Induction of the chick columella and its integration with the inner ear

Congenital ear malformation (CEM)

BACKGROUND

At present, there are not international unified standards and reports on Congenital Ear Malformation (CEM) in the world, which makes it difficult to transfer information and compare the literature.

AIMS/OBJECTIVES

Through the statistical analysis of a large sample of CEM, a unified standard of all aspects of CEM is proposed and the data are provided for reference, which is convenient for the international work and literature comparison in this field.

MATERIALS AND METHODS

Based on the author's 30 years of clinical and scientific research work on CEM and the relevant cases of 3231 (4714 ears) in our hospital, and combined with literature, statistical analysis was made.

RESULTS

This paper summarizes the classification, definition, epidemiology, embryonic development, pathogenic factors of CEM and elaborates on the clinical manifestations, examination and sequence therapy of representative Congenital Malformation of the Middle and Outer Ear (CMMOE). We also introduce malformation of the auricle and inner ear, so as to cover the outer, middle and inner ear. At the same time, we introduce our achievements and contributions in this field.

CONCLUSIONS AND SIGNIFICANCE

This study provides reference to the international unified standard and treatment principle of the CEM.

Distinct proliferative and middle ear skeletal-patterning functions for SHH-expressing epithelia in the chick hyoid arch

During chick craniofacial development, the second (hyoid) pharyngeal arch expands to close the neck and gives rise to skeletal elements, including the columella of the middle ear (a homologue of the mammalian stapes). Sonic hedgehog (SHH) signalling has been implicated in hyoid arch expansion and columella formation, but spatial and temporal aspects of these signalling interactions within the hyoid arch remain poorly understood. Here, we show that SHH is initially expressed in the posterior endoderm of the hyoid arch, and that this domain subsequently splits into a distal domain at the site of arch expansion (the posterior epithelial margin, PEM), and a proximal domain that lines the foregut (the proximal hyoid epithelium, PHE). Pharmacological manipulations and heterotopic grafting experiments demonstrate that SHH signalling is required for hyoid arch expansion and skeletogenesis, and reveal distinct roles for the PEM and PHE in these processes. The PEM promotes mesenchymal cell proliferation during arch expansion but is not sufficient to repattern the columella. Conversely, the PHE promotes mesenchymal cell survival, and PHE grafts induce partial duplication of the columella. This work demonstrates crucial and distinct roles for endodermal SHH signalling in hyoid arch morphogenesis and patterning of the middle ear skeleton.

Region-specific endodermal signals direct neural crest cells to form the three middle ear ossicles

Defects in the middle ear ossicles - malleus, incus and stapes - can lead to conductive hearing loss. During development, neural crest cells (NCCs) migrate from the dorsal hindbrain to specific locations in pharyngeal arch (PA) 1 and 2, to form the malleus-incus and stapes, respectively. It is unclear how migratory NCCs reach their proper destination in the PA and initiate mesenchymal condensation to form specific ossicles. We show that secreted molecules sonic hedgehog (SHH) and bone morphogenetic protein 4 (BMP4) emanating from the pharyngeal endoderm are important in instructing region-specific NCC condensation to form malleus-incus and stapes, respectively, in mouse. Tissue-specific knockout of Shh in the pharyngeal endoderm or Smo (a transducer of SHH signaling) in NCCs causes the loss of malleus-incus condensation in PA1 but only affects the maintenance of stapes condensation in PA2. By contrast, knockout of Bmp4 in the pharyngeal endoderm or Smad4 (a transducer of TGFβ/BMP signaling) in the NCCs disrupts NCC migration into the stapes region in PA2, affecting stapes formation. These results indicate that region-specific endodermal signals direct formation of specific middle ear ossicles.

Major evolutionary transitions and innovations: the tympanic middle ear

One of the most amazing transitions and innovations during the evolution of mammals was the formation of a novel jaw joint and the incorporation of the original jaw joint into the middle ear to create the unique mammalian three bone/ossicle ear. In this review, we look at the key steps that led to this change and other unusual features of the middle ear and how developmental biology has been providing an understanding of the mechanisms involved. This starts with an overview of the tympanic (air-filled) middle ear, and how the ear drum (tympanic membrane) and the cavity itself form during development in amniotes. This is followed by an investigation of how the ear is connected to the pharynx and the relationship of the ear to the bony bulla in which it sits. Finally, the novel mammalian jaw joint and versatile dentary bone will be discussed with respect to evolution of the mammalian middle ear.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

Differing contributions of the first and second pharyngeal arches to tympanic membrane formation in the mouse and chick

We have proposed that independent origins of the tympanic membrane (TM), consisting of the external auditory meatus (EAM) and first pharyngeal pouch, are linked with distinctive middle ear structures in terms of dorsal-ventral patterning of the pharyngeal arches during amniote evolution. However, previous studies have suggested that the first pharyngeal arch (PA1) is crucial for TM formation in both mouse and chick. In this study, we compare TM formation along the anterior-posterior axis in these animals using Hoxa2 expression as a marker of the second pharyngeal arch (PA2). In chick, the EAM begins to invaginate at the surface ectoderm of PA2, not at the first pharyngeal cleft, and the entire TM forms in PA2. Chick-quail chimera that have lost PA2 and duplicated PA1 suggest that TM formation is achieved by developmental interaction between a portion of the EAM and the columella auris in PA2, and that PA1 also contributes to formation of the remaining part of the EAM. By contrast, in mouse, TM formation is highly associated with an interdependent relationship between the EAM and tympanic ring in PA1.

