Intrinsic properties of the developing motor cortex in the rat: in vitro axons from the medial somatomotor cortex grow faster than axons from the lateral somatomotor cortex

The axons that originate in the medial somatomotor cortex of the rat depart, during development, after those from the lateral somatomotor cortex, yet they arrive in the cervical spinal cord first. Either the medially originating axons elongate faster, or the laterally originating ones pause along the descent pathway. To investigate the presence of an intrinsic difference of the axonal elongation velocity between the lateral and medial somatomotor cortical areas, we cultured explants taken from these areas for 2 days, and measured the length of the outgrowth. After 2 days the explants were surrounded by a radiate corona of axons of which the longest measured 1.95 mm. A significant difference was detected between the medial and lateral somatomotor cortical areas in vitro. Axons originating from explants taken from the medial somatomotor cortical area are, after 2 days in culture, on average 0.16 mm longer than those from the lateral somatomotor cortical area. Though the observed difference is not large enough to allow for the overtaking observed in vivo, it does indicate that intrinsic differences exist within the developing rat somatomotor cortex. This in turn indicates that intrinsic cortical traits not only influence regionalization and targeting behavior of cortical projection neurons, but also their axonal elongation speed.