Origin of the vertebrate inner ear: evolution and induction of the otic placode

Inner ear organoids: progress and outlook, with a focus on the vascularization

The inner ear is a complex organ that encodes sound, motion, and orientation in space. Given the complexity of the inner ear, it is not surprising that treatments are relatively limited despite the fact that, in 2015, hearing loss was the fourth leading cause of years lived with disability worldwide. Inner ear organoid models are a promising tool to advance the study of multiple aspects of the inner ear to aid the development of new treatments and validate drug-based therapies. The blood supply of the inner ear plays a pivotal role in growth, maturation, and survival of inner ear tissues and their physiological functions. This vasculature cannot be ignored in order to achieve a truly in vivo-like model that mimics the microenvironment and niches of organ development. However, this aspect of organoid development has remained largely absent in the generation of inner ear organoids. The current review focuses on three-dimensional inner ear organoid and how recent technical progress in generating in vitro vasculature can enhance the next generation of these models.

Morphology of the utricular otolith organ in the toadfish, Opsanus tau

The utricle provides the vestibular reflex pathways with the sensory codes of inertial acceleration of self-motion and head orientation with respect to gravity to control balance and equilibrium. Here we present an anatomical description of this structure in the adult oyster toadfish and establish a morphological basis for interpretation of subsequent functional studies. Light, scanning, and transmission electron microscopy techniques were applied to visualize the sensory epithelium at varying levels of detail, its neural innervation and its synaptic organization. Scanning electron microscopy was used to visualize otolith mass and morphological polarization patterns of hair cells. Afferent nerve fibers were visualized following labeling with biocytin, and light microscope images were used to make three-dimensional (3-D) reconstructions of individual labeled afferents to identify dendritic morphology with respect to epithelial location. Transmission electron micrographs were compiled to create a serial 3-D reconstruction of a labeled afferent over a segment of its dendritic field and to examine the cell-afferent synaptic contacts. Major observations are: a well-defined striola, medial and lateral extra-striolar regions with a zonal organization of hair bundles; prominent lacinia projecting laterally; dependence of hair cell density on macular location; narrow afferent dendritic fields that follow the hair bundle polarization; synaptic specializations issued by afferents are typically directed towards a limited number of 7-13 hair cells, but larger dendritic fields in the medial extra-striola can be associated with > 20 hair cells also; and hair cell synaptic bodies can be confined to only an individual afferent or can synapse upon several afferents.

The mouse Foxi3 transcription factor is necessary for the development of posterior placodes

The inner ear develops from the otic placode, one of the cranial placodes that arise from a region of ectoderm adjacent to the anterior neural plate called the pre-placodal domain. We have identified a Forkhead family transcription factor, Foxi3, that is expressed in the pre-placodal domain and down-regulated when the otic placode is induced. We now show that Foxi3 mutant mice do not form otic placodes as evidenced by expression changes in early molecular markers and the lack of thickened placodal ectoderm, an otic cup or otocyst. Some preplacodal genes downstream of Foxi3-Gata3, Six1 and Eya1-are not expressed in the ectoderm of Foxi3 mutant mice, and the ectoderm exhibits signs of increased apoptosis. We also show that Fgf signals from the hindbrain and cranial mesoderm, which are necessary for otic placode induction, are received by pre-placodal ectoderm in Foxi3 mutants, but do not initiate otic induction. Finally, we show that the epibranchial placodes that develop in close proximity to the otic placode and the mandibular division of the trigeminal ganglion are missing in Foxi3 mutants. Our data suggest that Foxi3 is necessary to prime pre-placodal ectoderm for the correct interpretation of inductive signals for the otic and epibranchial placodes.

Effects of music in combination with vibration in acupuncture points on the treatment of fibromyalgia

OBJECTIVE

Fibromyalgia (FM) is a disease that causes widespread pain and increased sensitivity to pain because of a dysfunction in the central nervous system. This study investigated the effect of music combined with vibration on acupuncture points for the treatment of FM.

