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 lat eral 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 pr operties, recording afferent activity with an electrode inserted direc tly into the neuromast. The innervation pattern changed during early m etamorphosis, the fiber thickness increasing after the reorganization. We found three different innervation patterns: in type A stitches, th e same two afferent fibers innervate all neuromasts; in type B stitche s, one or two fibers innervate more than one stitch: in type C stitche s, three to six fibers innervate a stitch. The distribution of the dif ferent types of stitches varied in different parts of the body. The fr equency of type A stitches differed between larval trunk and larval he ad. 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 indic ated that the sensitivity of an afferent fiber increases with the numb er of neuromasts it innervates. This increase and the variation in inn ervation patterns shows that the single afferent fiber, not the stitch , is the functional unit of the lateral line periphery.