Epithelial control of periotic mesenchyme chondrogenesis

Sculpting the skull through neurosensory epithelial-mesenchymal signaling

The vertebrate skull is a complex structure housing the brain and specialized sensory organs, including the eye, the inner ear, and the olfactory system. The close association between bones of the skull and the sensory organs they encase has posed interesting developmental questions about how the tissues scale with one another. Mechanisms that regulate morphogenesis of the skull are hypothesized to originate in part from the encased neurosensory organs. Conversely, the developing skull is hypothesized to regulate the growth of neurosensory organs, through mechanical forces or molecular signaling. Here, we review studies of epithelial-mesenchymal interactions during inner ear and olfactory system development that may coordinate the growth of the two sensory organs with their surrounding bone. We highlight recent progress in the field and provide evidence that mechanical forces arising from bone growth may affect olfactory epithelium development. Developmental Dynamics 248:88-97, 2019. © 2018 Wiley Periodicals, Inc.

Zebrafish skeleton development: High resolution micro-CT and FIB-SEM block surface serial imaging for phenotype identification

Although bone is one of the most studied living materials, many questions about the manner in which bones form remain unresolved, including fine details of the skeletal structure during development. In this study, we monitored skeleton development of zebrafish larvae, using calcein fluorescence, high-resolution micro-CT 3D images and FIB-SEM in the block surface serial imaging mode. We compared calcein staining of the skeletons of the wild type and nacre mutants, which are transparent zebrafish, with micro-CT for the first 30 days post fertilization embryos, and identified significant differences. We quantified the bone volumes and mineral contents of bones, including otoliths, during development, and showed that such developmental differences, including otolith development, could be helpful in identifying phenotypes. In addition, high-resolution imaging revealed the presence of mineralized aggregates in the notochord, before the formation of the first bone in the axial skeleton. These structures might play a role in the storage of the mineral. Our results highlight the potential of these high-resolution 3D approaches to characterize the zebrafish skeleton, which in turn could prove invaluable information for better understanding the development and the characterization of skeletal phenotypes.

Canonical Wnt signaling regulates the proliferative expansion and differentiation of fibrocytes in the murine inner ear

Otic fibrocytes tether the cochlear duct to the surrounding otic capsule but are also critically involved in maintenance of ion homeostasis in the cochlea, thus, perception of sound. The molecular pathways that regulate the development of this heterogenous group of cells from mesenchymal precursors are poorly understood. Here, we identified epithelial Wnt7a and Wnt7b as possible ligands of Fzd-mediated β-catenin (Ctnnb1)-dependent (canonical) Wnt signaling in the adjacent undifferentiated periotic mesenchyme (POM). Mice with a conditional deletion of Ctnnb1 in the POM exhibited a complete failure of fibrocyte differentiation, a severe reduction of mesenchymal cells surrounding the cochlear duct, loss of pericochlear spaces, a thickening and partial loss of the bony capsule and a secondary disturbance of cochlear duct coiling shortly before birth. Analysis at earlier stages revealed that radial patterning of the POM in two domains with highly condensed cartilaginous precursors and more loosely arranged inner mesenchymal cells occurred normally but that proliferation in the inner domain was reduced and cytodifferentiation failed. Cells with mis/overexpression of a stabilized form of Ctnnb1 in the entire POM mesenchyme sorted to the inner mesenchymal compartment and exhibited increased proliferation. Our analysis suggests that Wnt signals from the cochlear duct epithelium are crucial to induce differentiation and expansion of fibrocyte precursor cells. Our findings emphasize the importance of epithelial-mesenchymal signaling in inner ear development.

Expression and function of scleraxis in the developing auditory system

A study of genes expressed in the developing inner ear identified the bHLH transcription factor Scleraxis (Scx) in the developing cochlea. Previous work has demonstrated an essential role for Scx in the differentiation and development of tendons, ligaments and cells of chondrogenic lineage. Expression in the cochlea has been shown previously, however the functional role for Scx in the cochlea is unknown. Using a Scx-GFP reporter mouse line we examined the spatial and temporal patterns of Scx expression in the developing cochlea between embryonic day 13.5 and postnatal day 25. Embryonically, Scx is expressed broadly throughout the cochlear duct and surrounding mesenchyme and at postnatal ages becomes restricted to the inner hair cells and the interdental cells of the spiral limbus. Deletion of Scx results in hearing impairment indicated by elevated auditory brainstem response (ABR) thresholds and diminished distortion product otoacoustic emission (DPOAE) amplitudes, across a range of frequencies. No changes in either gross cochlear morphology or expression of the Scx target genes Col2A, Bmp4 or Sox9 were observed in Scx(-/-) mutants, suggesting that the auditory defects observed in these animals may be a result of unidentified Scx-dependent processes within the cochlea.

Retinoid signaling in inner ear development: A "Goldilocks" phenomenon

Retinoic acid (RA) is a biologically active derivative of vitamin A that is indispensable for inner ear development. The normal function of RA is achieved only at optimal homeostatic concentrations, with an excess or deficiency in RA leading to inner ear dysmorphogenesis. We present an overview of the role of RA in the developing mammalian inner ear, discussing both how and when RA may act to critically control a program of inner ear development. Molecular mechanisms of otic teratogenicity involving two members of the fibroblast growth factor family, FGF3 and FGF10, and their downstream targets, Dlx5 and Dlx6, are examined under conditions of both RA excess and deficiency. We term the effect of too little or too much RA on FGF/Dlx signaling a Goldilocks phenomenon. We demonstrate that in each case (RA excess, RA deficiency), RA can directly affect FGF3/FGF10 signaling within the otic epithelium, leading to downregulated expression of these essential signaling molecules, which in turn, leads to diminution in Dlx5/Dlx6 expression. Non-cell autonomous affects of the otic epithelium subsequently occur, altering transforming growth factor-beta (TGFβ) expression in the neighboring periotic mesenchyme and serving as a putative explanation for RA-mediated otic capsule defects. We conclude that RA coordinates inner ear morphogenesis by controlling an FGF/Dlx signaling cascade, whose perturbation by deviations in local retinoid concentrations can lead to inner ear dysmorphogenesis.

Chondrocyte-specific Smad4 gene conditional knockout results in hearing loss and inner ear malformation in mice

Smad4 is the central intracellular mediator of transforming growth factor-beta (TGF-beta) signaling, which plays crucial roles in tissue regeneration, cell differentiation, embryonic development, and regulation of the immune system. Conventional Smad4 gene knockout results in embryonic lethality, precluding its use in studies of the role of Smad4 in inner ear development. We used chondrocyte-specific Smad4 knockout mice (Smad4Co/Co) to investigate the function of Smad4 in inner ear development. Smad4Co/Co mice were characterized by a smaller cochlear volume, bone malformation, and abnormalities of the osseous spiral lamina and basilar membrane. The development of the hair cells was also abnormal, as evidenced by the disorganized stereocilia and reduced density of the neuronal processes beneath the hair cells. Auditory function tests revealed the homozygous Smad4Co/Co mice suffered from severe sensorineural hearing loss. Our results suggest that Smad4 is required for inner ear development and normal auditory function in mammals.

