Quantitative analysis of synaptogenesis in the cerebral cortex of the cat suprasylvian gyrus.

A quantitative study was made of the synapses in the association cortex of the suprasylvian gyrus and in the adjacent subcortical region, in kittens from birth (P0) to 103 days (P103), and in two adults. The quantitative data were corrected using the neuropil and the growth coefficients. At P0, the synaptic density of the cortex is only 7% of the adult density. The synapses are disposed preferentially in layers I, III and V. The rush of the synaptic density apparently starts at P9, or at P12 after correction using the growth coefficient. At this time, the cortico-cortical connections begin to form. During the rush, the synaptic peaks of the newborn animal disappear; the synaptic density reaches its maximum at P28-P35, exceeding the adult density by 53 to 82%. After P35, there appears a real, although very slow, elimination of synapses. This elimination touches the superficial layers (I to III) first, and then the deep layers. The young neuron, seeking a qualitative and quantitative equilibrium of its afferents and its efferents, seems to be responsible for this elimination. The superficial cortex always contains more synapses (60%) than the subjacent cortex, regardless of the animal's age. The rush is greatest here, as is the elimination. The question of senescence arises: since the majority of the synapses are very probably of cortico-cortical origin in this cortex, this process would seem to be due to a loss of association connections. Indeed, one of the two adults analyzed revealed a loss of synapses in the superficial layers of its cortex, as well as in layer VI. The synapses are present in the subcortical region, without discontinuity with the cortex in all animals. In the youngest, synapses were observed down to 2 mm below the cerebral cortex: the neurons establish a few rare contacts before reaching their definitive site. In the adult, the white matter is totally devoid of synapses. Since the elimination of synapses appears to be a process which continues throughout life, the term "maturity" cannot be attributed solely on the basis of the histologic components, but must result from an evaluation of behavior.