Coincidence of Schwann cell‐derived basal lamina development and loss of regenerative specificity of spinal motoneurons

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Abstract
The Schwann cell‐derived basal lamina forms a tube around single peripheral axons or small groups of axons that is continuous from the spinal cord to the target. In bullfrog tadpoles (Rana catesbeiana), motor axons transected at early developmental stages regenerate to the appropriate hindlimb region. In the present paper, we found that at these stages Schwann tubes are absent by morphological criteria, and individual axons are separated only by occasinal extensions of support cells. At stages when axons no longer regenerate to the correct hindlimb region, every axon is encased in a basal lamina tube. Schwann tubes persist in the distal stump after nerve transection, and regenerating axons grow within these tubes. These findings are consistent with previous results showing that the errors regenerating axons make in older animals are not random, but depend upon the course of the denervated Schwann tubes to which they ahve acces. In order to determine whether formation of the Schwann tube itself or interaction of its molecular constituents with growing axons was associated with loss of regenerative specificity, the expression during development of two major constituents of the basal lamina, laminin and heparan sulfate proteoglycan, was investigated. Immunoreactivity to both constituents was present both before and after the transition from specific to nonspecific regeneration, indicating that their expression per se was not sufficient to limit regnerative specificity. These data support the hypothesis that the physical constraint imposed by the Schwann cell‐derived basal lamina prevents regenerative specificity.