Distribution and innervation of lateral line organs in the axolotl


  • R. Glenn Northcutt

    1. Neurobiology Unit, Scripps Institution of Oceanography, and Department of Neurosciences, University of California, San Diego, La Jolla, California 92093–0201
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The lateral line system in axolotls consists of three types of receptors and the cranial nerves that innervate them. Superficial neuromasts, which are mechanoreceptors, are distributed in lines on both the head and trunk. Eight cephalic and three trunk lines can be distinguished on the basis of their innervation and differences in the orientation of the major axes of their neuromasts. A combination of histological techniques reveals that five separate pairs of cranial nerves innervate the neuromasts: anterodorsal lateral line nerves innervate cephalic supraorbital and infraorbital lines; anteroventral lateral line nerves innervate cephalic angular, oral, jugal, and preoperculomandibular lines of the cheek and lower jaw; middle and supratemporal lateral line nerves innervate the cephalic postotic lines; and posterior lateral line nerves innervate the trunk lines.

In addition to lines of neuromasts, a second class of mechanoreceptive lateral line organs, pit organs, also occur on the head. Pit organs are smaller than neuromasts but are innervated in a manner identical to that of neuromasts: the sensory hair cells of a single pit organ are innervated by two afferent fibers and a single efferent fiber. Although they occur in localized clusters rather than lines, the four distinct pit organ clusters occurring in axolotls are referred to as the anterior, middle, middle cheek, and gular pit lines as they appear to be homologous to the similarly named pit lines in fishes on the basis of their topography and innervation.

In addition to neuromasts and pit organs, ampullary organs comprise a third class of lateral line receptors and are restricted to the head. These electroreceptors occur singly or in small clusters adjacent to the neuromast lines and along the base of the external gills. The ampullary organs adjacent to the supraorbital and infraorbital lines are innervated by the anterodorsal lateral line nerves, whereas all other ampullary organs of the head are innervated by the anteroventral lateral line nerves. Thus postotic ampullary organs and neuromasts of the head are innervated by different lateral line nerves.

Examination of the sensory ganglia of the lateral line nerves with respect to the ganglia of the other cranial nerves indicates varying levels of fusion. The ganglia of the anterodorsal lateral line and profundal-trigeminal nerves are totally separate throughout their rostrocaudal extent. The ganglia of the anteroventral lateral line and facial nerves form a fused ganglionic complex with the larger pigmented cells of the anteroventral nerve occupying the dorsal portion of the ganglionic complex. The ganglia of the middle, supratemporal, and posterior lateral line nerves fuse with the ganglia of the glossopharyngeal and vagal nerves to form a single postotic ganglionic complex, where the large pigmented ganglionic cells of the postotic lateral line nerves are also restricted to the dorsal half of the complex.

Comparison of the lateral lines and their respective nerves in axolotls and other anamniotic vertebrates indicates that axolotls retain all lateral lines and nerves, except for an otic line of neuromasts, the posterior pit line, and the otic lateral line nerve. © 1992 Wiley-Liss, Inc.