Coordinated molecular control of otic capsule differentiation: functional role of Wnt5a signaling and opposition by sfrp3 activity

Wnt proteins constitute one of the major families of secreted ligands that function in developmental signaling, however, little is known of the role of Wnt5a during inner ear development. It is hypothesized that Wnt5a acts as a mediator of chondrogenesis in the developing otic capsule, a cartilaginous structure that surrounds the developing inner ear and presages the formation of the endochondral bony labyrinth. We report the pattern of expression of Wnt5a protein and mRNA in the developing mouse inner ear using immunohistochemistry, whole-mount in situ hybridization and RT-PCR, and the ability of exogenous Wnt5a to stimulate otic capsule chondrogenesis when added to high-density cultures of periotic mesenchyme containing otic epithelium (periotic mesenchyme + otic epithelium), a well-established model of otic capsule formation. We show that in the presence of secreted frizzled related protein 3 (sfrp3), a Wnt antagonist expressed in the developing inner ear, or Wnt5a-specific antisense oligonucleotide, which diminishes endogenous Wnt5a, otic capsule chondrogenesis is suppressed in culture. We determined by histological analysis and aggrecan immunoreactivity that chondrogenic differentiation is disturbed in Wnt5a null embryos, and provide evidence that the periotic mesenchyme + otic epithelium harvested from Wnt5a null mice is compromised in its ability to differentiate into cartilage when interacted in culture. We propose a model whereby sfrp3 and Wnt5a act antagonistically to ensure appropriate patterns of chondrogenesis and provide coordinated control of otic capsule formation. Our findings support Wnt5a and sfrp3 as regulators of otic capsule formation in the developing mouse inner ear.

Retinoic acid-induced inner ear teratogenesis caused by defective Fgf3/Fgf10-dependent Dlx5 signaling

BACKGROUND

Retinoic acid (RA) is essential for inner ear development. However, exposure to excess RA at a critical period leads to inner ear defects. These defects are associated with disruption in epithelial-mesenchymal interactions.

METHODS

This study investigates the role of Dlx5 in the epithelial-mesenchymal interactions that guide otic capsule chondrogenesis, as well as the effect of excess in utero RA exposure on Dlx5 expression in the developing mouse inner ear. Control of Dlx5 by Fgf3 and Fgf10 under excess RA conditions is investigated by examining the developmental window during which Fgf3 and Fgf10 are altered by in utero RA exposure and by testing the ability of Fgf3 and Fgf10 to mitigate the reduction in chondrogenesis and Dlx5 expression mediated by RA in high-density cultures of periotic mesenchyme containing otic epithelium, a model of epithelial-mesenchymal interactions in which chondrogenic differentiation of periotic mesenchyme ensues in response to induction by otic epithelium.

RESULTS

Dlx5 deletion alters expression of TGFbeta(1), important for otic capsule chondrogenesis, in the developing inner ear and compromises the ability of cultured periotic mesenchyme containing otic epithelium, harvested from Dlx5 null embryos, to differentiate into cartilage when compared with control cultures. Downregulation in Dlx5 ensues as a consequence of in utero RA exposure in association with inner ear dysmorphogenesis. This change in Dlx5 is noted at embryonic day 10.5 (E10.5), but not at E9.5, suggesting that Dlx5 is not a direct RA target. Before Dlx5 downregulation, Fgf3 and Fgf10 expression is modified in the inner ear by excess RA, with the ability of exogenous Fgf3 and Fgf10 to rescue chondrogenesis and Dlx5 expression in RA-treated cultures of periotic mesenchyme containing otic epithelium supporting these fibroblast growth factors (FGFs) as intermediary genes by which RA mediates its effects.

CONCLUSIONS

Disruption in an Fgf3, -10/Dlx5 signaling cascade is operant in molecular mechanisms of inner ear teratogenesis by excess RA.

Dlx5 and Dlx6 homeobox genes are required for specification of the mammalian vestibular apparatus

The mammalian inner ear is a complex organ that develops from a surface ectoderm into distinct auditory and vestibular components. Congenital malformation of these two components resulting from single or multiple gene defects is a common clinical occurrence and is observed in patients with split hand/split foot malformation, a malformation which is phenocopied by Dlx5/6 null mice. Analysis of mice lacking Dlx5 and Dlx6 homeobox genes identified their restricted and combined expression in the otic epithelium as a crucial regulator of vestibular cell fates. Otic induction initiates without incident in Dlx5/6(-/-) embryos, but dorsal otic derivatives including the semicircular ducts, utricle, saccule, and endolymphatic duct fail to form. Dlx5 and Dlx6 seem to influence vestibular cell fates by restricting Pax2 and activating Gbx2 and Bmp4 expression domains. Given their proximity to the disease locus and the observed phenotype in Dlx5/6 null mice, Dlx5/6 are likely candidates to mediate the inner ear defects observed in patients with split hand/split foot malformation.

Internal Auditory Canal Morphology in Children with Cochlear Nerve Deficiency

OBJECTIVE

To describe the internal auditory canal (IAC) and inner ear morphologic characteristics of children with cochlear nerve (CN) deficiency.

STUDY DESIGN

Retrospective case series.

SETTING

Tertiary referral center.

PATIENTS

Fourteen children with small or absent (deficient) CNs have been identified by means of high-resolution magnetic resonance imaging (MRI).

INTERVENTIONS

MRI of the brain. Clinical evaluation.

MAIN OUTCOME MEASURES

Review of medical records, audiological testing results, and imaging studies. Images were evaluated for the structure of the cochlear, vestibular and facial nerves, IACs and inner ears. Audiometric thresholds were evaluated in all subjects.

METHODS

Fourteen children with small or absent (deficient) CNs have been identified by means of high-resolution MRI. A review of the medical records, audiologic testing results, and imaging studies was undertaken. The images were evaluated for the structure of the cochlear, vestibular and facial nerves, IACs, and inner ears. The audiometric thresholds were evaluated in all subjects.

