Regressive events and selection in the differentiation of callosal axons

The growth stages described above lead to overproduction of callosal axons, branches in the white and in the gray matter, and synapses. They are followed by selection of axons, axonal branches, and synapses to be maintained and by elimination of the others. The elimination is massive. As was mentioned above, previous work has shown that at least 70% of the juvenile callosal axons are eliminated in both cat and monkey (Berbel and Innocenti, 1988; LaMantia and Rakic, 1990b). In addition, at the stage of subcortical branching, homo-topic callosal axons from the 17/18 border have established, on average, about 10 branching points in the contralateral white matter. These are reduced to two in the adult. The number of branches in the gray matter near the 17/18 border was found to exceed 500 at the beginning of the third postnatal month, and it was down to 240 in the adult. The number of boutons reached about 2100 at the beginning of the third postnatal month, and it was down to the average adult value of about 300 by the end of the fifth month. It should be noticed that these values are only suggestive because the number of serially reconstructed axons is limited.

The overproduction of synapses is a well-documented phenomenon in the development of cerebral cortex (see Bourgeois and Rakic, 1993, for data and references). The concept was established on the basis of electron-microscopic estimates of synaptic density in sample volumes of cortical gray matter. However, the implications of the synaptic overproduction and elimination for connectivity remained unclear. In particular, the origin of the supernumerary synapses could not be defined, and it could have been expected that the synaptic elimination would cause important rearrangements in the topography of cortical connections. The results reported above reject this possibility by showing that the overproduction of synapses does not blur much, if at all, the topographical specificity of the connections. Therefore, rather than sharpening the topography of the projections, the syn-aptic elimination might adjust the strength of the con nections between callosal axons and their postsynaptic targets.

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