The labyrinth of Myxine : anatomy, ultrastructure and electrophysiology

Abstract

The anatomy of the labyrinth of Myxine is described. A torus-shaped cartilaginous capsule contains the membranous labyrinth. There are two ampullae, each with a ring-shaped crista, and the ventro-medial wall of the toroidal space is covered by a single macula communis with an otolith consisting of statoconia in a porous matrix. The characteristic ciliated chambers found in the labyrinth of the lamprey are absent. The sensory cells differ strikingly from those of the lamprey and those of all gnathostomes by having a kinocilium lacking the two central filaments and the basal foot. There are also differences in the topographic arrangement of the kinocilium vis-à-vis the stereocilia. The hair-cell map of the macula communis resembles that of the lamprey maculae at either end, the hair cells pointing their kinocilia forward at the anterior and backward at the posterior end. The orientation is rather confused in the middle portion, and an area corresponding to the saccular macula of the lamprey is missing. There is no macula-neglecta. Recordings from parts of the eighth nerve innervating the cristae of the two ampullae show sensitive and characteristic responses to angular accelerations in all planes of space including the horizontal yawing plane. By dint of their orientation within the labyrinth and the uniform topographic arrangement of the hair cells on them, the two cristae are differentially sensitive to roll, pitch, and yaw. Positional responses can be recorded from nerve fibres innervating the macula communis. These resemble those obtained from the labyrinth of the lamprey (Lowenstein 1970) and cover tilts about the longitudinal and transverse body axes. No responses to vibrational stimuli were obtained from any branch of the eighth nerve, and this result may well fit in with the absence of a well-defined sacculus portion of the macula communis and also with the absence of a macula neglecta. Rhythmic discharges as recorded from the labyrinth of the lamprey (Lowenstein 1970) were not encountered.

The developing lamprey ear closely resembles the zebrafish otic vesicle:otx1expression can account for all major patterning differences

“…A similar pattern, with two semicircular canals, is found in the abundant agnathan fossil record, making it likely that this represents the ancestral condition (Mazan et al, 2000). Hagfish also have a single macula communis on the ventral floor of the inner ear, but differ in having a single toroidal semicircular canal, which nevertheless is associated with two cristae, and is probably a derived characteristic (Lowenstein and Thornhill, 1970) (for a review, see Lowenstein, 1971). Although the general appearance of the inner ear is symmetrical about the AP axis in adult agnathan fishes, the macula communis can be subdivided into at least three morphologically distinct regions, the anterior horizontal, vertical and posterior horizontal regions (Lowenstein et al, 1968;Lowenstein and Thornhill, 1970;Hagelin, 1974), and there have been several different interpretations of the homology of these regions to the various gnathostome maculae (de Burlet and Versteegh, 1930;Lowenstein et al, 1968;Thornhill, 1972;Hagelin, 1974).…”

“…Hagfish also have a single macula communis on the ventral floor of the inner ear, but differ in having a single toroidal semicircular canal, which nevertheless is associated with two cristae, and is probably a derived characteristic (Lowenstein and Thornhill, 1970) (for a review, see Lowenstein, 1971). Although the general appearance of the inner ear is symmetrical about the AP axis in adult agnathan fishes, the macula communis can be subdivided into at least three morphologically distinct regions, the anterior horizontal, vertical and posterior horizontal regions (Lowenstein et al, 1968;Lowenstein and Thornhill, 1970;Hagelin, 1974), and there have been several different interpretations of the homology of these regions to the various gnathostome maculae (de Burlet and Versteegh, 1930;Lowenstein et al, 1968;Thornhill, 1972;Hagelin, 1974). Several studies describe hair cell planar polarity patterns in the late larval and adult lamprey and hagfish macula communis as roughly symmetrical about the AP axis (Lowenstein et al, 1968;Lowenstein and Thornhill, 1970;Thornhill, 1972), and Hagelin (Hagelin, 1974) tentatively proposes that both the anterior and posterior ends of the lamprey macula correspond to the gnathostome utricular macula.…”

Functional architecture of vestibular primary afferents from the posterior semicircular canal of a turtle, Pseudemys (Trachemys) scripta elegans

“…Many small diameter afferents have g-ratios below the "ideal" range due to their thick myelin sheaths. lar primary afferents are myelinated in all vertebrates except cyclostomes (Lowenstein and Thornhill, 1970;Landolt et al, 1973;Dunn, 19781, as are most efferents (Warr, 19751, we presume that the unmyelinated fibers in the posterior ampullary nerve are primarily autonomic fibers, probably postganglionic axons supplying labyrinthine blood vessels (Ross, 1973;Spoendlin, 1981;Hozawa and Kimura, 1989). A few may be autonomic afferents (Carpenter and Sutin, 1983).…”

Sensory and Nonsensory Ciliated Cells In the Ear of the Sea Lamprey, <i>Petromyzon marinus</i>

“…However, the sensory hair cells in the lampreys differ from those of gnathostome fishes and many other vertebrates in that they usually have striated organelles located between the nucleus and the apical cell membrane [Hoshino, 1975;Lowenstein and Osborne, 1964;Lowenstein et al, 1968]. Striated organelles are not found in the hair cells of the lateral line of Lampetra [Lowenstein et al, 1968], and Lowenstein and Thornhill [1970] make no mention of striated organelles in their ul trastructural study of the hair cells of the ear of the hagfish Myxine glutinosa.…”