RESULTS

Among the 14 patients, 5 had known syndromes. MRI allowed an exact specification of the nervous structures within all ears with normal-size IACs. Precise characterization of the nerves in ears with small IACs was more difficult, requiring a consideration of both imaging findings and functional parameters. Five children had bilateral deficient CNs, whereas the remaining 9 subjects were affected unilaterally. Thus, 19 ears had CN deficiency (absent CN, 16; small CN, 3). Eleven ears had normal-size IACs and deficient CNs. Of the 9 ears with small IACs, 8 had deficient CNs (absent, 7; small, 1) on the basis of both MRI and functional assessments. Two ears with small IACs had clear morphologic and/or functional evidence for the presence of a CN: one had a small-size CN on MRI, whereas another had a single nerve in a small IAC with present facial and auditory functions.

CONCLUSION

The findings of this study suggest that CN deficiency is not an uncommon cause of congenital hearing loss. The findings that most ears with CN deficiency had normal IAC morphology and that two ears with small IACs had CNs present indicate that IAC morphology is an unreliable surrogate marker of CN integrity. On the basis of these findings, we think that high-resolution MRI, rather than CT imaging, should be performed in all cases of pediatric hearing loss, especially in those cases where profound hearing loss has been documented. For ears with small IACs, the resolution of MRI currently remains limiting. In these cases, the determination of CN status frequently requires a variety of anatomic (CT and MRI) and functional tests (auditory brainstem response, otoacoustic emissions, behavioral audiometry, and physical examination).

Retinoid signaling in inner ear development

The inner ear originates from an embryonic ectodermal placode and rapidly develops into a three-dimensional structure (the otocyst) through complex molecular and cellular interactions. Many genes and their products are involved in inner ear induction, organogenesis, and cell differentiation. Retinoic acid (RA) is an endogenous signaling molecule that may play a role during different phases of inner ear development, as shown from pathological observations. To gain insight into the function of RA during inner ear development, we have investigated the spatio-temporal expression patterns of major components of RA signaling pathway, including cellular retinoic acid binding proteins (CRABPs), cellular retinoid binding proteins (CRBPs), retinaldehyde dehydrogenases (RALDHs), catabolic enzymes (CYP26s), and nuclear receptors (RARs). Although the CrbpI, CrabpI, and -II genes are specifically expressed in the inner ear throughout development, loss-of-function studies have revealed that these proteins are dispensable for inner development and function. Several Raldh and Cyp26 gene transcripts are expressed at embryological day (E) 9.0-9.5 in the otocyst and show mainly complementary distributions in the otic epithelium and mesenchyme during following stages. From Western blot, RT-PCR, and in situ hybridization analysis, there is a low expression of Raldhs in the early otocyst at E9, while Cyp26s are strongly expressed. During the following days, there is an up-regulation of Raldhs and a down-regulation for Cyp26s. Specific RA receptor (Rar and Rxr) genes are expressed in the otocyst and during further development of the inner ear. At the otocyst stage, most of the components of the retinoid pathway are present, suggesting that the embryonic inner ear might act as an autocrine system, which is able to synthesize and metabolize RA necessary for its development. We propose a model in which two RA-dependent pathways may control inner ear ontogenesis: one indirect with RA from somitic mesoderm acting to regulate gene expression within the hindbrain neuroepithelium, and another with RA acting directly on the otocyst. Current evidence suggests that RA may regulate several genes involved in mesenchyme-epithelial interactions, thereby controlling inner ear morphogenesis. Our investigations suggest that RA signaling is a critical component not only of embryonic development, but also of postnatal maintenance of the inner ear.

Transforming growth factor-beta1 signaling participates in the physiological and pathological regulation of mouse inner ear development by all-trans retinoic acid

BACKGROUND

Retinoic acid (RA) is a vitamin A derivative that participates in patterning and regulation of inner ear development. Either excess RA or RA deficiency during a critical stage of inner ear development can produce teratogenic effects. Previous studies have shown that in utero exposure of the developing mouse inner ear to a high dose of all-trans RA (atRA) results in severe malformations of the inner ear that are associated with diminished levels of endogenous transforming growth factor-beta1 (TGF-beta(1)) protein.

METHODS

In this study, the effects of a teratogenic level of atRA on levels and patterns of expression of TGFbeta receptor II (TGFbetaRII) and Smad2, a downstream component of the TGFbeta signal transduction pathway, are investigated in the developing mouse inner ear. The expression pattern of endogenous RA receptor alpha (RARalpha) and the ability of an RARalpha(1)-specific antisense oligonucleotide (AS) to modulate otic capsule chondrogenesis are demonstrated in the inner ear and in culture.

RESULTS

Endogenous TGFbetaRII and Smad2 are downregulated in the inner ear following in utero atRA treatment. In addition, a reduction in endogenous TGFbeta(1) and a marked suppression of chondrogenesis occur in RARalpha(1) AS-treated cultures in comparison to untreated or oligonucleotide-treated control cultures. This chondrogenic suppression can be partially overcome by supplementation of RARalpha(1) AS-treated cultures with exogenous TGFbeta(1) protein.

CONCLUSIONS

Our findings support a role for TGFbeta in the physiological and pathological effects of RA on inner ear development.

In vitro growth and differentiation of mammalian sensory hair cell progenitors: a requirement for EGF and periotic mesenchyme

The sensory hair cells and supporting cells of the organ of Corti are generated by a precise program of coordinated cell division and differentiation. Since no regeneration occurs in the mature organ of Corti, loss of hair cells leads to deafness. To investigate the molecular basis of hair cell differentiation and their lack of regeneration, we have established a dissociated cell culture system in which sensory hair cells and supporting cells can be generated from mitotic precursors. By incorporating a Math1-GFP transgene expressed exclusively in hair cells, we have used this system to characterize the conditions required for the growth and differentiation of hair cells in culture. These conditions include a requirement for epidermal growth factor, as well as the presence of periotic mesenchymal cells. Lastly, we show that early postnatal cochlear tissue also contains cells that can divide and generate new sensory hair cells in vitro.

Fgf9 signaling regulates inner ear morphogenesis through epithelial–mesenchymal interactions

The mammalian inner ear comprises the cochleovestibular labyrinth, derived from the ectodermal otic placode, and the encasing bony labyrinth of the temporal bone. Epithelial-mesenchymal interactions are thought to control inner ear development, but the modes and the molecules involved are largely unresolved. We show here that, during the precartilage and cartilage stages, Fgf9 is expressed in specific nonsensory domains of the otic epithelium and its receptors, Fgfr1(IIIc) and Fgfr2(IIIc), widely in the surrounding mesenchyme. To address the role of Fgf9 signaling, we analyzed the inner ears of mice homozygous for Fgf9 null alleles. Fgf9 inactivation leads to a hypoplastic vestibular component of the otic capsule and to the absence of the epithelial semicircular ducts. Reduced proliferation of the prechondrogenic mesenchyme was found to underlie capsular hypoplasticity. Semicircular duct development is blocked at the initial stages, since fusion plates do not form. Our results show that the mesenchyme directs fusion plate formation and they give direct evidence for the existence of reciprocal epithelial-mesenchymal interactions in the developing inner ear. In addition to the vestibule, in the cochlea, Fgf9 mutation caused defects in the interactions between the Reissner's membrane and the mesenchymal cells, leading to a malformed scala vestibuli. Together, these data show that Fgf9 signaling is required for inner ear morphogenesis.