Development and Integration of the Ear

The perception of our environment via sensory organs plays a crucial role in survival and evolution. Hearing, one of our most developed senses, depends on the proper function of the auditory system and plays a key role in social communication, integration, and learning ability. The ear is a composite structure, comprised of the external, middle, and inner ear. During development, the ear is formed from the integration of a number of tissues of different embryonic origin, which initiate in distinct areas of the embryo at different time points. Functional connections between the components of the hearing apparatus have to be established and maintained during development and adulthood to allow proper sound submission from the outer to the middle and inner ear. This highly organized and intimate connectivity depends on intricate spatiotemporal signaling between the various tissues that give rise to the structures of the ear. Any alterations in this chain of events can lead to the loss of integration, which can subsequently lead to conductive hearing loss, in case of outer and middle ear defects or sensorineural hearing loss, if inner ear structures are defective. This chapter aims to review the current knowledge concerning the development of the three ear compartments as well as mechanisms and signaling pathways that have been implicated in the coordination and integration process of the ear.

A technique to increase accessibility to late-stage chick embryos for in ovo manipulations

BACKGROUND

During early development, avian embryos are easily accessible in ovo for transplantations and experimental perturbations. However, these qualities of the avian embryonic model rapidly wane shortly after embryonic day (E)4 when the embryo is obscured by extraembryonic membranes, making it difficult to study developmental events that occur at later stages in vivo.

RESULTS

In this study, we describe a multistep method that involves initially windowing eggs at E3, followed by dissecting away extraembryonic membranes at E5 to facilitate embryo accessibility in ovo until later stages of development. The majority of the embryos subjected to this technique remain exposed between E5 and E8, then become gradually displaced by the growing allantois from posterior to anterior regions.

CONCLUSIONS

Exposed embryos are viable and compatible with embryological and modern developmental biology techniques including tissue grafting and ablation, gene manipulation by electroporation, and protein expression. This technique opens up new avenues for studying complex cellular interactions during organogenesis and can be further extrapolated to regeneration and stem cell studies.

Developmental origin and fate of middle ear structures

Results from developmental and phylogenetic studies have converged to facilitate insight into two important steps in vertebrate evolution: (1) the ontogenetic origin of articulating elements of the buccal skeleton, i.e., jaws, and (2) the later origins of middle ear impedance-matching systems that convey air-borne sound to the inner ear fluids. Middle ear ossicles and other skeletal elements of the viscerocranium (i.e., gill suspensory arches and jaw bones) share a common origin both phylogenetically and ontogenetically. The intention of this brief overview of middle-ear development is to emphasize the intimate connection between evolution and embryogenesis. Examples of developmental situations are discussed in which cells of different provenance, such as neural crest, mesoderm or endoderm, gather together and reciprocal interactions finally determine cell fate. Effects of targeted mutagenesis on middle ear development are described to illustrate how the alteration of molecularly-controlled morphogenetic programs led to phylogenetic modifications of skeletal development. Ontogenetic plasticity has enabled the diversification of jaw elements as well as middle ear structures during evolution. This article is part of a special issue entitled "MEMRO 2012".

The origin of the stapes and relationship to the otic capsule and oval window

BACKGROUND

The stapes, an ossicle found within the middle ear, is involved in transmitting sound waves to the inner ear by means of the oval window. There are several developmental problems associated with this ossicle and the oval window, which cause hearing loss. The developmental origin of these tissues has not been fully elucidated.

RESULTS

Using transgenic reporter mice, we have shown that the stapes is of dual origin with the stapedial footplate being composed of cells of both neural crest and mesodermal origin. Wnt1cre/Dicer mice fail to develop neural crest-derived cartilages, therefore, have no middle ear ossicles. We have shown in these mice the mesodermal stapedial footplate fails to form and the oval window is induced but underdeveloped.

CONCLUSIONS

If the neural crest part of the stapes fails to form the mesodermal part does not develop, indicating that the two parts are interdependent. The stapes develops tightly associated with the otic capsule, however, it is not essential for the positioning of the oval window, suggesting that other tissues, perhaps within the inner ear are needed for oval window placement.