METHODS

A total of 120 patients with FM were allocated randomly to four groups (30 patients each). One group listened to a sequence of Bach's compositions. The second group was subjected to vibratory stimuli on a combination of acupuncture points on the skin. The third group (complete) underwent both procedures in a simultaneous and synchronized manner, with inclusion of binaural beats. The fourth group (control) received no stimulation. The participants underwent the experimental procedure during five sessions performed on alternate days. They were assessed by the Fibromyalgia Impact Questionnaire (FIQ) and the Health Assessment Questionnaire (HAQ) before the first session and after the last session (20 days).

RESULTS

All groups showed a significant improvement in FIQ and HAQ scores at the evaluation after the intervention. The complete group exhibited the best result on both the FIQ and HAQ (p<0.001), and the improvement in HAQ score was significant (p<0.004).

CONCLUSIONS

The results suggest that the placebo effect in FM may be substantial. However, comparison between groups revealed that the complete group had the greatest reduction in both FIQ and HAQ, with a significant improvement in HAQ, suggesting that the combined use of music and vibration exerts a greater effect on FM symptoms.

The role of foxi family transcription factors in the development of the ear and jaw

The mammalian outer, middle, and inner ears have different embryonic origins and evolved at different times in the vertebrate lineage. The outer ear is derived from first and second branchial arch ectoderm and mesoderm, the middle ear ossicles are derived from neural crest mesenchymal cells that invade the first and second branchial arches, whereas the inner ear and its associated vestibule-acoustic (VIIIth) ganglion are derived from the otic placode. In this chapter, we discuss recent findings in the development of these structures and describe the contributions of members of a Forkhead transcription factor family, the Foxi family to their formation. Foxi transcription factors are critical for formation of the otic placode, survival of the branchial arch neural crest, and developmental remodeling of the branchial arch ectoderm.

Segregating neural and mechanosensory fates in the developing ear: patterning, signaling, and transcriptional control

The vertebrate inner ear is composed of multiple sensory receptor epithelia, each of which is specialized for detection of sound, gravity, or angular acceleration. Each receptor epithelium contains mechanosensitive hair cells, which are connected to the brainstem by bipolar sensory neurons. Hair cells and their associated neurons are derived from the embryonic rudiment of the inner ear epithelium, but the precise spatial and temporal patterns of their generation, as well as the signals that coordinate these events, have only recently begun to be understood. Gene expression, lineage tracing, and mutant analyses suggest that both neurons and hair cells are generated from a common domain of neural and sensory competence in the embryonic inner ear rudiment. Members of the Shh, Wnt, and FGF families, together with retinoic acid signals, regulate transcription factor genes within the inner ear rudiment to establish the axial identity of the ear and regionalize neurogenic activity. Close-range signaling, such as that of the Notch pathway, specifies the fate of sensory regions and individual cell types. We also describe positive and negative interactions between basic helix-loop-helix and SoxB family transcription factors that specify either neuronal or sensory fates in a context-dependent manner. Finally, we review recent work on inner ear development in zebrafish, which demonstrates that the relative timing of neurogenesis and sensory epithelial formation is not phylogenetically constrained.

Setting appropriate boundaries: fate, patterning and competence at the neural plate border

The neural crest and craniofacial placodes are two distinct progenitor populations that arise at the border of the vertebrate neural plate. This border region develops through a series of inductive interactions that begins before gastrulation and progressively divide embryonic ectoderm into neural and non-neural regions, followed by the emergence of neural crest and placodal progenitors. In this review, we describe how a limited repertoire of inductive signals-principally FGFs, Wnts and BMPs-set up domains of transcription factors in the border region which establish these progenitor territories by both cross-inhibitory and cross-autoregulatory interactions. The gradual assembly of different cohorts of transcription factors that results from these interactions is one mechanism to provide the competence to respond to inductive signals in different ways, ultimately generating the neural crest and cranial placodes.

