The inner ear of the common rhea (Rhea americana L.).

Abstract

The morphology of the inner ear in rheas was examined by light and electron microscopy. The shape is typically bird-like with very long semicircular canals. The anterior and posterior cristae have small septa cruciata. The vestibular sensory epithelia contain two main types of hair cell innervation; bouton-innervated hair cells and calyceal hair cells characterized by a surrounding nerve calyx. The utricular macula has a single zone of calyceal hair cells, while all other previously examined birds, except the mute swan, have 2 zones. The height of the tallest sensory hairs of the cristae is 20-30 microns. In the utricular and lagenar macula, the hairs are 5-7 microns in the striola and 10-20 microns in the main parts of the sense organs. Along the edges of the maculae the longest hairs may reach 20-30 microns. The number of stereovilli on mature vestibular hair cells is 40-60. The sensory hairs of the hearing organ, the basilar papilla, are generally shorter but more numerous than the vestibular sensory hairs. In the proximal end, the tallest of the 175-200 stereovilli are 2.8-3.7 microns; in the distal end of the papilla, the number of stereovilli decrease to 65-100, and their height increases gradually to 7.3-8.7 microns. The neural sensory hairs are generally taller than those of the abneural side.