THE CAT - A QUANTITATIVE ELECTRON-MICROSCOPIC ANALYSIS

We studied the development of corticotectal synaptic terminal boutons and synapses by making injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into area 17 of visual cortex in k ittens ranging from newborn to 12 weeks of age and in adults. The loca tion and extent of the injection site, and labeled corticotectal axon terminals in the superficial layers of the superior colliculus were de monstrated histochemically with the cobalt-glucose oxidase diaminobenz idine reaction. During the first 2 weeks after birth, the majority of labeled profiles resembled axonal growth cones, or structures intermed iate in morphology between growth cones and synaptic terminals, while very few corticotectal axon terminals forming well-defined synaptic co ntacts were observed. Labeled synaptic terminals in kittens at 1 and 2 weeks of age were small, contained very few synaptic vesicles, which were usually restricted to the contact zone, and exhibited few mitocho ndria. By 4 and 6 weeks after birth, a well-developed population of sy naptic terminals was established; however, growth cones and intermedia te profiles were still numerous. At 8 weeks of age synaptic terminals were morphologically mature, and growth cone-like profiles were no lon ger observed. To study quantitative changes in synapse development we used the disector method to obtain unbiased estimates of the density a nd number of corticotectal synaptic terminals and synapses; both the d ensity and number of terminals and synapses increased steadily through out postnatal development. These results suggest that the corticotecta l projection develops by the progressive elaboration of synapses, as o pposed to synapse overproduction and subsequent elimination. (C) 1993 Wiley-Liss, Inc.