Saccule

Abstract: The molecular mechanisms dictating the morphogenesis and differentiation of the mammalian inner ear are largely unknown. To better elucidate the normal development of this organ, two approaches were taken. First, the membranous labyrinths of mouse inner ears ranging from 10.25 to 17 d postcoitum (dpc) were filled with paint to reveal their gross development. Particular attention was focused on the developing utricle, saccule, and cochlea. Second, we used bone morphogenetic protein 4 (BMP4) and lunatic fringe (Fng) as molecular markers to identify the origin of the sensory structures. Our data showed that BMP4 was an early marker for the superior, lateral, and posterior cristae, whereas Fng served as an early marker for the macula utriculi, macula sacculi, and the sensory portion of the cochlea. The posterior crista was the first organ to appear at 11.5 dpc and was followed by the superior crista, the lateral crista, and the macula utriculi at 12 dpc. The macula sacculi and the cochlea were present at 12 dpc but became distinguishable from each other by 13 dpc. Based on the gene expression patterns, the anterior and lateral cristae may share a common origin. Similarly, three sensory organs, the macula utriculi, macula sacculi, and cochlea, seem to arise from a single region of the otocyst.

Abstract: Adaptation in a vestibular organ, the bullfrog's sacculus, was studied in vivo and in vitro. In the in vivo experiments, the discharge of primary saccular neurons and the extracellular response of saccular hair cells were recorded during steps of linear acceleration. The saccular neurons responded at the onset of the acceleration steps, then adapted fully within 10-50 msec. The extracellular (microphonic) response of the hair cells adapted with a similar time course, indicating that the primary sources of the neural adaptation are peripheral to the afferent synapse--in the hair cell, its mechanical inputs, or both. Evidence for hair cell adaptation was provided by 2 in vitro preparations: after excising the sacculus and removing the accessory structures, we recorded either the extracellular hair cell response to displacement of the otolithic membrane or the intracellular hair cell response to hair bundle displacement. In both cases the response to a step stimulus adapted. The adaptation involved a shift in the displacement-response curve along the displacement axis, so that the cell's operating point was reset toward the static position of its hair bundle. This displacement shift occurred in response to both depolarizing and hyperpolarizing stimuli. Its time course varied among cells, from tens to hundreds of milliseconds, and also varied with the concentration of Ca2+ bathing the apical surfaces of the hair cells. Voltage-clamp experiments suggested that the displacement shift does not depend simply on ion entry through the hair cell's transduction channels and can occur at a fixed membrane potential. The possible role of the displacement-shift process in the function of the frog's sacculus as a very sensitive vibration detector is discussed.

Abstract: Recordings were made from single afferent fibers in the inferior vestibular nerve, which innervates the saccule and posterior semicircular canal. A substantial portion of the fibers with irregular background activity increased their firing in response to moderately intense clicks and tones. In responsive fibers, acoustic clicks evoked action potentials with minimum latencies of 80 dB SPL) caused synchronization of spikes to preferred phases of the tone cycle. PUSH and PULL fibers had preferred response phases approximately 180 degrees apart. These two response classes are consistent with fibers that innervate hair cells having opposite morphological polarizations, an arrangement found in the saccule. With low-frequency tone bursts, sound levels of > or = 90 dB SPL evoked increases in mean spike rate. Spike rates increased monotonically with sound level without saturating at levels < or = 115 dB SPL. Contraction of the middle-ear muscles decreased responses to sound, consistent with the sound transmission path being through the middle ear. Several fibers were labeled with biocytin and traced. All labeled fibers had bipolar cell bodies in the inferior vestibular ganglion with peripheral processes extending toward the saccular nerve and central processes in the vestibular nerve. Two fibers were traced to the saccular epithelium. One fiber was traced centrally and arborized extensively in vestibular nuclei and a region ventromedial to the cochlear nucleus. Our results confirm and extend previous suggestions that the mammalian saccule responds to sound at frequencies and levels within the normal range of human hearing. We suggest a number of auditory roles that these fibers may play in the everyday life of mammals.

Abstract: The mammalian inner ear contains two sensory organs, the cochlea and vestibule. Their sensory neuroepithelia are characterized by a mosaic of hair cells and supporting cells. Cochlear hair cells differentiate in four rows: a single row of inner hair cells (IHCs) and three rows of outer hair cells (OHCs). Recent studies have shown that Math1, a mammalian homolog of Drosophila atonal is a positive regulator of hair cell differentiation. The basic helix-loop-helix (bHLH) genes Hes1 and Hes5 (mammalian hairy and Enhancer-of-split homologs) can influence cell fate determination by acting as negative regulators to inhibit the action of bHLH-positive regulators. We show by using reverse transcription-PCR analysis that Hes1, Hes5, and Math1 are expressed in the developing mouse cochleae. In situ hybridization revealed a widespread expression of Hes1 in the greater epithelial ridge (GER) and in lesser epithelial ridge (LER) regions. Hes5 is predominantly expressed in the LER, in supporting cells, and in a narrow band of cells within the GER. Examination of cochleae from Hes1(-/-) mice showed a significant increase in the number of IHCs, whereas cochleae from Hes5(-/-) mice showed a significant increase in the number of OHCs. In the vestibular system, targeted deletion of Hes1 and to a lesser extent Hes5 lead to formation of supernumerary hair cells in the saccule and utricle. The supernumerary hair cells in the mutant mice showed an upregulation of Math1. These data indicate that Hes1 and Hes5 participate together for the control of inner ear hair cell production, likely through the negative regulation of Math1.