Cortical layer VII and persistent subplate cells in mammalian brains.

Layer VII is the deepest cortical layer in rats, and consists of a thin layer of persistent subplate cells overlain by a cell-sparse, myelin-rich stratum through which many corticocortical axons travel. Layer VII neurons participate in local and long-distance corticocortical connections. The present study was undertaken to determine whether layer VII is a typical feature in rodent brains, and to determine which other mammalian taxa exhibit a layer VII. The adult brains of 144 species from 22 orders were examined. Of these, 43 species in 6 orders exhibit a layer VII. These include the sciurognath Rodentia, Insectivora, Paucituberculata, Paramelemorphia, some Xenarthra, and some Chiroptera. In all taxa interstitial cells were observed scattered throughout the white matter. The observed distribution of layer VII in this sample of mammalian taxa suggests that layer VII is a typical feature in some orders, but is not present in most orders. The heterogeneous distribution of layer VII in the Rodentia and Chiroptera suggests that species-level developmental dynamics are involved. It is hypothesized that the timing of subplate apoptosis in relation to the establishment of corticocortical connections is the major factor that determines whether layer VII is present in the adult stage.