High morphological variation of vestibular system accompanies slow and infrequent locomotion in three-toed sloths

The semicircular canals (SCs), part of the vestibular apparatus of the inner ear, are directly involved in the detection of angular motion of the head for maintaining balance, and exhibit adaptive patterns for locomotor behaviour. Consequently, they are generally believed to show low levels of intraspecific morphological variation, but few studies have investigated this assumption. On the basis of high-resolution computed tomography, we present here, to our knowledge, the first comprehensive study of the pattern of variation of the inner ear with a focus on Xenarthra. Our study demonstrates that extant three-toed sloths show a high level of morphological variation of the bony labyrinth of the inner ear. Especially, the variation in shape, relative size and angles of their SCs greatly differ from those of other, faster-moving taxa within Xenarthra and Placentalia in general. The unique pattern of variation in three-toed sloths suggests that a release of selection and/or constraints on their organ of balance is associated with the observed wide range of phenotypes. This release is coincident with their slow and infrequent locomotion and may be related, among other possible factors, to a reduced functional demand for a precise sensitivity to movement.     

Bony labyrinth morphometry indicates locomotor adaptations in the squirrel-related clade (Rodentia,Mammalia)

The semicircular canals (SCs) of the inner ear detect angular acceleration and are located in the bony labyrinth of the petrosal bone. Based on high-resolution computed tomography, we created a size-independent database of the bony labyrinth of 50 mammalian species especially rodents of the squirrel-related clade comprising taxa with fossorial, arboreal and gliding adaptations. Our sampling also includes gliding marsupials, actively flying bats, the arboreal tree shrew and subterranean species. The morphometric anatomy of the SCs was correlated to the locomotion mode. Even if the phylogenetic signal cannot entirely be excluded, the main significance for functional morphological studies has been found in the diameter of the SCs, whereas the radius of curvature is of minor interest. Additionally, we found clear differences in the bias angle of the canals between subterranean and gliding taxa, but also between sciurids and glirids. The sensitivity of the inner ear correlates with the locomotion mode, with a higher sensitivity of the SCs in fossorial species than in flying taxa. We conclude that the inner ear of flying and gliding mammals is less sensitive due to the large information flow into this sense organ during locomotion.     

Convergence vs. Specialization in the ear region of moles (mammalia)

We investigated if and how the inner ear region undergoes similar adaptations in small, fossorial, insectivoran‐grade mammals, and found a variety of inner ear phenotypes. In our sample, afrotherian moles (Chrysochloridae) and the marsupial Notoryctes differ from most other burrowing mammals in their relatively short radii of semicircular canal curvature; chrysochlorids and fossorial talpids share a relatively long interampullar width. Chrysochlorids are unique in showing a highly coiled cochlea with nearly four turns. Extensive cochlear coiling may reflect their greater ecological dependence on low frequency auditory cues compared to talpids, tenrecids, and the marsupial Notoryctes. Correspondingly, the lack of such extensive coiling in the inner ear of other fossorial species may indicate a greater reliance on other senses to enable their fossorial lifestyle, such as tactile sensation from vibrissae and Eimer's organs. The reliance of chrysochlorids on sound is evident in the high degree of coiling and in the diversity of its mallear types, and may help explain the lack of any semiaquatic members of that group. The simplest mallear types among chrysochlorids are not present in the basal‐most members of that clade, but all extant chrysochlorids investigated to date exhibit extensive cochlear coiling. The chrysochlorid ear region thus exhibits mosaic evolution; our data suggest that extensive coiling evolved in chrysochlorids prior to and independently of diversification in middle ear ossicle size and shape. J. Morphol. 276:900–914, 2015. © 2015 Wiley Periodicals, Inc.     

雌激素及其受体在内耳发育和功能上的作用

雌激素在生殖系统、认知记忆系统、骨骼和神经的发育及其功能维持等多种生理功能中扮演了重要的作用。近年来,在内耳发育及其功能研究过程中,许多学者发现在听力和平衡系统功能上的性别差异可能归根因于不同性别的雌激素水平差异。这些研究表明,雌激素及其受体在内耳发育、听力和平衡系统功能维持上也具有重要作用。该文用一个新的视角聚焦于雌激素及其受体在内耳发育和功能上的研究进展。该综述能为进一步研究雌激素在听力和平衡系统中的作用机制及相关疾病的临床治疗提供参考。     

What did Morganucodon hear?

The structure of the middle and inner ear of Morganucodon , one of the oldest known mammals, is reviewed and compared to the structure of the ears of extant mammals, reptiles and birds with known auditory capabilities. Specifically, allometric relationships between ear dimensions (basilar-membrane length, tympanic-membrane area and stapes-footplate area) and specific features of the audiogram are defined in extant ears. These relationships are then used to make several predictions of auditory function in Morganucodon. The results point out that the ear structures of Morganucodon–Art similar in dimensions to ear structures in both extant small mammals–with predominantly high-frequency (10 kHz) auditory capabilities, and reptiles and birds- with better low and middle-frequency hearing (< 5 kHz). Although the allometric analysis cannot by itself determine whether Morganucodon heard more like present-day small mammals, or birds and reptiles, the apparent stiffness of the Morganucodon middle ear is both more consistent with the high-frequency mammalian middle ear and would act to decrease the sensitivity of a bird-reptile middle ear to low-frequency sound. Several likely hearing scenarios for Morganucodon are defined, including a scenario in which these animals had ears like those of modern small mammals that are selectively sensitive to high-frequency sounds, and a second scenario in which the Morganucodon ear was moderately sensitive to sounds of a narrow middle-frequency range (5–7 kHz) and relatively insensitive to sounds of higher or lower frequency. The evidence needed to substantiate either scenario includes some objective measure of the stiffness of the Morganucodon ossicular system, while a key datum needed to distinguish between the two hypotheses includes confirmation of the presence or absence of a cochlear lamina in the Morganucodon inner ear.     