Shaping sound in space: the regulation of inner ear patterning

The inner ear is one of the most morphologically elaborate tissues in vertebrates, containing a group of mechanosensitive sensory organs that mediate hearing and balance. These organs are arranged precisely in space and contain intricately patterned sensory epithelia. Here, we review recent studies of inner ear development and patterning which reveal that multiple stages of ear development - ranging from its early induction from the embryonic ectoderm to the establishment of the three cardinal axes and the fine-grained arrangement of sensory cells - are orchestrated by gradients of signaling molecules.

Sprouty1 and Sprouty2 limit both the size of the otic placode and hindbrain Wnt8a by antagonizing FGF signaling

Multiple signaling molecules, including Fibroblast Growth Factor (FGF) and Wnt, induce two patches of ectoderm on either side of the hindbrain to form the progenitor cell population for the inner ear, or otic placode. Here we report that in Spry1, Spry2 compound mutant embryos (Spry1⁻/⁻; Spry2⁻/⁻ embryos), the otic placode is increased in size. We demonstrate that the otic placode is larger due to the recruitment of cells, normally destined to become cranial epidermis, into the otic domain. The enlargement of the otic placode observed in Spry1⁻/⁻; Spry2⁻/⁻ embryos is preceded by an expansion of a Wnt8a expression domain in the adjacent hindbrain. We demonstrate that both the enlargement of the otic placode and the expansion of the Wnt8a expression domain can be rescued in Spry1⁻/⁻; Spry2⁻/⁻ embryos by reducing the gene dosage of Fgf10. Our results define a FGF-responsive window during which cells can be continually recruited into the otic domain and uncover SPRY regulation of the size of a putative Wnt inductive center.

Expression patterns of astrocyte elevated gene-1 (AEG-1) during development of the mouse embryo

Expression of astrocyte elevated gene-1 (AEG-1) is elevated in multiple human cancers including brain tumors, neuroblastomas, melanomas, breast cancers, non-small cell lung cancers, liver cancers, prostate cancers, and esophageal cancers. This gene plays crucial roles in tumor cell growth, invasion, angiogenesis and progression to metastasis. In addition, over-expression of AEG-1 protects primary and transformed cells from apoptosis-inducing signals by activating PI3K-Akt signaling pathways. These results suggest that AEG-1 is intimately involved in tumorigenesis and may serve as a potential therapeutic target for various human cancers. However, the normal physiological functions of AEG-1 require clarification. We presently analyzed the expression pattern of AEG-1 during mouse development. AEG-1 was expressed in mid-to-hindbrain, fronto-nasal processes, limbs, and pharyngeal arches in the early developmental period from E8.5 to E9.5. In addition, at stages of E12.5-E18.5 AEG-1 was localized in the brain, and olfactory and skeletal systems suggesting a role in neurogenesis, as well as in skin, including hair follicles, and in the liver, which are organ sites in which AEG-1 has been implicated in tumor development and progression. AEG-1 co-localized with Ki-67, indicating a role in cell proliferation, as previously revealed in tumorigenesis. Taken together, these results suggest that AEG-1 may play a prominent role during normal mouse development in the context of cell proliferation as well as differentiation, and that temporal regulation of AEG-1 expression may be required during specific stages and in specific tissues during development.

Identification of a primary target of thalidomide teratogenicity

Half a century ago, thalidomide was widely prescribed to pregnant women as a sedative but was found to be teratogenic, causing multiple birth defects. Today, thalidomide is still used in the treatment of leprosy and multiple myeloma, although how it causes limb malformation and other developmental defects is unknown. Here, we identified cereblon (CRBN) as a thalidomide-binding protein. CRBN forms an E3 ubiquitin ligase complex with damaged DNA binding protein 1 (DDB1) and Cul4A that is important for limb outgrowth and expression of the fibroblast growth factor Fgf8 in zebrafish and chicks. Thalidomide initiates its teratogenic effects by binding to CRBN and inhibiting the associated ubiquitin ligase activity. This study reveals a basis for thalidomide teratogenicity and may contribute to the development of new thalidomide derivatives without teratogenic activity.

