Hair cell development

The expression of p27 in the adult vestibular sensory organs and its possible roles

Vestibular hair cells (HCs) located in the inner ear are the receptors of vestibular sensory, which facilitates the human sense of balance. The detailed differentiation pattern and maturation process of the vestibular HCs are unclear now. p27, a cyclin/CDK inhibitor, plays a critical role in regulating the exit of cell cycle. We found that p27 was continuously expressed in the terminally differentiated and mature vestibular HCs using p27-P2A-iCreER/+; Rosa26-LSL-tdTomato/+ mice, suggesting p27 might have novel roles independent of its CDK inhibitory action. p27 is also reported to be associated with cell differentiation, cell migration and cell survival. We further explored the difference of p27 expression between two subtypes of vestibular HCs, and found that the proportion of p27-tdTomato positive type I vestibular HCs increased gradually along the subtype determination and maturation of vestibular HCs, suggesting that p27 might play a role in the HC subtype differentiation, maturation and function acquirement.

The cochlear sensory epithelium derives from Wnt responsive cells in the dorsomedial otic cup

Wnt1 and Wnt3a secreted from the dorsal neural tube were previously shown to regulate a gene expression program in the dorsal otic vesicle that is necessary for vestibular morphogenesis (Riccomagno et al., 2005. Genes Dev. 19, 1612-1623). Unexpectedly, Wnt1(-/-); Wnt3a(-/-) embryos also displayed a pronounced defect in the outgrowth of the ventrally derived cochlear duct. To determine how Wnt signaling in the dorsal otocyst contributes to cochlear development we performed a series of genetic fate mapping experiments using two independent Wnt responsive driver strains (TopCreER and Gbx2(CreER)) that when crossed to inducible responder lines (Rosa(lacZ) or Rosa(zsGreen)) permanently labeled dorsomedial otic progenitors and their derivatives. Tamoxifen time course experiments revealed that most vestibular structures showed some degree of labeling when recombination was induced between E7.75 and E12.5, consistent with continuous Wnt signaling activity in this tissue. Remarkably, a population of Wnt responsive cells in the dorsal otocyst was also found to contribute to the sensory epithelium of the cochlear duct, including auditory hair and support cells. Similar results were observed with both TopCreER and Gbx2(CreER) strains. The ventral displacement of Wnt responsive cells followed a spatiotemporal sequence that initiated in the anterior otic cup at, or immediately prior to, the 17-somite stage (E9) and then spread progressively to the posterior pole of the otic vesicle by the 25-somite stage (E9.5). These lineage-tracing experiments identify the earliest known origin of auditory sensory progenitors within a population of Wnt responsive cells in the dorsomedial otic cup.

Hair cell recovery in mitotically blocked cultures of the bullfrog saccule

Hair cells in many nonmammalian vertebrates are regenerated by the mitotic division of supporting cell progenitors and the differentiation of the resulting progeny into new hair cells and supporting cells. Recent studies have shown that nonmitotic hair cell recovery after aminoglycoside-induced damage can also occur in the vestibular organs. Using hair cell and supporting cell immunocytochemical markers, we have used confocal and electron microscopy to examine the fate of damaged hair cells and the origin of immature hair cells after gentamicin treatment in mitotically blocked cultures of the bullfrog saccule. Extruding and fragmenting hair cells, which undergo apoptotic cell death, are replaced by scar formations. After losing their bundles, sublethally damaged hair cells remain in the sensory epithelium for prolonged periods, acquiring supporting cell-like morphology and immunoreactivity. These modes of damage appear to be mutually exclusive, implying that sublethally damaged hair cells repair their bundles. Transitional cells, coexpressing hair cell and supporting cell markers, are seen near scar formations created by the expansion of neighboring supporting cells. Most of these cells have morphology and immunoreactivity similar to that of sublethally damaged hair cells. Ultrastructural analysis also reveals that most immature hair cells had autophagic vacuoles, implying that they originated from damaged hair cells rather than supporting cells. Some transitional cells are supporting cells participating in scar formations. Supporting cells also decrease in number during hair cell recovery, supporting the conclusion that some supporting cells undergo phenotypic conversion into hair cells without an intervening mitotic event.

