FURTHER-STUDIES ON THE MECHANICS OF THE COCHLEAR PARTITION IN THE MOUSTACHED BAT .1. ULTRASTRUCTURAL OBSERVATIONS ON THE TECTORIAL MEMBRANEAND ITS ATTACHMENTS

From semithin and ultrathin sections of the mustached bat cochlea, bas o-apical gradients in ultrastructural composition, shape and attachmen t site of thr tectorial membrane (TM) were determined in relation to g radients in hair cell size and stereocilia size. These provide a data base for estimates of the mechanical properties of the organ of Corti as they relate to specialized aspects of the cochlear frequency map (K ossl and Vater, 1996). As in other mammals, the TM is composed of type A and type B protofibrils. Measurements of the packing density of typ e A protofibrils reveal gradients in both the radial and longitudinal direction. Distinct variations in packing density of type A protofibri ls across the radial extent of the TM allow the definition of more sub regions than in other mammals. Throughout the cochlea, packing density is highest in the 'stripe' region located close to the spiral limbus. The centrally located 'core' region of the middle zone contains disti nctly fewer type A protofibrils than the laterally located 'mantle' re gion of the middle zone. The TM in the specialized basal turn (first a nd second half-turns) features a higher packing density of type A prot ofibrils in the 'mantle' than the TM in the apical cochlea (upper thir d to fifth half-turns), and an incorporation of longitudinally directe d type A protofibrils in the marginal zone. Among cochlear turns, ther e are pronounced changes in cross-sectional area of the TM and the ext ent of its limbal attachment site. Within the densely innervated secon d half-turn that contains an expanded representation of the 60 kHz con stant frequency (CF) component of the echolocation signal, both the cr oss-sectional area (see also Henson and Henson, 1991) and the attachme nt site of the TM are enlarged. An extended limbal attachment site is also observed in the densely innervated region of the lower first half -turn that represents the upper harmonics of the call. Within the spar sely innervated region of the upper first half-turn, the limbal attach ment site of the TM is significantly diminished. Size of outer hair ce lls (OHC) ranges between 12 and 13 mu m throughout the basal 80% of co chlear length and reaches maximal values of 20 mu m in the apex. Size of OHC stereocilia ranges between 0.7 and 0.8 mu m throughout the basa l 60% of cochlear length and reaches a maximal size of 2.2 mu min the apex. These data corroborate and extend previous notions that morpholo gical specializations of the TM in concert with specializations of the basilar membrane and perilymphatic spaces play an integral role in cr eating specialized cochlear tuning in the mustached bat.