Innervation of a lizard auditory organ having gap junctions between most hair cells: a serial transmission electron microscopy study.

Two apical unidirectional and 16 basal bidirectional papillar hair cells of the yucca night lizard, Xantusia vigilis, were serially sectioned for transmission electron microscopy (TEM) to determine the pattern of hair cell innervation. The 16 bidirectional hair cells (central group) were sectioned across the entire width of the papilla and consisted of four complete hair cells in each of the first three rows and the upper (or neural) half of the four hair cells in the fourth or last row. Both hair cell types were nonexclusively innervated, i.e., each afferent nerve fiber innervated two or more hair cells. The apical unidirectional hair cells were innervated by six or seven different afferent nerve fibers and five or six efferent fibers. The afferent nerve fibers made an average of 52.5 synapses/hair cell. In the central group of 16 bidirectional hair cells, 25 different afferent nerve fibers innervated an average of 4.5 hair cells. The average number of hair cells innervated by the eight afferent nerve fibers limited to the central group was 5.4. An unusual finding was the presence of gap junctions directly interconnecting more than half the hair cells in both papillar segments. In the bidirectional hair cell region, it was possible to count the number of gap junctions between 24 contiguous hair cells. The average number of gap junctions was four per hair cell, and all bidirectional hair cells were either directly or indirectly interconnected by gap junctions. The possible functions of a nonexclusive type of hair cell innervation and the presence of large numbers of gap junctions are discussed.