Neuron addition is one means whereby the nervous system can compensate for increased body size. Neurons can be added either by mitosis of stem cells or by late differentiation of committed precursors. Previously, the doubling of hind limb dorsal root ganglion (DRG) neurons in postmetamorphic bullfrogs (Rana catesbeiana) was found to occur in the absence of neuron proliferation (St. Wecker and Farel [1994] J. Comp. Neurol. 342:430-438). In the present study, we identify a population of cells in the DRGs of juvenile frogs that lack the appearance typical of sensory neurons yet are immunoreactive to a neuron-specific probe for neurofilament protein. These less differentiated (type-L neurons) could not be labeled retrogradely with horseradish peroxidase from the periphery or dorsal root. Despite their apparent immaturity, type-L neurons appear to have extended axons both centrally and toward the periphery, because axon number in dorsal roots and peripheral nerves was similar in juvenile and adult frogs. These findings are consistent with the existence in juvenile frogs of a population of incompletely differentiated DRG neurons that lack the physiological properties and appearance typical of mature neurons.
Antisera were raised to a tridecapeptide, Ser-Asp-Val-Thr-Lys-Arg-Gln-His-Pro-Gly-Arg-Arg-Phe, that was synthesized based on the sequence (residues 166-178) of a proposed cDNA for pro-TRH reported by Lechan et al. With this antiserum, immunostaining of Western blots of rat brain extracts revealed two major proteins with mol wt (Mr = 39,000 and 52,000) considerably larger than that of the largest protein (Mr = 29,000) that could be encoded by the cDNA of Lechan et al. Because these observations suggested the possibility of novel TRH precursors, we studied the immunocytochemical distribution of pro-TRH (39-52K) in rat brain. Our anatomical findings were 4-fold. 1) The distributions of 29K pro-TRH and 39-52K pro-TRH are not identical. 2) TRH is found only in regions containing 29K pro-TRH, 39-52K pro-TRH, or both. 3) There are regions that contain both 29K pro-TRH and 39-52K pro-TRH, but no TRH. 4) Regions containing only 39-52K pro-TRH do not contain 29K pro-TRH mRNA as mapped by Segerson et al. From these electrophoretic and anatomical observations, we postulate the existence of at least one and possibly two additional precursors that can be processed to TRH in rat brain.
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 occasional 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 have access. 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 regenerative specificity. These data support the hypothesis that the physical constraint imposed by the Schwann cell-derived basal lamina prevents regenerative specificity.
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