Mears S C, Frank E
Department of Neurobiology, University of Pittsburgh School of Medicine, PA 15261, USA.
Int J Dev Neurosci. 1996 Oct;14(6):731-7. doi: 10.1016/s0736-5748(96)00061-5.
During development, the projections that sensory neurons make within the spinal cord are influenced by the specific targets they contact in the periphery. If sensory ganglia normally supplying principally cutaneous targets are forced to grow into limb muscles, in early stage tadpoles, many sensory neurons within these ganglia innervate limb muscles and subsequently develop spinal projections appropriate for muscle spindle afferents. If the same procedure is performed with adult frogs, however, these novel projections do not form. In this study, we have determined the developmental stages at which this sensitivity to peripheral targets exists. Axons from sensory neurons in thoracic (largely cutaneous) dorsal root ganglia were re-routed into the front leg at various stages through metamorphosis, and the central spinal projections of these re-routed fibers were assessed with HRP labeling. We found that thoracic sensory axons could be made to project to limb muscles throughout development, but that the central projections of these neurons were only appropriate for spindle afferents if the fibers were re-routed before stage XVIII, shortly before metamorphic climax. Because sensory neurons can regenerate specifically into the appropriate spinal laminae even in adult frogs, these results suggest that changes in either the DRG or the arm musculature occur by stage XVII so that DRG neurons cannot respond to novel peripheral targets.
在发育过程中，感觉神经元在脊髓内形成的投射受其在周围所接触的特定靶标的影响。如果在早期蝌蚪中，通常主要为皮肤靶标提供神经支配的感觉神经节被迫生长到肢体肌肉中，这些神经节内的许多感觉神经元会支配肢体肌肉，并随后形成适合肌梭传入纤维的脊髓投射。然而，如果对成年青蛙进行同样的操作，这些新的投射则不会形成。在本研究中，我们确定了这种对周围靶标敏感的发育阶段。在变态发育的各个阶段，将来自胸段（主要为皮肤感觉）背根神经节感觉神经元的轴突重新引导至前腿，并通过辣根过氧化物酶（HRP）标记评估这些重新引导的纤维的中枢脊髓投射。我们发现，胸段感觉轴突在整个发育过程中都可以被引导至肢体肌肉，但只有在变态发育高潮前不久的第十八阶段之前重新引导纤维，这些神经元的中枢投射才适合肌梭传入纤维。由于即使在成年青蛙中感觉神经元也能特异性地再生到合适的脊髓板层，这些结果表明，在第十七阶段DRG或臂部肌肉组织发生了变化，使得DRG神经元无法对新的周围靶标做出反应。
