Postnatal growth of intrinsic connections in mouse barrel cortex

Surprisingly little is known about the development of connections within a functional area of the cerebral cortex. We examined the postnatal growth of connections in mouse barrel cortex during the second and third weeks after birth, coinciding with the period of rapid synaptogenesis that occurs just after the barrels first form. A barrel is a group of neurons in layer 4 of somatosensory cortex that is part of a cortical column. Each whisker/barre l column is linked anatomically and functionally to a homotopic whisker on the contralateral face. Radial groups of cortical neurons were labeled with the neuronal tracer biotinylated dextran amine in mice ranging in age from postnatal day 8 (P8; P0 is the date of birth) to adulthood. The spatial di stributions of retrogradely labeled neurons in different laminae were analy zed. The barrel map in layer 4 was used as a template to compare quantitati ve data from different animals and to account for substantial changes in ba rrel and barrel field size during development. Intrinsic projections 1) inn ervate increasingly more distant targets within barrel cortex up to 3 weeks of age; 2) continue to form in targets after 3 weeks, effectively strength ening existing connections; 3) follow a timetable for growth that is layer- specific; 4) link more distant barrel columns in layer 4 from neurons that are found preferentially in the barrel side and the septa between barrels; and 5) form over the shortest distances between the barrel columns. These d ata indicate that intrinsic connections in mouse barrel cortex develop by t he progressive addition of neuronal connections rather than by sculpting pr eliminary connections. We describe statistically significant changes in con nectivity during development that may be applied to model and assess the de velopment of connections after a variety of experimental perturbations, suc h as to the environment and/or the genome: J. Comp. Neurol. 436:17-31, 2001 . (C) 2001 Wiley-Liss, Inc.