Defining classes of spinal interneuron and their axonal projections in hatchling Xenopus laevis tadpoles

Neurobiotin was injected into individual spinal interneurons in the Xenopus tadpole to discern their anatomical features and complete axonal projectio n patterns. Four classes of interneuron are described, with names defining their primary axon projection: Dorsolateral ascending and commissural inter neurons are predominantly multipolar cells with somata and dendrites exclus ively in the dorsal half of the spinal cord. Ascending interneurons have un ipolar somata located in the dorsal half, but their main dendrites are loca ted in the ventral half of the spinal cord. Descending interneurons show bi gger variance in their anatomy, but the majority are unipolar, and they all have a descending primary axon. Dorsolateral commissural interneurons are clearly defined using established criteria, but the others are not, so clus ter analysis was used. Clear discriminations can be made, and criteria are established to characterize the three classes of interneuron with ipsilater al axonal projections. With identifying criteria established, the distribut ion and axonal projection patterns of the four classes of interneuron are d escribed. By using data from gamma -aminobutyric acid immunocytochemistry, the distribution of the population of ascending interneurons is defined. To gether with the results from the axonal projection data, this allows the as cending interneuron axon distribution along the spinal cord to be estimated . By making simple assumptions and using existing information about the som a distributions of the other interneurons, estimates of their axon distribu tions are made. The possible functional roles of the four interneuron class es are discussed. (C) 2001 Wiley-Liss, Inc.