VISUALIZATION OF NEWT ARAGONITIC OTOCONIAL MATRICES USING TRANSMISSION ELECTRON-MICROSCOPY

Otoconia are calcified protein matrices within the gravity-sensing org ans of the vertebrate vestibular system. These protein matrices are th ought to originate from the supporting or hair cells in the macula dur ing development. Previous studies of mammalian calcitic, barrel-shaped otoconia revealed an organized protein matrix consisting of a thin pe ripheral layer, a well-defined organic core and a flocculent matrix in between. No studies have reported the microscopic organization of the aragonitic otoconial matrix, despite its protein characterization. Pot e et al. (1993b) used densitometric methods and inferred that prismati c (aragonitic) otoconia have a peripheral protein distribution, compar ed to that described for the barrel-shaped, calcitic otoconia of birds , mammals, and the amphibian utricle. By using tannic acid as a negati ve stain, we observed three kinds of organic matrices in preparations of fixed, decalcified saccular otoconia from the adult newt: (1) fusif orm shapes with a homogenous electron-dense matrix; (2) singular and m ultiple strands of matrix; and (3) more significantly, prismatic shape s outlined by a peripheral organic matrix. These prismatic shapes rema in following removal of the gelatinous matrix, revealing an internal a rray of organic matter. We conclude that prismatic otoconia have a lar gely peripheral otoconial matrix, as inferred by densitometry.