Competence, specification and commitment to an olfactory placode fate

The nasal placode shares a common origin with other sensory placodes within a pre-placodal domain at the cranial neural plate border. However, little is known about early events in nasal placode development as it segregates from prospective lens, neural tube and epidermis. Here, Dlx3, Dlx5, Pax6 and the pan-neuronal marker Hu serve as molecular labels to follow the maturation of olfactory precursors over time. When competence to form olfactory placode was tested by grafting ectoderm from different axial levels to the anterior neural fold, we found that competence is initially broad for head, but not trunk, ectoderm and declines rapidly with time. Isolated olfactory precursors are specified by HH10, concomitant with their complete segregation from other placodal, epidermal and neural progenitors. Heterotopic transplantation of olfactory progenitors reveals they are capable of autonomous differentiation only 12 hours later, shortly before overt placode invagination at HH14. Taken together, these results show that olfactory placode development is a step-wise process whereby signals from adjacent tissues specify competent ectoderm at or before HH10, followed by gradual commitment just prior to morphological differentiation.

Deciphering the function of canonical Wnt signals in development and disease: conditional loss- and gain-of-function mutations of beta-catenin in mice

Wnt signaling is one of a handful of powerful signaling pathways that play crucial roles in the animal life by controlling the genetic programs of embryonic development and adult homeostasis. When disrupted, these signaling pathways cause developmental defects, or diseases, among them cancer. The gateway of the canonical Wnt pathway, which contains >100 genes, is an essential molecule called beta-catenin (Armadillo in Drosophila). Conditional loss- and gain-of-function mutations of beta-catenin in mice provided powerful tools for the functional analysis of canonical Wnt signaling in many tissues and organs. Such studies revealed roles of Wnt signaling that were previously not accessible to genetic analysis due to the early embryonic lethality of conventional beta-catenin knockout mice, as well as the redundancy of Wnt ligands, receptors, and transcription factors. Analysis of conditional beta-catenin loss- and gain-of-function mutant mice demonstrated that canonical Wnt signals control progenitor cell expansion and lineage decisions both in the early embryo and in many organs. Canonical Wnt signaling also plays important roles in the maintenance of various embryonic or adult stem cells, and as recent findings demonstrated, in cancer stem cell types. This has opened new opportunities to model numerous human diseases, which have been associated with deregulated Wnt signaling. Our review summarizes what has been learned from genetic studies of the Wnt pathway by the analysis of conditional beta-catenin loss- and gain-of-function mice.

Lateral line, otic and epibranchial placodes: developmental and evolutionary links?

Two embryonic cell populations, the neural crest and cranial ectodermal placodes, between them give rise to many of the unique characters of vertebrates. Neurogenic placode derivatives are vital for sensing both external and internal stimuli. In this speculative review, we discuss potential developmental and evolutionary relationships between two placode series that are usually considered to be entirely independent: lateral line placodes, which form the mechanosensory and electroreceptive hair cells of the anamniote lateral line system as well as their afferent neurons, and epibranchial placodes (geniculate, petrosal and nodose), which form Phox2b(+) visceral sensory neurons with input from both the external and internal environment. We illustrate their development using molecular data we recently obtained in shark embryos, and we describe their derivatives, including the possible geniculate placode origin of a mechanosensory sense organ associated with the first pharyngeal pouch/cleft (the anamniote spiracular organ/amniote paratympanic organ). We discuss how both lateral line and epibranchial placodes can be related in different ways to the otic placode (which forms the inner ear and its afferent neurons), and how both are important for protective somatic reflexes. Finally, we put forward a highly speculative proposal about the original function of the cells whose evolutionary descendants today include the derivatives of the lateral line, otic and epibranchial placodes, namely that they produced sensory receptors and neurons for Phox2b-dependent protective reflex circuits. We hope this review will stimulate both debate and a fresh look at possible developmental and evolutionary relationships between these seemingly disparate and independent placodes.