Analysis of genes from inner ear developmental-stage cDNA subtraction reveals molecular regionalization of the otic capsule

Although the gross embryology of inner ear development has been documented for several different vertebrate species at a descriptive level, our understanding of the molecular mechanisms involved remains rudimentary. Therefore, we have used cDNA subtraction and normalization procedures to define genes upregulated in the 13.5dpc mouse inner ear, a developmental stage where inner ear morphogenesis and tissue remodeling is active and differentiation of future hair cells is being initiated. We recovered 33 different genes from this subtraction and using gene-specific primers have confirmed the transcriptional upregulation of 26 of these in the 13.5dpc inner ear. Northern analyses were used to investigate splicing differences between the inner ear and the whole embryo at 13.5dpc. Spatial localization of expression was determined through whole-ear in situ hybridization analysis, and selected genes were analyzed in more detail through in situ hybridization of tissue sections. These data illustrate that the genes isolated in this study are expressed in the developing otic capsule and/or neuroepithelium. Furthermore, the expression patterns also reveal molecular heterogeneity in the developing capsule and indicate that for some genes, the chondrogenic otic capsule is composed of distinct domains of gene expression.

Lack of pendrin expression leads to deafness and expansion of the endolymphatic compartment in inner ears of Foxi1 null mutant mice

Mice that lack the winged helix/forkhead gene Foxi1 (also known as Fkh10) are deaf and display shaker/waltzer behavior, an indication of disturbed balance. While Foxi1 is expressed in the entire otic vesicle at E9.5, it becomes gradually restricted to the endolymphatic duct/sac epithelium and at E16.5 Foxi1 expression in the inner ear is confined to this epithelium. Histological sections, paintfill experiments and whole-mount hybridizations reveal no abnormality in inner ear development of Foxi1(-/-) mice before E13.5. Between E13.5 and E16.5 the membranous labyrinth of inner ears from null mutants starts to expand as can be seen in histological sections, paint-fill experiments and three-dimensional reconstruction. Postnatally, inner ears of Foxi1(-/-) mice are extremely expanded, and large irregular cavities, compressing the cerebellum and the otherwise normal middle ear, have replaced the delicate compartments of the wild-type inner ear. This phenotype resembles that of the human sensorineural deafness syndrome Pendred syndrome, caused by mutations in the PDS gene. In situ hybridization of Foxi1(-/-) endolymphatic duct/sac epithelium shows a complete lack of the transcript encoding the chloride/iodide transporter pendrin. Based on this, we would like to suggest that Foxi1 is an upstream regulator of pendrin and that the phenotype seen in Foxi1 null mice is, at least in part, due to defective pendrin-mediated chloride ion resorption in the endolymphatic duct/sac epithelium. We show that this regulation could be mediated by absence of a specific endolymphatic cell type--FORE (forkhead related) cells--expressing Foxi1, Pds, Coch and Jag1. Thus, mutations in FOXI1 could prove to cause a Pendred syndrome-like human deafness.

Bone morphogenetic protein 4 (BMP4): a regulator of capsule chondrogenesis in the developing mouse inner ear

Formation of the cartilaginous otic capsule is directed by otic epithelial-periotic mesenchymal interactions. In response to induction by otic epithelium, condensations of mesenchyme appear in the periotic region and form a chondrified otic capsule that serves as the template for the subsequent formation of the endochondral bony labyrinth. Previous studies indicate that members of the transforming growth factor beta superfamily, including transforming growth factor beta(1), participate in guiding these tissue interactions. In this study, we report the localization of bone morphogenetic protein 4 (BMP4) to the mesenchymal and epithelial-derived tissues of the mouse inner ear between 10.5 and 14 days of embryonic development. We demonstrate modulation of chondrogenesis in cultured mouse periotic mesenchyme by exogenous BMP4 protein and investigate the function of endogenous BMP4 in otic capsule chondrogenesis. We show that in the presence of the BMP antagonist, Noggin, otic capsule chondrogenesis is suppressed in culture in a dose-dependent manner. Consistent with this finding, addition of BMP4-specific antisense oligonucleotide to cultures of mouse periotic mesenchyme containing otic epithelium decreases levels of endogenous BMP4 protein and suppresses the chondrogenic response of the cultured periotic mesenchyme, providing evidence of the necessity for BMP4 in mediating otic capsule chondrogenesis. Supplementation of either Noggin- or BMP4 antisense oligonucleotide-treated cultures with BMP4 protein can restore the extent of chondrogenesis to normal levels. Our findings support BMP4 as an essential mediator of chondrogenesis in the developing otic capsule in situ.

Sonic hedgehog regulates otic capsule chondrogenesis and inner ear development in the mouse embryo

Development of the cartilaginous capsule of the inner ear is dependent on interactions between otic epithelium and its surrounding periotic mesenchyme. During these tissue interactions, factors endogenous to the otic epithelium influence the differentiation of the underlying periotic mesenchyme to form a chondrified otic capsule. We report the localization of Sonic hedgehog (Shh) protein and expression of the Shh gene in the tissues of the developing mouse inner ear. We demonstrate in cultures of periotic mesenchyme that Shh alone cannot initiate otic capsule chondrogenesis. However, when Shh is added to cultured periotic mesenchyme either in combination with otic epithelium or otic epithelial-derived fibroblast growth factor (FGF2), a significant enhancement of chondrogenesis occurs. Addition of Shh antisense oligonucleotide (AS) to cultured periotic mesenchyme with added otic epithelium decreases levels of endogenous Shh and suppresses the chondrogenic response of the mesenchyme cells, while supplementation of Shh AS-treated cultures with Shh rescues cultures from chondrogenic inhibition. We demonstrate that inactivation of Shh by targeted mutation produces anomalies in the developing inner ear and its surrounding capsule. Our results support a role for Shh as a regulator of otic capsule formation and inner ear development during mammalian embryogenesis.

