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The cells of origin of the trochlear nerve of urodeles, anurans and gymnophionans were labelled with HRP in order to compare the location and morphology of trochlear motoneurons and to find evidence for sensory fibers in the trochlear nerve of amphibians. Trochlear motoneuron perikarya were found in a ventral tegmental position predominantly on the contralateral side, but an ipsilateral cell was present in some specimens of urodeles and anurans. About 19 motoneurons were labelled in Ambystoma, about 60 motoneurons in Xenopus, and a maximum of 7 cells in Ichthyophis. Decussation of trochlear nerve fibers showed only in Xenopus a highly variable pattern. In urodeles, selective filling of the trochlear nerve labelled in addition to trochlear motoneurons a caudo-medical tectal group of about 20 neurons of the nucleus of the mesencephalic root of the trigeminal nerve. Gymnophionans showed also labelled cells of the mesencephalic trigeminal root in the caudal midbrain close to the trochlear nerve root. In some frogs, a few cells of the mesencephalic trigeminal root were labelled in the caudal tectum and occasionally in the velum medullare anterius. Comparison of the numbers of trochlear nerve fibers with HRP-labelled motoneurons revealed in Xenopus a proportion of 1.2:1, but of 2.7:1 in Ambystoma. However, counting both labelled motoneurons and cells of the mesencephalic trigeminal root resulted in a trochlear nerve fiber to labelled neuron proportion of 1.3:1 in Ambystoma much like in Xenopus. The numbers of superior oblique muscle fibers and of trochlear nerve fibers, but not of HRP-labelled motoneurons, increased significantly with size in Xenopus laevis. We suggest that increased peripheral branching of individual fibers within the trochlear nerve with size rather than differentiation of additional motoneurons takes place in growing postmetamorphic Xenopus. In contrast to other vertebrates studied so far, the trochlear nerve is a mixed nerve in Ambystoma and perhaps in Ichthyophis. Whether this reflects a primitive or a derived condition is at present unclear.
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