Spalt4 mediates invagination and otic placode gene expression in cranial ectoderm

Vertebrate placodes are regions of thickened head ectoderm that contribute to paired sensory organs and cranial ganglia. We demonstrate that the transcription factor Spalt4 (also known as Sall4) is broadly expressed in chick preplacodal epiblast and later resolves to otic, lens and olfactory placodes. Ectopic expression of Spalt4 by electroporation is sufficient to induce invagination of non-placodal head ectoderm and prevent neurogenic placodes from contributing to cranial ganglia. Conversely, loss of Spalt4 function in the otic placode results in abnormal otic vesicle development. Intriguingly, Spalt4 appears to initiate a placode program appropriate for the axial level but is not involved in later development of specific placode fates. Fgfs can regulate Spalt4, since implantation of Fgf2 beads into the area opaca induces its expression. The results suggest that Spalt4 is involved in early stages of placode development, initiating cranial ectodermal invagination and region-specific gene regulatory networks.

Identification and developmental expression of two Tbx1/10-related genes in the agnathan Lethenteron japonicum

We have identified two Tbx1/10-related genes, LjTbx1/10A and LjTbx1/10B, from the Japanese river lamprey Lethenteron japonicum. We used in situ hybridization to characterize their expression pattern during embryonic development. LjTbx1/10A and LjTbx1/10B shared common expression in the pharyngeal arches and otic vesicle, although their levels and timing of expression differed markedly. LjTbx1/10A was highly expressed in the mesodermal core of pharyngeal arches and the adjacent endoderm throughout pharyngeal arch development, whereas LjTbx1/10B expression was only transiently upregulated in forming pharyngeal pouches. LjTbx1/10A transcripts first appeared at stage 25 in otic vesicles, whereas LjTbx1/10B transcripts could already be detected in the developing otic placode at stage 20. These results suggest that lamprey LjTbx1/10A and LjTbx1/10B may play distinct roles in the patterning and development of the pharyngeal apparatus. It appears that lamprey Tbx1/10 genes have undergone subfunctionalization independent from gnathostomes, with regard to both regulation and function.

The developing lamprey ear closely resembles the zebrafish otic vesicle: otx1 expression can account for all major patterning differences

The inner ear of adult agnathan vertebrates is relatively symmetric about the anteroposterior axis, with only two semicircular canals and a single sensory macula. This contrasts with the highly asymmetric gnathostome arrangement of three canals and several separate maculae. Symmetric ears can be obtained experimentally in gnathostomes in several ways, including by manipulation of zebrafish Hedgehog signalling, and it has been suggested that these phenotypes might represent an atavistic condition. We have found, however, that the symmetry of the adult lamprey inner ear is not reflected in its early development; the lamprey otic vesicle is highly asymmetric about the anteroposterior axis, both morphologically and molecularly, and bears a striking resemblance to the zebrafish otic vesicle. The single sensory macula originates as two foci of hair cells, and later shows regions of homology to the zebrafish utricular and saccular maculae. It is likely, therefore, that the last common ancestor of lampreys and gnathostomes already had well-defined otic anteroposterior asymmetries. Both lamprey and zebrafish otic vesicles express a target of Hedgehog signalling, patched, indicating that both are responsive to Hedgehog signalling. One significant distinction between agnathans and gnathostomes, however, is the acquisition of otic Otx1 expression in the gnathostome lineage. We show that Otx1 knockdown in zebrafish, as in Otx1(-/-) mice, gives rise to lamprey-like inner ears. The role of Otx1 in the gnathostome ear is therefore highly conserved; otic Otx1 expression is likely to account not only for the gain of a third semicircular canal and crista in gnathostomes, but also for the separation of the zones of the single macula into distinct regions.