Hmx2 homeobox gene control of murine vestibular morphogenesis

Development of the vertebrate inner ear is characterized by a series of genetically programmed events involving induction of surface ectoderm, preliminary morphogenesis, specification and commitment of sensory, nonsensory and neuronal cells, as well as outgrowth and restructuring of the otocyst to form a complex labyrinth. Hmx2, a member of the Hmx homeobox gene family, is coexpressed with Hmx3 in the dorsolateral otic epithelium. Targeted disruption of Hmx2 in mice demonstrates the temporal and spatial involvement of Hmx2 in the embryonic transition of the dorsal portion (pars superior) of the otocyst to a fully developed vestibular system. In Hmx2 null embryos, a perturbation in cell fate determination in the lateral aspect of the otic epithelium results in reduced cell proliferation in epithelial cells, which includes the vestibular sensory patches and semicircular duct fusion plates, as well as in the adjacent mesenchyme. Consequently, enlargement and morphogenesis of the pars superior of the otocyst to form a complex labyrinth of cavities and ducts is blocked, as indicated by the lack of any distinguishable semicircular ducts, persistence of the primordial vestibular diverticula, significant loss in the three cristae and the macula utriculus, and a fused utriculosaccular chamber. The developmental regulators Bmp4, Dlx5 and Pax2 all play a critical role in inner ear ontogeny, and the expression of each of these genes is affected in the Hmx2 null otocyst suggesting a complex regulatory role for Hmx2 in this genetic cascade. Both Hmx2 and Hmx3 transcripts are coexpressed in the developing central nervous system including the neural tube and hypothalamus. A lack of defects in the CNS, coupled with the fact that not all of the Hmx2-positive regions in developing inner ear are impaired in the Hmx2 null mice, suggest that Hmx2 and Hmx3 have both unique and overlapping functions during embryogenesis.

Remarks on the inner ear of elasmobranchs and its interpretation from skeletal labyrinth morphology

The structure and function of the craniate inner ear is reviewed, with 33 apomorphic characters of the membranous labyrinth and associated structures identified in craniates, gnathostomes, and elasmobranchs. Elasmobranchs are capable of low-frequency semi-directional phonoreception, even in the absence of any pressure-to-displacement transducer such as ear ossicles. The endolymphatic (parietal) fossa, semicircular canals, and crista (macula) neglecta are all adapted toward phonoreception. Some (but not all) of the morphological features associated with phonoreception can be inferred from the elasmobranch skeletal labyrinth. Endocranial spaces such as the skeletal labyrinth also provide suites of morphological characters that may be incorporated into phylogenetic analyses, irrespective of how closely these spaces reflect underlying soft anatomy. The skeletal labyrinths of Squalus and Notorynchus are compared using silicone endocasts and high-resolution CT-scanning. The latter procedure offers several advantages over other techniques; it is more informative, nondestructive, preserves relationships of surrounding structures, and it can be applied both to modern and fossil material.

Development of the vertebrate inner ear

The inner ear, also called the membranous labyrinth, contains the cochlea, which is responsible for the sense of hearing, and the vestibular apparatus, which is necessary for the sense of balance and gravity. The inner ear arises in the embryo from placodes, which are epithelial thickenings of the cranial ectoderm symmetrically located on either side of hindbrain rhombomeres 5 and 6. Placode formation in mice is first visible at the 12-somite stage and is controlled by surrounding tissues, the paraxial mesoderm and neural ectoderm. Diffusible molecules such as growth factors play an important role in this process. The activity of several genes confers the identity to the placodal cells. Subsequent cellular proliferation processes under influences from the adjacent hindbrain cause the inner ear epithelium to invaginate and form a vesicle called the otocyst. Combinatorial expression of several genes and diffusible factors secreted from the vesicle epithelium and hindbrain control specification of distinct inner ear compartments. Transplantation studies and inner ear in vitro cultures show that each of these compartments is already committed to develop unique inner ear structures. Later developmental periods are principally characterized by intrinsic differentiation processes. In particular, sensory patches differentiate into fully functional sensory epithelia, and the semicircular canals along with the cochlear duct are elaborated and ossified.

Fdp, a new fibrocyte-derived protein related to MIA/CD-RAP, has an in vitro effect on the early differentiation of the inner ear mesenchyme

During the course of a study aimed at isolating transcripts specifically or preferentially expressed in the inner ear, we identified a novel gene, encoding a fibrocyte-derived protein, that we named Fdp. Fdp is predicted to be a secreted 128-amino acid protein, which is highly homologous to the melanoma-inhibiting activity/cartilage-derived retinoic acid-sensitive protein (MIA/CD-RAP), a cartilage-specific protein also expressed in several tumors. Fdp and MIA/CD-RAP thus define a new family of proteins. Fdp is expressed from embryonic day 10.5 in the mesenchyme surrounding the otic epithelium. During development, these cells progressively aggregate, condense, and differentiate into cartilaginous cells forming the otic capsule, which no longer expresses Fdp, and into fibrocytes surrounding the epithelia, which strongly express Fdp. In order to address the function of Fdp, we developed an in vitro antisense oligonucleotide approach using microdissected periotic mesenchyme micromass cultures, and showed that Fdp antisense oligonucleotide treatment results in a significant reduction in chondrogenesis. Our results demonstrate that Fdp plays a role in the initiation of periotic mesenchyme chondrogenesis. Accordingly, Fdp and its human ortholog FDP, which map to chromosome 2 and band 20p11, respectively, could be candidate genes for forms of deafness associated with malformations of the otic capsule.

Treatment with all-trans-retinoic acid decreases levels of endogenous TGF-beta(1) in the mesenchyme of the developing mouse inner ear

BACKGROUND

Previous studies have shown that in utero exposure of the mouse embryo to high doses of all-trans-retinoic acid (atRA) produces defects of the developing inner ear and its surrounding cartilaginous capsule, while exposure of cultured periotic mesenchyme plus otic epithelium to high doses of exogenous atRA results in an inhibition of otic capsule chondrogenesis.

METHODS

In this study, we examine the effects of atRA exposure on the endogenous expression of transforming growth factor-beta(1) (TGF-beta(1)), a signaling molecule that mediates the epithelial-mesenchymal interactions that guide the development of the capsule of the inner ear.

RESULTS

Our results demonstrate a marked reduction in immunostaining for TGF-beta(1) in the periotic mesenchyme of atRA-exposed embryos of age E10.5 and E12 days in comparison with control specimens. Consistent with these in vivo findings, high-density cultures of E10.5 periotic mesenchyme plus otic epithelium, treated with doses of atRA that suppress chondrogenesis, showed significantly decreased levels of TGF-beta(1), as compared with TGF-beta(1) levels in untreated control cultures. Furthermore, we demonstrate a rescue of cultured periotic mesenchyme plus otic epithelium from atRA-induced chondrogenic suppression by supplementation of cultures with excess TGF-beta(1).

CONCLUSIONS

Our results support the hypothesis that TGF-beta(1) plays a role in mechanisms of atRA teratogenicity during inner ear development.

