Afferent nerve ending development and synaptogenesis in the vestibular epithelium of human fetuses

Molecular characterization and prospective isolation of human fetal cochlear hair cell progenitors

Sensory hair cells located in the organ of Corti are essential for cochlear mechanosensation. Their loss is irreversible in humans resulting in permanent hearing loss. The development of therapeutic interventions for hearing loss requires fundamental knowledge about similarities and potential differences between animal models and human development as well as the establishment of human cell based-assays. Here we analyze gene and protein expression of the developing human inner ear in a temporal window spanning from week 8 to 12 post conception, when cochlear hair cells become specified. Utilizing surface markers for the cochlear prosensory domain, namely EPCAM and CD271, we purify postmitotic hair cell progenitors that, when placed in culture in three-dimensional organoids, regain proliferative potential and eventually differentiate to hair cell-like cells in vitro. These results provide a foundation for comparative studies with otic cells generated from human pluripotent stem cells and for establishing novel platforms for drug validation.

Hearing requires mechanosensitive hair cells in the organ of Corti, which derive from progenitors of the cochlear duct. Here the authors examine human inner ear development by studying key developmental markers and describe organoid cultures from human cochlear duct progenitors for in vitro hair cell differentiation.

Anatomical and physiological development of the human inner ear

We describe the development of the human inner ear with the invagination of the otic vesicle at 4 weeks gestation (WG), the growth of the semicircular canals from 5 WG, and the elongation and coiling of the cochlea at 10 WG. As the membranous labyrinth takes shape, there is a concomitant development of the sensory neuroepithelia and their associated structures within. This review details the growth and differentiation of the vestibular and auditory neuroepithelia, including synaptogenesis, the expression of stereocilia and kinocilia, and innervation of hair cells by afferent and efferent nerve fibres. Along with development of essential sensory structures we outline the formation of crucial accessory structures of the vestibular system - the cupula and otolithic membrane and otoconia as well as the three cochlea compartments and the tectorial membrane. Recent molecular studies have elaborated on classical anatomical studies to characterize the development of prosensory and sensory regions of the fetal human cochlea using the transcription factors, PAX2, MAF-B, SOX2, and SOX9. Further advances are being made with recent physiological studies that are beginning to describe when hair cells become functionally active during human gestation. This article is part of a Special Issue entitled <Annual Reviews 2016>.

Preliminary characterization of voltage-activated whole-cell currents in developing human vestibular hair cells and calyx afferent terminals

We present preliminary functional data from human vestibular hair cells and primary afferent calyx terminals during fetal development. Whole-cell recordings were obtained from hair cells or calyx terminals in semi-intact cristae prepared from human fetuses aged between 11 and 18 weeks gestation (WG). During early fetal development (11–14 WG), hair cells expressed whole-cell conductances that were qualitatively similar but quantitatively smaller than those observed previously in mature rodent type II hair cells. As development progressed (15–18 WG), peak outward conductances increased in putative type II hair cells but did not reach amplitudes observed in adult human hair cells. Type I hair cells express a specific low-voltage activating conductance, G_K,L. A similar current was first observed at 15 WG but remained relatively small, even at 18 WG. The presence of a “collapsing” tail current indicates a maturing type I hair cell phenotype and suggests the presence of a surrounding calyx afferent terminal. We were also able to record from calyx afferent terminals in 15–18 WG cristae. In voltage clamp, these terminals exhibited fast inactivating inward as well as slower outward conductances, and in current clamp, discharged a single action potential during depolarizing steps. Together, these data suggest the major functional characteristics of type I and type II hair cells and calyx terminals are present by 18 WG. Our study also describes a new preparation for the functional investigation of key events that occur during maturation of human vestibular organs.

