HAIR-CELL COUNTS AND AFFERENT INNERVATION PATTERNS IN THE CRISTAE AMPULLARES OF THE SQUIRREL-MONKEY WITH A COMPARISON TO THE CHINCHILLA

1. The numbers of type I and type II hair cells were estimated by dissector techniques applied to semithin, stained sections of the horizontal, superior, and posterior cristae in the squirrel monkey and the chinchilla. 2. The crista in each species was divided into concentrically arranged central, intermediate, and peripheral zones of equal areas. The three zones con be distinguished by the sizes of individual hair cells and calyx endings, by the density of hair cells, and by the relative frequency of calyx endings innervating single or multiple type I hair cells. 3. In the monkey crista, type I hair cells outnumber type II hair cells by a ratio of almost 3:1. The ratio decreases from 4-5:1 in the central and intermediate zones to under 2:1 in the peripheral zone. For the chinchilla, the ratio is near 1:1 For the entire crista and decreases only slightly between the central and peripheral zones. 4. Nerve fibers supplying the cristae in the squirrel monkey were labeled by extracellular injections of horseradish peroxidase (HRP) into the vestibular nerve, peripheral terminations of individual fibers were reconstructed and related to the zones of the cristae they innervated and to the sizes of their parent axons. Results were similar for the horizontal, superior, and posterior cristae. 5. Axons seldom bifurcate below the neuroepithelium. Most Fibers begin branching shortly after crossing the basement membrane. Their terminal arbors are compact, usually extending no more than 50-100 mu m from the parent axon. A small number of long intraepithelial fibers enter the intermediate and peripheral zones of the cristae near its base, then Nn unbranched for long distances through the neuroepithelium to reach the central zone. 6. There are three classes of afferent fibers innervating the monkey crista. Calyx fibers terminate exclusively on type I hair cells, and bouton fibers end only on type II hair cells. Dimorphic fibers provide a mixed innervation, including calyx endings to type I hair cells and bouton endings to type II hair cells. Long intraepithelial fibers are calyx and dimorphic units, whose terminal fields are similar to those of other fibers. The central zone is innervated by calyx and dimorphic fibers; the peripheral zone, by bouton and dimorphic fibers; and the intermediate zone, by all three kinds of fibers. Internal (axon) diameters are largest for calyx fibers and smallest for bouton fibers. Of the entire sample of 286 labeled fibers, 52% were dimorphic units, 40% were calyx units, and 8% were bouton units. 7. Most calyx units in the monkey are unbranched, giving rise to a single calyx ending that innervates one to three adjacent type I hair cells. The terminal arbors of bouton fibers consist of fine collaterals that extend for 30-100 mu m from the parent axon and contain 15-70 endings, Dimorphic fibers have thick branches terminating as calyx endings and fine collaterals with bouton endings. Individual dimorphs have 1-4 calyx endings and from 1 to >100 bouton endings; those supplying the central zone have larger axons and simpler trees than those terminating in the peripheral zone. 8. Afferent branching patterns in the monkey cristae were compared with those previously described in the chinchilla cristae. Long intraepithelial fibers were seen in the monkey, but not in the chinchilla. In other respects, the afferent innervation is qualitatively similar in the two species. There are quantitative differences in the proportions of calyx, dimorphic, and bouton fibers, in the number of bouton endings provided by dimorphic fibers and in the number of type I hair cells contacted by calyx and dimorphic fibers. The differences serve to match the afferent innervation to the complement of hair cells found in each species.