Dlx5 regulates regional development of the branchial arches and sensory capsules

We report the generation and analysis of mice homozygous for a targeted deletion of the Dlx5 homeobox gene. Dlx5 mutant mice have multiple defects in craniofacial structures, including their ears, noses, mandibles and calvaria, and die shortly after birth. A subset (28%) exhibit exencephaly. Ectodermal expression of Dlx5 is required for the development of olfactory and otic placode-derived epithelia and surrounding capsules. The nasal capsules are hypoplastic (e.g. lacking turbinates) and, in most cases, the right side is more severely affected than the left. Dorsal otic vesicle derivatives (e. g. semicircular canals and endolymphatic duct) and the surrounding capsule, are more severely affected than ventral (cochlear) structures. Dlx5 is also required in mandibular arch ectomesenchyme, as the proximal mandibular arch skeleton is dysmorphic. Dlx5 may control craniofacial development in part through the regulation of the goosecoid homeobox gene. goosecoid expression is greatly reduced in Dlx5 mutants, and both goosecoid and Dlx5 mutants share a number of similar craniofacial malformations. Dlx5 may perform a general role in skeletal differentiation, as exemplified by hypomineralization within the calvaria. The distinct focal defects within the branchial arches of the Dlx1, Dlx2 and Dlx5 mutants, along with the nested expression of their RNAs, support a model in which these genes have both redundant and unique functions in the regulation of regional patterning of the craniofacial ectomesenchyme.

Targeted mutagenesis of the POU-domain gene Brn4/Pou3f4 causes developmental defects in the inner ear

Targeted mutagenesis in mice demonstrates that the POU-domain gene Brn4/Pou3f4 plays a crucial role in the patterning of the mesenchymal compartment of the inner ear. Brn4 is expressed extensively throughout the condensing mesenchyme of the developing inner ear. Mutant animals displayed behavioral anomalies that resulted from functional deficits in both the auditory and vestibular systems, including vertical head bobbing, changes in gait, and hearing loss. Anatomical analyses of the temporal bone, which is derived in part from the otic mesenchyme, demonstrated several dysplastic features in the mutant animals, including enlargement of the internal auditory meatus. Many phenotypic features of the mutant animals resulted from the reduction or thinning of the bony compartment of the inner ear. Histological analyses demonstrated a hypoplasia of those regions of the cochlea derived from otic mesenchyme, including the spiral limbus, the scala tympani, and strial fibrocytes. Interestingly, we observed a reduction in the coiling of the cochlea, which suggests that Brn-4 plays a role in the epithelial-mesenchymal communication necessary for the cochlear anlage to develop correctly. Finally, the stapes demonstrated several malformations, including changes in the size and morphology of its footplate. Because the stapes anlage does not express the Brn4 gene, stapes malformations suggest that the Brn4 gene also plays a role in mesenchymal-mesenchymal signaling. On the basis of these data, we suggest that Brn-4 enhances the survival of mesodermal cells during the mesenchymal remodeling that forms the mature bony labyrinth and regulates inductive signaling mechanisms in the otic mesenchyme.

Eya1 expression in the developing ear and kidney: towards the understanding of the pathogenesis of Branchio-Oto-Renal (BOR) syndrome

Branchio-Oto-Renal (BOR) syndrome is an autosomal dominant, early developmental defect characterised by varying combinations of branchial (fistulas, sinuses, and cysts), outer, middle and inner ear, and renal anomalies. The gene underlying this syndrome, EYA1, is homologous to the Drosophila developmental gene eyes absent which encodes a transcriptional co-activator required for eye specification. We report here the temporal and spatial pattern of expression of the murine homologue, Eya1, throughout ear and kidney development in relation to the anomalies of BOR syndrome. The expression of Eya1 in the branchial arch apparatus (namely in the 2nd, 3rd, and 4th branchial clefts and pharyngeal pouches) at embryonic day (E)10.5, can be correlated with the branchial fistulas, sinuses, and cysts but not with the outer and middle ear anomalies. In contrast, Eya1 is expressed during the slightly more advanced stage of outer and middle ear morphogenesis at E13.5, in the mesenchyme adjacent to the first branchial cleft (the cleft will give rise to the external auditory canal and the surrounding mesenchyme to the auricular hillocks) and surrounding the primordia of the middle ear ossicles, and in the epithelium of the tubotympanic recess (the future tympanic cavity). During early inner ear development, Eya1 is expressed in the ventromedial wall of the otic vesicle (the site of the future sensory epithelia), in the statoacoustic ganglion, and in the periotic mesenchyme, consistent with the cochlear anomalies and sensorineural hearing loss of BOR syndrome. Subsequently, Eya1 expression is observed in the differentiating hair and supporting cells of the sensory epithelia, as well as in the associated ganglia, and persists after differentiation has taken place. This suggests that, in addition to a role in the morphogenetic process, Eya1 could also be implicated in the differentiation and/or survival of these inner ear cell populations. Finally, Eya1 expression in the condensing mesenchymal cells of the kidney is consistent with the excretory and collecting system anomalies of BOR syndrome. From the comparison of the Eya1 and Pax2 expression patterns during ear and kidney development, a contribution of these two genes to the same regulatory pathway can only be suggested in the mesenchymal-epithelial transition directing renal tubule formation.

Prx1 and Prx2 in skeletogenesis: roles in the craniofacial region, inner ear and limbs

Prx1 and Prx2 are closely related paired-class homeobox genes that are expressed in very similar patterns predominantly in mesenchyme. Prx1 loss-of-function mutants show skeletal defects in skull, limbs and vertebral column (Martin, J. F., Bradley, A. and Olson, E. N. (1995) Genes Dev. 9, 1237–1249). We report here that mice in which Prx2 is inactivated by a lacZ insertion had no skeletal defects, whereas Prx1/Prx2 double mutants showed many novel abnormalities in addition to an aggravation of the Prx1 single mutant phenotype. We found defects in external, middle and inner ear, reduction or loss of skull bones, a reduced and sometimes cleft mandible, and limb abnormalities including postaxial polydactyly and bent zeugopods. A single, or no incisor was present in the lower jaw, and ectopic expression of Fgf8 and Pax9 was found medially in the mandibular arch. A novel method to detect β-galactosidase activity in hydroxyethylmethacrylate sections allowed detailed analysis of Prx2 expression in affected structures. Our results suggest a role for Prx genes in mediating epitheliomesenchymal interactions in inner ear and lower jaw. In addition, Prx1 and Prx2 are involved in interactions between perichondrium and chondrocytes that regulate their proliferation or differentiation in the bones of the zeugopods.