Inner ear formation during the early larval development of Xenopus laevis

The formation of the eight independent endorgan compartments (sacculus, utricle, horizontal canal, anterior canal, posterior canal, lagena, amphibian papilla, and basilar papilla) of the Xenopus laevis inner ear is illustrated as the otic vesicle develops into a complex labyrinthine structure. The morphology of transverse sections and whole-mounts of the inner ear was assessed in seven developmental stages (28, 31, 37, 42, 45, 47, 50) using brightfield and laser scanning confocal microscopy. The presence of mechanosensory hair cells in the sensory epithelia was determined by identification of stereociliary bundles in cryosectioned tissue and whole-mounts of the inner ear labeled with the fluorescent F-actin probe Alexa-488 phalloidin. Between stages 28 and 45, the otic vesicle grows in size, stereociliary bundles appear and increase in number, and the pars inferior and pars superior become visible. The initial formation of vestibular compartments with their nascent stereociliary bundles is seen by larval stage 47, and all eight vestibular and auditory compartments with their characteristic sensory fields are present by larval stage 50. Thus, in Xenopus, inner ear compartments are established between stages 45 and 50, a 2-week period during which the ear quadruples in length in the anteroposterior dimension. The anatomical images presented here demonstrate the morphological changes that occur as the otic vesicle forms the auditory and vestibular endorgans of the inner ear. These images provide a resource for investigations of gene expression patterns in Xenopus during inner ear compartmentalization and morphogenesis.

Ultrastructural analysis of [3H]thymidine-labeled cells in the rat utricular macula

Ototoxic drugs stimulate cell proliferation in adult rat vestibular sensory epithelia, as does the infusion of transforming growth factor alpha (TGFalpha) plus insulin. We sought to determine whether new hair cells can be regenerated by means of a mitotic pathway. Previously, studies have shown that the nuclei of some newly generated cells are located in the lumenal half of the sensory epithelium, suggesting that some may be newly generated sensory hair cells. The aim of this study was to examine the ultrastructural characteristics of newly proliferated cells after TGFalpha stimulation and/or aminoglycoside damage in the utricular sensory epithelium of the adult rat. The cell proliferation marker tritiated-thymidine was infused, with or without TGFalpha plus insulin, into the inner ears of normal or aminoglycoside-damaged rats for 3 or 7 days by means of osmotic pumps. Autoradiographic techniques and light microscopy were used to identify cells synthesizing DNA. Sections with labeled cells were re-embedded, processed for transmission electron microscopy, and the ultrastructural characteristics of the labeled cells were examined. The following five classes of tritiated-thymidine labeled cells were identified in the sensory epithelium: (1) labeled cells with synaptic specializations that appeared to be newly generated hair cells, (2) labeled supporting cells, (3) labeled leukocytes, (4) labeled cells that we have classified as "active cells" in that they are relatively nondescript but contain massive numbers of polyribosomes, and (5) labeled degenerating hair cells. These findings suggest that new hair cells can be generated in situ by means of a mitotic mechanism in the vestibular sensory epithelium of adult mammals.

Postnatal development of type I and type II hair cells in the mouse utricle: acquisition of voltage-gated conductances and differentiated morphology

The type I and type II hair cells of mature amniote vestibular organs have been classified according to their afferent nerve terminals: calyx and bouton, respectively. Mature type I and type II cells also have different complements of voltage-gated channels. Type I cells alone express a delayed rectifier, gK,L, that is activated at resting potential. We report that in mouse utricles this electrophysiological differentiation occurs during the first postnatal week. Whole-cell currents were recorded from hair cells in denervated organotypic cultures and in acutely excised epithelia. From postnatal day 1 (P1) to P3, most hair cells expressed a delayed rectifier that activated positive to resting potential and a fast inward rectifier, gK1. Between P4 and P8, many cells acquired the type I-specific conductance gK,L and/or a slow inward rectifier, gh. By P8, the percentages of cells expressing gK,L and gh were at mature levels. To investigate whether the electrophysiological differentiation correlated with morphological changes, we fixed utricles at different times between P0 and P28. Ultrastructural criteria were developed to classify cells when calyces were not present, as in cultures and neonatal organs. The morphological and electrophysiological differentiation followed different time courses, converging by P28. At P0, when no hair cells expressed gK,L, 33% were classified as type I by ultrastructural criteria. By P28, approximately 60% of hair cells in acute preparations received calyx terminals and expressed gK,L. Data from the denervated cultures showed that neither electrophysiological nor morphological differentiation depended on ongoing innervation.

