DEVELOPMENT OF LOCAL CONNECTIONS IN FERRET SOMATOSENSORY CORTEX

Ferrets have become recognized as a useful and interesting model for s tudy of neocortical development. Because of their immaturity at birth, it is possible to study very early events in the ontogeny of the brai n. We used living slices of ferret somatosensory cortex to study the f ormation and development of intrinsic elements within the neocortex. A small number of fixed, hemisected brains injected with -dioctadecyl-3 ,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) were also used. The slices were obtained from ferret kits aged postnatal day (P)1 to P62 and maintained in a chamber; each slice received injections of flu orescent-labeled dextrans. The injections were made at different ages in several distinct sites, which included the proliferative ventricula r zone, the intervening white matter (or intermediate zone), and diffe rent sites of developing cortex, including the deeper cortical plate, which incorporated the subplate in young animals, and more superficial cortical sites, depending on the age of the animal. Several animals a lso received injections into the ventrobasal thalamus. Injections into young animals (P1-7) produced a dominant radial pattern that extended from the ventricular zone into the cortex. Injections into the ventri cular zone labeled many cells that appeared morphologically like radia l glia as well as presumptive neurons. Although the predominant patter n was radial, injections in the ventricular zone often produced tangen tially oriented cells and horizontally arranged fibers at the outer ed ge of the proliferative zone. These cells and fibers may provide a sub strate for tangential dispersion of neurons within the neocortex. More superficial injections within the slice labeled lines of cells that a ppeared to be stacked upon one another in a radial pile in the cortex; the cortical plate received very few lateral projections. Data obtain ed from more mature slices indicated that, although the overall patter n of staining remained radial, the precise character of the pattern ch anged to include more lateral spread into surrounding cortex, which ev entually refined and developed into distinct patches by P28, when the overall cortical architecture appeared adult like. The data involving thalamocortical connections were more limited, but they indicated that the thalamus projects precisely to the somatosensory cortex in a poin t-to-point fashion from the earliest date studied (P0) and that the ve ntrobasal nucleus terminates upon the somatosensory cortex in a patchy manner during the early postnatal days of development. This study of the development of the somatosensory cortex confirms the ubiquitous na ture of column-like connections throughout the neocortex and provides a novel view of the radial nature of early neocortical maturation. (C) 1996 Wiley-Lies, Inc.