Expression of TGFbeta family in the developing internal ear of rat embryos

In order to investigate the expression patterns of the transforming growth factor (TGF)beta isoforms in the internal ear, an immunohistochemical study of rat embryos was performed. Rat embryos were taken on the 13th, 15th, 17th, and 19th day after conception and their internal ears were immunohistochemically stained against TGF beta1, beta2, and beta3. As a result, the 13-day-old embryo showed a very weak positivity to TGF beta1. After the 15th day of pregnancy, no reactivity to TGF beta1 was defected. Immunoreactivity to TGF beta2 was observed from the 15th day of pregnancy throughout the rest of the period. The ampulla of the semicircular canal and the cochlear duct showed a notably strong immunohistochemical reaction. A strong reaction to TGF beta3 was observed on the 15th day of pregnancy. However, no positive reactions were observed thereafter. A strong immunoreactivity was observed especially on the apical cytoplasms, the surfaces of the epithelial cells, and basement membranes of the cochlear duct, as well as the semicircular canals of the developing internal ear of rat embryo.

Development and evolution of lateral line placodes in amphibians I. Development

Lateral line placodes are specialized regions of the ectoderm that give rise to the receptor organs of the lateral line system as well as to the sensory neurons innervating them. The development of lateral line placodes has been studied in amphibians since the early 1900s. This paper reviews these older studies and tries to integrate them with more recent findings. Lateral line placodes are probably induced in a multistep process from a panplacodal area surrounding the neural plate. The time schedule of these inductive processes has begun to be unravelled, but little is known yet about their molecular basis. Subsequent pattern formation, morphogenesis and differentiation of lateral line placodes proceeds in most respects relatively autonomously: Onset and polarity of migration of lateral line primordia, the type, spacing, size and number of receptor organs formed, as well as the patterned differentiation of different cell types occur normally even in ectopic locations. Only the pathways for migration of lateral line primordia depend on external cues. Thus, lateral line placodes act as integrated and relatively context-insensitive developmental modules.

Tissues and signals involved in the induction of placodal Six1 expression in Xenopus laevis

Ectodermal placodes, from which many cranial sense organs and ganglia develop, arise from a common placodal primordium defined by Six1 expression. Here, we analyse placodal Six1 induction in Xenopus using microinjections and tissue grafts. We show that placodal Six1 induction occurs during neural plate and neural fold stages. Grafts of anterior neural plate but not grafts of cranial dorsolateral endomesoderm induce Six1 ectopically in belly ectoderm, suggesting that only the neural plate is sufficient for inducing Six1 in ectoderm. However, extirpation of either anterior neural plate or of cranial dorsolateral endomesoderm abolishes placodal Six1 expression indicating that both tissues are required for its induction. Elevating BMP-levels blocks placodal Six1 induction, whereas ectopic sources of BMP inhibitors expand placodal Six1 expression without inducing Six1 ectopically. This suggests that BMP inhibition is necessary but needs to cooperate with additional factors for Six1 induction. We show that FGF8, which is expressed in the anterior neural plate, can strongly induce ectopic Six1 in ventral ectoderm when combined with BMP inhibitors. In contrast, FGF8 knockdown abolishes placodal Six1 expression. This suggests that FGF8 is necessary and together with BMP inhibitors sufficient to induce placodal Six1 expression in cranial ectoderm, implicating FGF8 as a central component in generic placode induction.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

Hierarchy of regulatory events in sensory placode development

Cranial placodes are a uniquely vertebrate characteristic; they form the paired sense organs of the eyes, ears and nose, in addition to the distal parts of some of the cranial sensory ganglia. These focal ectodermal thickenings have been studied from an embryological perspective in a diversity of organisms, revealing tissue interactions that are crucial for the morphological formation of the different placodes. In recent times, there has been a renewed interest in understanding the induction and differentiation of these deceptively simple ectodermal regions. This has led to a wealth of information on the molecular cues governing these processes. In particular, the integration of signals at the level of 'placode-specific' enhancers is beginning to provide a glimpse into the complexity of genetic networks that function within this embryonic cell population to generate key components of the peripheral nervous system.