Spatiotemporal development of cochlear innervation and hair cell differentiation in the rat

The apical cytoskeleton of cochlear hair cells is largely comprised of actin microfilaments and actin-associated proteins, of which fodrin is one of the most prominent. We studied the development of this mechanosensory apical portion of cochlear hair cells of the rat by fluorescence microscopy using rhodamine conjugated phalloidin to detect F-actin and an antibody against alpha-fodrin. An antibody against the 160 kDa neurofilament polypeptide was used for tracing nerve fibers. The first sign of differentiation of the mechanosensory region, actin-containing stereocilia, was observed on the 19th gestational day in the inner hair cells of the basal coil. The appearance of expression of cytoskeletal actin in the cochlear hair cells proceeded gradientally from basal to apical coil and from inner to outer hair cells. Corresponding maturation sequences were observed in the development of fodrin immunoreactivity in the cuticular plates, but the first evidence of this reactivity was found one day later than the appearance of stereocilia in the hair cells at the same location. Also the penetration of neurofilament-positive neurites into the sensory epithelium followed the same kind of longitudinal and radial maturation gradients throughout the cochlea. Fibers were revealed beneath the sensory cells shortly before the first appearance of differentiation of their mechanosensory region. The results suggest that ingrowing nerve fibers may influence the timing of the apical cytoskeleton differentiation in cochlear hair cells or that both these processes could be controlled by the same external signals that are gradientally expressed throughout the cochlea.

Vestibular site of action of hypothyroidism in the pigmented rat

The vestibular cell type affected by congenital hypothyroidism (CH) was investigated by measuring the activity of glutamate decarboxylase (GAD) and choline acetyltransferase (ChAT), synthesizing enzymes of putative afferent (GABA) and efferent (acetylcholine, ACh) neurotransmitters and thus, respectively, hair cell I and II (HC-I, HC-II), and efferent terminal (ET) marker enzymes, in vestibular homogenates of control, congenitally hypothyroid rats (CHR) and in thyroxine-replaced CHR (CHR-T4) whose postnatal age ranged from 20 to 60 days old. In the vestibule, CH-II and its efferent cholinergic contacting bouton mature prior to thyroid function whereas HC-I-differentiation and its efferent synapse arrival are the latest events in vestibular maturation. Therefore, a differential effect of CH upon GAD and ChAT in CHR could be anticipated. In control rats as in CHR the magnitude of GAD was the same with time starting on the 20th day. In CHR, ChAT gradually diminished beginning on day 28 to become 45% decreased with respect to control on the 60th postnatal day. Prevention of ChAT decrease in CHR by early administration of thyroxine (T4), a striking diminution of T4 and triiodothyronine (T3) in CHR serum and a normal level of these hormones found in CHR-T4 corroborated thyroid involvement. These results confirm the preference of hypothyroidism to affect cholinergic cell types (or compartments) of late maturation (HC-I-containing ET and hence 45% ChAT decrease) leaving HC-I, HC-II and HC-II-connecting ET untouched, supported by a 55% remanent ChAT and a constant GAD activity regardless of time and treatment.

Differentiation of the inner ear sensory epithelia of Proteus anguinus (Urodela, amphibia)

The stages of differentiation of the inner ear sensory epithelia of the neotenous cave urodele, Proteus anguinus, was studied with light and electron microscopy. Comparative ultrastructural analysis among specimens of different sizes confirms that new sensory cells may be generated throughout life, particularly along the periphery of the saccular macula. The inner ear of Proteus contains at least four types of sensory cells that differ in their apical ciliary part. The lungs and air-filled buccal cavity may function as transducers of sound pressure in underwater conditions. Sound waves might be transmitted from the buccal cavity to the connected oval window. The very complex orientation of the sensory hair cells of the saccular macula and the large overlying saccular otoconial mass suggest that this macula facilitates orientation of Proteus in its underground aqueous habitat.

Ultrastructural study of striated organelles in vestibular sensory cells of human fetuses

The presence of striated organelles in the vestibular receptor of human fetuses of 10 to 22 weeks gestation was investigated. They were very frequent under the cuticular plate of the sensory cells, especially in type-I hair cells. Cross-sections of striated organelles showed that there were twice as many filaments in the dark bands as in the light bands. These filaments were often arranged in broken lines. Striated organelles were frequently associated with microtubules, mitochondria, and endoplasmic reticulum, and were fused with the plasma membrane forming an incomplete ring in the cytoplasm of the cells. We discuss the possibility that in adults, these striated organelles participate in an active mechanism that regulates the transduction of stimuli by means of a feedback control of the apical part of the calyx.