Expression of BDNF and TrkB mRNAs in the crista neurosensory epithelium and vestibular ganglia following ototoxic damage

Following ototoxic lesion with the aminoglycoside gentamicin, the vestibular neurosensory epithelia undergo degeneration and then limited spontaneous regeneration. The spatio-temporal expression of brain-derived neurotrophic factor (BDNF) and of its high affinity receptor (trkB) mRNA was investigated in the vestibular end organs and ganglia of chinchillas following gentamicin ototoxicity. In the vestibular ganglia of untreated chinchillas, the level of expression of BDNF mRNA is low. At 1 and 2 weeks after intraotic treatment with gentamicin, BDNF mRNA levels in the vestibular ganglia were elevated significantly compared to untreated chinchillas and chinchillas 4 weeks after treatment. At 4 weeks after gentamicin treatment, BDNF mRNA levels were at intact levels of expression. In the crista ampullaris, high levels of BDNF transcripts were found in the untreated chinchillas. At 1 and 2 weeks after treatment, when only supporting cells are present in the crista, BDNF mRNA was undetectable. Four weeks after aminoglycoside treatment BDNF mRNA was present in the epithelium but at lower levels than in the intact epithelium. In contrast to its ligand, high levels of trkB mRNA hybridization were present in the vestibular ganglia of untreated chinchillas and trkB mRNA levels did not change following gentamicin treatment. In the vestibular epithelia, trkB mRNA was not detected either in the intact epithelium or after gentamicin ototoxicity. These data suggest that BDNF may be involved in the maintenance of the vestibular ganglia and contribute to neurite outgrowth to new and repaired hair cells following ototoxic damage.

Nerve dependency of developing and mature sensory receptor cells

Old and recent data concerning development of sensory cells and trophic interdependency of sensory neurons and sensory cells is reviewed for the ear, the lateral line system, the electroreceptive system, and the taste system. All sensory neurons originate from placodes. However, only most ear, lateral line and electrosensory cells derive from placodes, while the taste sensory cell originate locally. All sensory cells apparently are nerve independent for their formation, and at least sensory cells in the ear and the taste system share the neurotrophic support for their specific sensory neurons. Later, most of these sensory cells appear to depend, to a variable degree, on some innervation for maintenance. While the molecular nature of the signal cascade from sensory cells to sensory neurons is known in at least two systems, nothing is known about the molecular nature of the signal cascade from the sensory neurons back to the sensory cells.

Hair cells and supporting cells share a common progenitor in the avian inner ear

Sensory organs of the vertebrate inner ear contain two major cell types: hair cells (HCs) and supporting cells (SCs). To study the lineage relationships between these two populations, replication-defective retroviral vectors encoding marker genes were delivered to the otic vesicle of the chicken embryo. The resulting labeled clones were analyzed in the hearing organ of the chicken, called the basilar papilla (BP), after cellular differentiation. BPs were allowed to develop for 2 weeks after delivery of the retrovirus, were removed, and were processed histochemically as whole mounts to identify clones of cells. Clusters of labeled cells were evident in the sensory epithelium, the nonsensory epithelium, and in adjacent tissues. Labeled cell types included HCs, two morphologically distinct types of SCs, homogene cells, border cells, hyaline cells, ganglion cells, and connective tissue cells. Each clone was sectioned and cell-type identification was performed on sensory clones expressing retrovirally transduced beta-galactosidase. Cell composition was determined for 41 sensory clones, most of which contained both HCs and SCs. Clones containing one HC and one SC were observed, suggesting that a common progenitor exists that can remain bipotential up to its final mitotic division. The possibility that these two cell types may also arise from a mitotic precursor during HC regeneration in the mature basilar papilla is consistent with their developmental history.