Molecular anatomy of placode development in Xenopus laevis

We analyzed the spatiotemporal pattern of expression of 15 transcription factors (Six1, Six4, Eya1, Sox3, Sox2, Pax6, Pax3, Pax2, Pax8, Dlx3, Msx1, FoxI1c, Tbx2, Tbx3, Xiro1) during placode development in Xenopus laevis from neural plate to late tail bud stages. Out of all genes investigated, only the expression of Eya1, Six1, and Six4 is maintained in all types of placode (except the lens) throughout embryonic development, suggesting that they may promote generic placodal properties and that their crescent-shaped expression domain surrounding the neural plate defines a panplacodal primordium from which all types of placode originate. Double-labeling procedures were employed to reveal the precise position of this panplacodal primordium relative to neural plate, neural crest, and other placodal markers. Already at neural plate stages, the panplacodal primordium is subdivided into several subregions defined by particular combinations of transcription factors allowing us to identify the approximate regions of origin of various types of placode. Whereas some types of placode were already prefigured by molecularly distinct areas at neural plate stages, the epibranchial, otic, and lateral line placodes arise from a common posterior placodal area (characterized by Pax8 and Pax2 expression) and acquire differential molecular signatures only after neural tube closure. Our findings argue for a multistep mechanism of placode induction, support a combinatorial model of placode specification, and suggest that different placodes evolved from a common placodal primordium by successive recruitment of new inducers and target genes.

Ringing in the new ear: resolution of cell interactions in otic development

The vertebrate inner ear is a marvel of structural and functional complexity, which is all the more remarkable because it develops from such a simple structure, the otic placode. Analysis of inner ear development has long been a fascination of experimental embryologists, who sought to understand cellular mechanisms of otic placode induction. More recently, however, molecular and genetic approaches have made the inner ear a useful model system for studying a much broader range of basic developmental mechanisms, including cell fate specification and differentiation, axial patterning, epithelial morphogenesis, cytoskeletal dynamics, stem cell biology, neurobiology, physiology, etc. Of course, there has also been tremendous progress in understanding the functions and processes peculiar to the inner ear. The goal of this review is to recount how historical approaches have shaped our understanding of the signaling interactions controlling early otic development; to discuss how new findings have led to fundamental new insights; and to point out new problems that need to be resolved in future research.

Development of the basement membrane and formation of collagen fibrils below the placodes in the head of anuran larvae

The development of the basement membrane and collagen fibrils below placodes, including the corneal region of the ectoderm, lens epithelium, nasal plate, and auditory vesicle in anuran larvae was observed by transmission electron microscopy and compared with that in nonplacodal regions such as the epidermis, neural tube, and optic vesicle. In the corneal region the lamina densa becomes thick concomitantly with the development of the connecting apparatuses such as hemidesmosomes and anchoring fibrils. The collagen fibrils increase in number and form a multilayered structure, showing similar morphology to the connective tissues below the epidermis. These two areas, i.e., the corneal region and epidermis, possess much collagenous connective tissue below them. On the other hand, the neural tube and ophthalmic vesicle that originated from the neural tube each have a thin lamina densa and a small number of underlying collagen fibrils. The lamina densa does not thicken and the number of collagen fibrils do not significantly increase during development. These two areas possess little extracellular matrix. The nasal plate and auditory vesicle show intermediate characteristics between the epidermis-type and the neural tube-type areas. In these areas, the lamina densa becomes thick and hemidesmosomes and anchoring fibrils develop. The number of collagen fibrils increases during development, but does not show an orderly arrangement; rather, they are randomly distributed. It is thought that the difference in the arrangement of collagen fibrils in different tissues is due to differences in the extracellular matrix around the collagen fibrils. Placodal epithelia have the same origin as epidermis, but during development their morphological characteristics differ and they are not associated with the pattern of extracellular matrix with characteristics of epidermal and corneal multilayered collagen fibril areas.