Changes in the subcellular localization of the Brn4 gene product precede mesenchymal remodeling of the otic capsule

To better understand the genetic mechanisms that regulate the formation of the temporal bone, we have characterized the developmental expression pattern of the mouse gene, Brn4/Pou3f4, which plays a central role in bony labyrinth formation. Expression of this gene is initially detected in the ventral aspect of the otic capsule at 10.5 days post coitus (dpc), and correlates with the onset of mesenchymal condensation in the otic capsule. As the otic capsule condenses further and surrounds the entire otic vesicle, the Brn4 gene product is detected throughout the inner ear in the mesenchyme of both the cochlear and vestibular aspects. Early in otic embryogenesis, the Brn4 gene product is localized to the nucleus of the vast majority of cells in which it is expressed. The Brn4 gene product remains nuclear in those regions of the otic capsule that eventually give rise to the mature bony labyrinth. However, the subcellular localization of the Brn4 gene product shifts from strictly nuclear to perinuclear in those regions of the otic capsule that will cavitate to form acellular regions in the temporal bone, such as the scala tympani, scala vestibuli, and the internal auditory meatus. These data provide a detailed analysis of the expression pattern of the Brn4 gene, and provide insight into the role of the Brn4 gene product and its regulation during otic capsule formation.

Role of FGF3 in otic capsule chondrogenesis in vitro: an antisense oligonucleotide approach

Initiation of otic capsule chondrogenesis depends on interactions between the otocyst and surrounding periotic mesenchyme. We previously reported localization of endogenous basic fibroblast growth factor (FGF2) to the epithelium of the mouse otocyst, and initiation of chondrogenesis in cultured periotic mesenchyme by this epithelial-derived signaling molecule. We now report that FGF3, related to FGF2, can also initiate otic capsule chondrogenesis. We show localization of endogenous FGF3 to the otocyst, and suppression of chondrogenesis by antisense oligonucleotides complementary to different regions of the murine FGF3 gene. Our results support a role for FGF3 in otic capsule formation in situ.

Effect of retinoic acid on otic capsule chondrogenesis in high-density culture suggests disruption of epithelial-mesenchymal interactions

Previous studies have shown that in utero exposure of the mouse embryo to nonphysiological levels of all-trans retinoic acid (RA) produces malformations of the epithelial-derived auditory and vestibular receptors of the inner ear and its surrounding cartilaginous capsule. In this study, we demonstrate the effects of all-trans RA in high-density cultures of the periotic mesenchyme fated to form the otic capsule. Our results demonstrate an inhibition of chondrogenesis in cultured periotic mesenchyme + otic epithelium of embryonic age E10.5 days (E10.5) in response to all-trans RA exposure. However, at later stages of development (i.e., E12, E14), when epithelial-mesenchymal interactions are no longer required for initiation of chondrogenesis, exposure to this teratogen has no effect on the chondrogenic process. Two analogues of all-trans RA, i.e., cis-RA and trans-retinol, were investigated for their biological activity in chondrogenic cultures of inner ear mesenchyme + epithelium. Moreover, we tested the inductive capability and responsiveness of in utero RA-exposed inner ear tissues when cultured with inner ear tissues that were not exposed to this teratogen. Our results support the hypothesis that all-trans RA disrupts otic capsule formation by interfering with the tissue interactions required for its normal differentiation and development.

Fine structure of extracellular matrix and basal laminae in two types of abnormal collagen production: L-proline analog-treated otocyst cultures and disproportionate micromelia (Dmm/Dmm) mutants

L-Azetidine-2-carboxylic acid (LACA), a naturally occurring vegetable imino acid, can be incorporated into mammalian proteins in place of proline, thereby eliciting an inhibitory effect on collagen secretion. Exposure of explants of the embryonic mouse inner ear to LACA reduces the number of collagen fibrils in the otic capsule, gives rise to a dose-dependent derangement of the basal lamina, and ultimately results in dysmorphogenesis and retarded differentiation of the inner ear. Disproportionate micromelia (Dmm) is an incomplete dominant form of dwarfism characterized by a reduced quantity of type II collagen in the cartilaginous extracellular matrix (ECM). Abnormal morphogenesis in homozygotic Dmm mice resembles the abnormal morphogenesis observed in LACA-exposed otic explants, resulting in malformed inner ears with a bulky cartilaginous capsule and a lack or reduction of defined perilymphatic spaces (Van De Water and Galinovic-Schwartz, 1987). In this study, we examined by ultrastructural analysis LACA-exposed otic explants and inner ears of Dmm/Dmm mouse embryos for abnormalities in the collagenous constituents of the basal laminae and capsular ECM. We demonstrate, in comparison to normal embryonic mouse inner ears, a reduction in collagen fibrils and irregular cytodifferentiation of chondrocytes in the ECM of LACA-exposed and Dmm/Dmm inner ears as well as in the basal laminae of LACA-exposed specimens. In addition, we provide evidence of dysmorphogenesis of the otic capsule and perilymphatic spaces in LACA-exposed explants. Moreover, while previous studies demonstrated the anomalous development of sensory structures in otocyst explants following LACA exposure, in this study we provide evidence of the normal morphogenesis of otic epithelial-derived sensory structures in homozygotic Dmm/Dmm mouse embryos.

Retinoic acid-induced embryopathy of the mouse inner ear

Retinoic acid (RA) is an active metabolite of vitamin A that is teratogenic when present in excess during mammalian embryogenesis. We have investigated the effect of embryonic exposure to nonphysiological levels of all-trans RA on the development of the mouse inner ear. Dysmorphogenesis of both vestibular and auditory portions of the inner ear, and abnormal formation of the surrounding capsule are produced by exposure to teratogenic levels of RA at an embryonic age of 9 days (E9). There was no observable teratogenic effect of RA when administered at earlier (i.e., E7 or E8) or later (i.e., E10) stages of otic morphogenesis. We hypothesize that exposure to high levels of RA during a critical period of early otic morphogenesis interferes with the inductive tissue interactions required for inner ear development.

The gene for the homeodomain-containing protein Cart-1 is expressed in cells that have a chondrogenic potential during embryonic development

We have examined the expression of the gene for Cart-1, a new homeodomain-containing protein, during rat embryonic development. In early embryos, Cart-1 RNA was detected at high levels in head mesenchyme, lateral mesoderm, sclerotomes and limb bud mesenchyme. These tissues contain prechondrocytic mesenchymal cells responsible for the formation of the cartilaginous skeleton. In addition, Cart-1 RNA was also found in lung buds, tendons and mesonephros. Cells in these tissues have the potential of undergoing chondrogenesis either in explants for mesonephros and tendons, or in vivo for tendons and the precursors of bronchi cartilages. No hybridization was observed in brain, spinal cord, heart, spleen, gastrointestinal tract, liver and muscle. Our results support the hypothesis that Cart-1 may play a role in the pathway of chondrogenesis. The gene for Cart-1 was mapped to a segment of mouse chromosome 10 between the genes for phenylalanine hydroxylase and interferon gamma.

