Similar synapse density in layer IV columns of the primary somatosensory cortex of transgenic mice with different brain size: implications for mechanisms underlying the differential allocation of cortical space

Abstract

The relative dimension of the areas constituting the cerebral cortex differs greatly in the brains of different mammalian species. The mechanisms by which such an evolutionary remodeling has occurred is not well understood. To begin exploring possible mechanisms, we took advantage of a transgenic mouse model in which the area of the primary somatosensory cortex (S1) shifts, to some extent independent from the area of the cortex as a whole, as a result of differences in the availability of insulin-like growth factor I (IGF-I). Electron microscopy estimations of synapse density in D3 and C3 cortical columns of the S1 layer IV revealed that this parameter was similar among wild type and transgenic mice with higher and lower availability of IGF-I. Because D3 and C3 columns were larger and smaller than normal in mice with higher and lower IGF-I availability, the total number of synapses contained in the average area of D3 and C3 columns increased and decreased, respectively. No differences in the number and overall arrangement of S1 columns were observed among animal groups. These results suggest that: 1) synapse density is a constant factor within the S1 cortical column structure; 2) the mechanisms and factors regulating cell number and synaptogenesis are affected as columns and cortical areas modify their relative dimensions; 3) altered availability of neurotrophic factors might be associated with changes in areal dimensions; and 4) changes in cortical areal dimensions within single lineages might result from the addition of minicolumns to preexisting columns.