The development of the vertebrate inner ear

The inner ear is a complex sensory organ responsible for balance and sound detection in vertebrates. It originates from a transient embryonic structure, the otic vesicle, that contains all of the information to develop autonomously into the mature inner ear. We review here the development of the otic vesicle, bringing together classical embryological experiments and recent genetic and molecular data. The specification of the prospective ectoderm and its commitment to the otic fate are very early events and can be related to the expression of genes with restricted expression domains. A combinatorial gene expression model for placode specification and diversification, based on classical embryological evidence and gene expression patterns, is discussed. The formation of the otic vesicle is dependent on inducing signals from endoderm, mesoderm and neuroectoderm. Ear induction consists of a sequence of discrete instructions from those tissues that confer its final identity on the otic field, rather than a single all-or-none process. The important role of the neural tube in otic development is highlighted by the abnormalities observed in mouse mutants for the Hoxa1, kreisler and fgf3 genes and those reported in retinoic acid-deficient quails. Still, the nature of the relation between the neural tube and otic development remains unclear. Gene targeting experiments in the mouse have provided evidence for genes potentially involved in regional and cell-fate specification in the inner ear. The disruption of the mouse Brn3.1 gene identifies the first mutation affecting sensory hair-cell specification, and mutants for Pax2 and Nkx5.1 genes show their requirement for the development of specific regions of the otic vesicle. Several growth-factors contribute to the patterned cell proliferation of the otic vesicle. Among these, IGF-I and FGF-2 are expressed in the otic vesicle and may act in an autocrine manner. Finally, little is known about early mechanisms involved in guiding ear innervation. However, targeted disruption of genes coding for neurotrophins and Trk receptors have shown that once synaptic contacts are established, they depend on specific trophic interactions that involve these two gene families. The accessibility of new cellular and molecular approaches are opening new perspectives in vertebrate development and are also starting to be applied to ear development. This will allow this classical and attractive model system to see a rapid progress in the near future.

Use of the teleost saccule to identify genes involved in inner ear function

The vertebrate inner ear sensory epithelia contain different types of hair cells and supporting cells. The teleost saccule is anatomically similar to the mammalian saccule and is primarily involved in the detection of translational acceleration and orientation with respect to gravity. To facilitate molecular studies of the teleost saccule cDNA libraries were constructed from microdissected Lepomis macrochirus (bluegill sunfish) saccular maculae. To our knowledge, this is the first report of cDNA libraries constructed from the saccule. In one instance, a non-polymerase chain reaction-based method of amplifying a mRNA population from limited amounts of starting tissue was employed that allowed construction of cDNA libraries from nanogram amounts of tissue mRNA. Conventional cDNA libraries were constructed from the sunfish saccular maculae as well. These cDNA libraries enriched in hair cell and supporting cell transcripts should facilitate molecular biological studies of inner ear sensory epithelia. As an example of their utility, efforts to identify tyrosine kinases expressed in the saccular endorgan using low-stringency hybridization screening of these cDNA libraries and the partial sequence of a cDNA found to encode an erbB-2-related tyrosine kinase are also reported.

Supporting cells in isolated sensory epithelia of avian utricles proliferate in serum-free culture

Sheets of sensory epithelia were isolated from the utricles of chicks and cultured in serum-free media and in media that contained serum. The proliferation of epithelial supporting cells was assayed using the mitotic tracer bromodeoxyuridine. Similar levels of supporting cell proliferation were observed in epithelia maintained in serum-free and serum-containing media. The results suggest that the vestibular epithelia of birds contain whatever mitogens are necessary for the continued proliferation of epithelial supporting cells.

Histochemical and scanning electron microscopic studies of supernumerary hair cells in embryonic rat cochlea in vitro

In the embryonic organ of Corti supernumerary hair cells were observed when developed in organotypic cultures. Hair cells ranging in up to two rows of inner hair cells (IHCs) and up to nine rows of outer hair cells (OHCs), were observed by phalloidin histochemistry. The total number of hair cells may double in some explanted cochleae compared to control ones. Cuticular plates of hair cells displayed an actin-free zone corresponding to the kinocilium location, differently located and indicating different degrees of differentiation and maturation. Moreover, some hair cells had a small apical surface area and a centrally located kinocilium, revealing immaturity. Under scanning electron microscopy, stereocilia appeared to differentiate normally, as compared to the in vivo development. The staircase pattern of the stereociliary bundles was reached on most of the hair cells with a 'V' shape on the OHCs and hemispherical one on the IHCs. Hair cell polarity was not homogeneous along the length of the tissue. Organs of Corti explanted at birth developed a weaker number of supernumerary hair cells showing a decrease of supernumerary hair cells with the developmental stage of the explant. These results provide evidence for supernumerary hair cells in the mammalian cochlea in culture, without loss or injury to preexisting hair cells.