Innervation patterns of the lateral line stitches of the clawed frog, Xenopus laevis, and their reorganization during metamorphosis.

We quantitatively examined the afferent innervation pattern of the lateral line stitches of both larval and postmetamorphotic clawed frogs, Xenopus laevis, using a silver staining technique. We also studied the relevance of the number of neuromasts in a stitch to physiological properties, recording afferent activity with an electrode inserted directly into the neuromast. The innervation pattern changed during early metamorphosis, the fiber thickness increasing after the reorganization. We found three different innervation patterns: in type A stitches, the same two afferent fibers innervate all neuromasts; in type B stitches, one or two fibers innervate more than one stitch; in type C stitches, three to six fibers innervate a stitch. The distribution of the different types of stitches varied in different parts of the body. The frequency of type A stitches differed between larval trunk and larval head. For both larvae and juveniles, type B stitches were more frequent on ventral than dorsal areas, while type C stitches were more frequent on the head than on the trunk. Electrophysiological experiments indicated that the sensitivity of an afferent fiber increases with the number of neuromasts it innervates. This increase and the variation in innervation patterns shows that the single afferent fiber, not the stitch, is the functional unit of the lateral line periphery.