Quantitative studies of auditory hair cells and nerves in lizards

Because the lizard cochlear duct is anatomically accessible as well as relatively simple in structure it is an excellent model in which to study auditory hair cells, nerve fibers, and innervational patterns. The objectives of this study were to determine the intra- and interspecific variations of auditory hair cell and nerve fiber numbers, nerve fiber/hair cell ratios, and nerve fiber sizes in a varied of lizard species and to relate these to auditory function and phylogeny. Hair cell numbers were determined by SEM and serial frontal sections of the papilla basilaris and nerve fiber numbers and diameters by use of a Zeiss TGZ3 particle counter. The coefficient of variation of hair cell numbers varied from 3.2 to 16.6 (171 specimens, 15 species) and of nerve fiber numbers from 1.2 to 14.4 (381 specimens, 35 species). There was no correlation between hair cell or nerve fiber number and age or sex. The nerve fiber number/hair cell number ratio was 3.5-11.1/1 in small papillae basilares of the iguanid-agamid-anguid type, 2.4-3.2/1 in the teiid type, and 0.6-1.5/1 in the larger specialized papillae of the scincid and gekkonid types. Nerve fibers varied in diameter from 0.8 to 6.0 microns (largest percentage were 2-4 microns) and were unimodally distributed. Larger nerve fibers usually supplied the unidirectionally oriented hair cells of the papilla basilaris. Variations in hair cell and nerve fiber numbers in other vertebrate classes and the functional and phylogenetic aspects of lizard papilla basilaris structure and innervation are discussed.