Evolution of the tetrapod ear: an analysis and reinterpretation

The dominant view of tetrapod otic evolution–the “standard view”–holds that the tympanum developed very early in tetrapod history and is homologous in all tetrapods and that the opercular process of the rhipidistian hyomandibula is homologous to the tympanic process of the stapes in lower tetrapods. Under that view, the labyrinthodont amphibians of the Paleozoic are usually considered ancestral to reptiles, and thus the “otic notch” of labyrinthodonts and the tympanum it presumably contained form the starting-point for middle ear evolution in reptiles. Four problems have classically been identified with the standard view: the differing relationships of the internal mandibular branch of N. VII (chorda tympani) to the processes of the stapes in amniotes and anurans; the differing orientations of the stapes in key fossil and living groups; the location of the tympanum in early fossil reptiles; and the transferral of the tympanum, during the origin of mammals, from the stapes to the articular bone of the lower jaw. An examination of these problems and of the solutions proposed under the standard view reveals the ad hoc, and therefore unsatisfactory, nature of the proposed solutions. To organize and review alternative hypotheses of otic evolution an analytical table is constructed, using three characters (tympanic process, Nerve VII, tympanum), each with two possible states. A total of eight hypotheses about middle ear evolution are possible under this system, one of which is the standard view. The seven “non-standard” hypotheses, only five of which have been argued in the literature, are briefly examined. Six of the “non-standard” hypotheses appear unattractive for various reasons, including reliance on ad hoc arguments. The seventh was first proposed by Gaupp in 1898. It is today almost universally ignored but apparently largely for historical rather than scientific reasons. This hypothesis, her called the “alternative view”, appears to rest on assumptions equally as plausible as those of the standard view. Moreover, it offers a solution of the problems associated with the standard view without, apparently, raising any similarly serious problems. This paper compares the standard and alternative views of middle ear evolution in detail. Comparison proceeds on two levels. On one level, they are compared in terms of the hypotheses of phyletic tetrapod relationships each promotes and how strongly each supports its hypothesis. Both views promote the same hypothesis of tetrapod relationships. The alternative view is the more parsimonious, but the difference is not considered sufficient to provide a choice. On another level, the two views are compared in terms of their implications for: (1) the evolution of relative and absolute auditory perceptive ability; (2) the origin of reptiles; (3) the evolution of the suspensorium and cranial kinesis; and (4) the origin and evolution of recent amphibians. The nature of the data required for a test of the implications of the two views is specified in each case. Where data are available. the alternative view is consistent and the standard view is inconsistent with these data. We conclude that the alternative view is the preferable hypothesis of middle-ear evolution. This conclusion implies the following: the tympanic membranes and the tympanic processes of the stapes in recent mammals, reptiles + birds. and frogs. are not homologous; the evolution of “special periotic systems” in the ancestors of amphibians and amniotes were independent events and preceded the evolution of tympanic membranes; the amphibian tympanic membrane. probably including that of labyrinthodonts. is not ancestral to that of amniotes. and that labyiinthodonts with an otic notch are not suitable as amniote ancestors; the stapes of early reptiles functioned primarily as part of the jaw suspension rather than in hearing; the mechanisms and abilities of sound perception in recent tetrapods are likely to be diverse rather than forming parts of a cline; and the lack of a tympanum in Gymnophiona and Caudata may be a retention of a primitive condition.     

干细胞治疗感音神经性耳聋

治疗内耳疾病的主要困难之一是找到耳蜗毛细胞或者螺旋神经元丢失所导致的听力损失的治疗方法。本文讨论使用干细胞替代感觉细胞丢失为目的的几个治疗策略。作者最近在成年内耳中发现了可以分化为毛细胞的干细胞,发现了胚胎干细胞可在体外转化为毛细胞并表达毛细胞标记物。在动物模型中,成年内耳干细胞、神经干细胞和胚胎干细胞来源的前体细胞可分化成为毛细胞和神经细胞。本文将讨论使用干细胞再生损伤毛细胞的不同方法,介绍几种可行的动物模型,并讨论发展基于干细胞的细胞替代疗法治疗内耳损伤中存在的困难。     

STRUCTURE AND FUNCTION IN WHALE EARS

ABSTRACT

Ultrasonic echolocation abilities are well documented in several dolphin species, but hearing characteristics are unknown for most whales. Vocalization data suggest whale hearing spans infra- to ultrasonic ranges. This paper presents an overview of whale ear anatomy and analyzes 1) how whale ears are adapted for underwater hearing and 2) how inner ear differences relate to different hearing capacities among whales.

Whales have adaptations for rapid, deep diving and long submersion; e.g., broad- bore Eustachian tubes, no pinnae, and no air-filled external canals, that impact sound reception. In odontocetes, two soft tissue channels conduct sound to the ear. In mysticetes, bone and soft tissue conduction are likely. The middle ear is air-filled but has an extensible mucosa. Cochlear structures are hypertrophied and vestibular components are reduced. Auditory ganglion cell densities are double land mammal averages (2000–4000/mm). Basilar membrane lengths range 20–70 mm; gradients are larger than in terrestrial mammals. Odontocetes have 20–60% bony membrane support and basal ratios >0.6, consistent with hearing >150 kHz. Mysticetes have apical ratios <0.002 and no bony lateral support, implying acute infrasonic hearing. Cochlear hypertrophy may be adaptive for high background noise. Vestibular loss is consistent with cervical fusion. Exceptionally high auditory fiber counts suggest both mysticetes and odontocetes have ears “wired” for more complex signal processing mechanisms than most land mammals.