Observation in inner ear of tree shrew using scanning electron microscope and the Atoh1 distribution in cochlea

This study aimed to observe the ultrastructure on the surface of the inner ear of a normal tree shrew using scanning electron microscope (SEM). The specimens of cochlea, macula utriculi, macula sacculi, and crista ampullaris of the normal adult tree shrew were collected and observed by SEM. We used immunofluorescence for cochlear protein Atoh1 staining. We observed that cochlea of the tree shrew is centered on the cochlear axis, circling about 3.5 times from bottom to top of the cochlea. The organ of Corti is located between medial and lateral grooves, including inner and outer hair cells as well as supporting cells. Maculae staticae include macula of saccule and macula of utricle, and the surface of macula is covered with a large number of otoliths. We found a gelatinous layer below the otoliths, followed by the layer of the honeycomb structure. The hair cell cilia of macula and crista ampullaris include one kinocilium and more stereocilia. There is no obvious cross structure but numerous hair cell cilia on semicircular canal crista ampullaris. Immunofluorescence staining showed that protein Atoh1 is mainly distributed in the nucleus of the cochlea's inner and outer hair cells. The observation of the inner ear structure under SEM elucidate the fine surface morphological structure of the entire cochlea, the vestibular maculae staticae, and crista ampullaris, providing new insight into the structure and function of the inner ear of tree shrew. HIGHLIGHTS: This article is the first to describe the inner ear ultrastructure of a small primate tree shrew by scanning electron microscopy (SEM). Under an SEM, the phalangeal processes of Deiter cells in tree shrews were observed to be connected to the tip of a neighboring hair cell, which was different from that of Deiters' cells in guinea pigs, and this crossed one hair cell, and connected to the tip of the third hair cell. It was observed that the crista ampullaris of tree shrews were horseshoe-shaped, and similar to that of humans and monkeys, this had no obvious "cross-shaped hump" structure. Tree shrew's ABR threshold value curve conforms to the mammalian U-shaped curve, wave III is the main wave of ARB, its sensory frequency may be higher 8 kHz, and the characteristics of the stereocilia of tree shrew we have observed may be related to the perception of higher frequency hearing.