Appearance and distribution of neuron-specific enolase and calbindin (CaBP 28 kDa) in the developing human inner ear

The onset and development of neuron-specific enolase (NSE) and calbindin immunoreactivities were studied in the inner ear of human fetuses aged from 6-7 to 14 weeks of gestation. NSE occurred very early in ganglion neurons. Its appearance in vestibular sensory cells at 8 weeks coincided with the formation of the first afferent synapses, and showed an apex/base gradient in the cristae. Calbindin was found in vestibular ganglion neurons at 6-7 weeks and in the cochlear ganglion neurons at 8-9 weeks. Vestibular sensory cells and the whole ventral wall of the cochlear duct were stained from 8-9 weeks. At 14 weeks, calbindin staining occurred only in the sensory cells of the cochlear neuroepithelium. Non-neuronal secretory structures, i.e. Kölliker's organ and some cells of the transitional zone of the utricle, were also reactive. Staining appeared in Kölliker's organ with a base to apex gradient and disappeared from it with an internal to external gradient. Calbindin appeared in vestibular sensory cells later than NSE staining, synapse formation and sensory hair bundle differentiation. By contrast in the cochlea, calbindin staining appeared in the neuroepithelium before sensory cell differentiation, but remained only in the hair cells after they had differentiated and been contacted by the afferent fibers.

Early innervation and differentiation of hair cells in the vestibular epithelia of mouse embryos: SEM and TEM study

Early afferent innervation and differentiation of sensory vestibular cells were studied in mouse embryos from gestation day (GD) 13 to 16. Afferent neurites were found as early as GD 13 in the epithelium when there were no clearly differentiated sensory cells. By GD 14 the earliest sensory cells which exhibited short hair bundles at their luminal pole were then contacted by afferent endings at their basal part. On GD 15 nerve endings establishing specialized synaptic contacts, characterized by asymmetrical membrane densities and synaptic bodies, were observed. At this stage, microtubules contacting the presynaptic membranes, as well as coated vesicles were found. On GD 16 the hair cells were multi-afferented and numerous synaptic bodies were found. These results showing a concomitance between the hair cell differentiation and the establishment of nerve contacts are discussed with particular respect to nerve-hair cell interactions during sensory differentiation. This study does not point to a primary induction of vestibular hair cell differentiation by nerve endings, but it is consistent with the possibility that the ingrowth of nerve fibers is one of many factors that influence the differentiation of receptor cells. With respect to synapse formation, it is assumed that the location of synaptic bodies at presynaptic densities is determined by the arrival of afferent nerve endings.

Ontogenetic approach to cellular localization of neurotransmitters in the chick vestibule

Gamma-aminobutyric acid (GABA) and acetylcholine (Ach) have been implicated in afferent and efferent neurotransmission, respectively, in the vestibular sensory periphery. Assuming that glutamate decarboxylase (GAD), the GABA-synthesizing enzyme, and choline acetyltransferase (ChAT), the enzyme for synthesis of acetylcholine, are located in distinct cell types of the inner ear whose maturation occurs at different times during ontogenesis, we measured these enzymes in the ampullary cristae of embryonic chicks at different stages of development. By making these measurements in parallel with electron-microscopic studies of the different cell elements of the chick vestibular sensory periphery, we found that the values of GAD activity were nearly the same from the earliest stage studied, i.e., the 13th day of ontogeny to day 18 of embryonic development, paralleling the morphologically mature appearance of the hair cells and their afferent synapses. A slight increase in enzymatic activity from day 19 of ontogeny to one day after hatching corresponded to a rise in the number of afferent synapses. In contrast, ChAT activity was practically undetectable up to day 17 of embryonic development, but rose suddenly on the 19th day, reaching 1-day-old levels by day 20 of ontogenesis in coincidence with an elevation in the number of well-developed efferent boutons. These results are in accord with the localization of GAD in the sensory cells and a localization of ChAT in the efferent nerve endings. These findings suggest that GABA and Ach are the respective neuromediators for the afferent and efferent systems.