Induction of chondrogenesis: requirement for synergistic interaction of basic fibroblast growth factor and transforming growth factor-beta

Interactions between the epithelial anlage of the developing mouse inner ear and its associated periotic mesenchyme control the differentiation of the cartilaginous otic capsule. Transforming growth factor-beta 1 (TGF-beta 1) is a naturally occurring signal peptide that is present in these tissues at times of active differentiation and morphogenesis. Previous studies have shown that TGF-beta 1 alone is not a sufficient stimulus to initiate chondrogenesis in cultured periotic mesenchyme. In this study, we provide evidence that basic fibroblast growth factor (bFGF) can elicit a specific but limited chondrogenic response in cultured periotic mesenchymal cells. We also demonstrate that simultaneous addition of bFGF and TGF-beta 1 to cultured periotic mesenchyme results in a full chondrogenic response comparable to that which occurs when periotic mesenchyme is grown in the presence of its natural inductor tissue (i.e. otic epithelium). Utilizing antibodies directed against bFGF, we show localization of endogenous bFGF in the otic epithelium in vivo and in mixed epithelial-mesenchymal cultures. Additionally, we demonstrate the presence of FGF-like activity in medium conditioned by otic epithelium. Blocking of epithelial elicited chondrogenesis by a combination of both alpha bFGF and alpha TGF-beta 1 antibodies provides further evidence of the necessity for these growth factors in the chondrogenic differentiation of periotic mesenchyme in vitro. Our results suggest a role for both bFGF and TGF-beta 1 in the regulation of chondrogenesis during otic capsule formation in situ.

Two rhombomeres are altered in Hoxa-1 mutant mice

This study provides a detailed description of the anatomical defects in the Hoxa-1−/− mutant mice previously generated in our laboratory (T. Lufkin, A. Dierich, M. LeMeur, M. Mark and P. Chambon, 1991; Cell 66, 1105–1119). Three-dimensional reconstructions of the Hoxa-1−/− rhombencephalon reveals that it bears only five rhombomeric structures (ie. morphological segments) instead of the normal seven. The first three of these rhombomeres appear normal as judged from the distribution pattern of CRABPI transcripts in the neurectoderm and from the histological analysis of the cranial nerve components derived from these structures. In contrast, the neural-crest-cell-free region normally located opposite rhombomere 5 is lacking in Hoxa-1−/− embryos, and motor neurons of the facial and abducens nerves, which normally differentiate within rhombomeres 4, 5 and 6, are missing in Hoxa-1−/− fetuses. These morphological data, combined with the determination of the molecular positional identities of the rhombomeres 4 and 5 (P. Dolle, T. Lufkin, R. Krumlauf, M. Mark, D. Duboule and P. Chambon, 1993; Proc. Natl. Acad. Sci. USA, in press), suggest that rhombomere 4 is markedly reduced, whereas rhombomere 5 is almost absent. Thus, the remnants of rhombomeres 4 and 5 appear to be fused caudally with rhombomere 6 to form a single fourth rhombomeric structure. Moreover, the migration of neural crest cells contributing to the glossopharyngeal and vagus nerves occurs in a more rostral position, resulting in abnormalities of these cranial nerves, which were visualized by whole-mount anti-neurofilament immunostaining. The mutual relationship along the rostrocaudal axis between the otic pit and the neuroepithelial site of int-2 protein secretion (a putative otogenic cue) is not significantly changed in Hoxa-1−/− embryos. However, the abnormal relationship between the rhombencephalon and the epithelial inner ear may account for the aplasia and faulty differentiation of the membranous labyrinth, the disruption of the cartilaginous otic capsule and the disorganisation of some middle ear structures. This phenotype is compared with that of the Hoxa-1−/− mutants generated by O. Chisaka, T. S. Musci and M. R. Capecchi, 1992 (Nature 335, 516–520) and with that of the mice homozygous for the kreisler mutation.

Transforming growth factor beta 1 is an epithelial-derived signal peptide that influences otic capsule formation

Interactions between epithelial and mesenchymal tissues in the developing inner ear direct the formation of its cartilaginous capsule. Recent work indicates that many growth factors are distributed in the early embryo in vivo in a temporal-spatial pattern that correlates with sites of ongoing morphogenetic events. We report here that the localization of transforming growth factor beta 1 (TGF-beta 1) in both epithelial and mesenchymal tissues of the mouse inner ear between 10 and 16 days of embryonic development (E10-E16). In addition, utilizing a high-density culture system as an in vitro model of otic capsule chondrogenesis, we show that modulation of chondrogenesis by TGF-beta 1 in cultured mouse periotic mesenchyme mimics the in vitro effects of otic epithelium on the expression of chondrogenic potential. We provide evidence of a causal relationship of this growth factor to otic capsule formation in situ by demonstrating that the actual sequence of chondrogenic events that occur in the developing embryo is reproduced in culture by the addition of exogenous TGF-beta 1 peptide. Furthermore, in cultures of mesenchyme containing otic epithelium, we demonstrate the localization of endogenous TGF-beta 1, first within the epithelial tissue and later within both the epithelium and its surrounding periotic mesenchyme, contrasted to an absence of endogenous TGF-beta 1 in cultures of mesenchyme alone. Our results suggest that TGF-beta 1 is one of the signal molecules that mediate the effects of otic epithelium in influencing the formation of the cartilaginous otic capsule.

Interdigital chondrogenesis and extra digit formation in the duck leg bud subjected to local ectoderm removal

In the chick embryo the interdigital tissue in the stages previous to cell death exhibits in vitro a high chondrogenic potential, and forms extra digits when subjected in vivo to local ectodermal removal. In the present work we have analyzed the chondrogenic potential both in vivo and in vitro of the interdigital mesenchyme of the duck leg bud. As distinct from the chick, the interdigital mesenchyme of the duck leg bud exhibits a low degree of degeneration, resulting in the formation of webbed digits. Our results show that duck interdigital mesenchyme exhibits also a high chondrogenic potential in vitro until the stages in which cell death starts. Once cell death is finished chondrogenesis becomes negative and the interdigital mesenchyme forms a fibroblastic tissue. In vivo the interdigital mesenchyme of the duck leg bud subjected to ectoderm removal forms ectopic foci of chondrogenesis with a range of incidence similar to that in the chick. Unlike those of the chick the ectopic cartilages of the duck are rounded and smaller, and appear to be located at the distal margin of the interdigital mesenchyme. Formation of extra digits in the duck occurs with a lower incidence than in the chick. It is concluded that ectopic chondrogenesis and formation of extra digits is related to the intensity of interdigital cell death. The non-degenerating interdigital mesenchymal cells destined to form the interdigital webs of the duck appear to contribute very little to the formation of interdigital cartilages.