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 TGFß/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.

Fetal development of the elastic-fiber-mediated enthesis in the human middle ear.

In the human middle ear, the annular ligament of the incudostapedial joint and the insertions of the tensor tympani and stapedius muscles contain abundant elastic fibers; i.e., the elastic-fiber-mediated entheses. Hyaluronan also coexists with the elastic fibers. In the present study using immunohistochemistry, we demonstrated the distribution of elastin not only in the incudostapedial joint but also in the other two joints of the middle ear in adults and fetuses. In adults, the expression of elastin did not extend out of the annular ligament composed of mature elastic fibers but clearly overlapped with it. Electron microscopic observations of the annular ligament demonstrated a few microfibrils along the elastic fibers. Thus, in contrast to the vocal cord, the middle ear entheses seemed not to contain elaunin and oxytalan fibers. In mid-term fetuses (at approximately 15-16 weeks of gestation) before opening of the external acoustic meatus, the incudostapedial joint showed abundant elastic fibers, but the incudomalleolar and stapediovestibular joints did not. At this stage, hyaluronan was not colocalized, but distributed diffusely in loose mesenchymal tissues surrounding the ear ossicles. Therefore, fetal development of elastin and elastic fibers in the middle ear entheses is unlikely to require acoustic oscillation. In late-stage fetuses (25-30 weeks), whose ear ossicles were almost the same size as those in adults, we observed bundling and branching of elastic fibers. However, hyaluronan expression was not as strong as in adults. Colocalization between elastic fibers and hyaluronan appeared to be a result of postnatal maturation of the entheses.

Crural attachment to promontory case report: implications for stapes development.

Understanding of the embryologic origin of the stapes remains controversial. Theories diverge upon whether the entirety of the stapes arises from a single source versus the footplate and suprastructure arising from distinct sources. A 12-year-old boy with left-sided conductive hearing loss had computed tomography of the temporal bone, revealing an inferiorly displaced left stapes, and a nonspecific density in the left Prussak's space. Exploratory tympanotomy revealed the crura of the stapes to be attached to the promontory. The stapes footplate was located in the oval window and was mobile.

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.

Thyroid hormone receptors control developmental maturation of the middle ear and the size of the ossicular bones.

Thyroid hormone is critical for auditory development and has well-known actions in the inner ear. However, less is known of thyroid hormone functions in the middle ear, which contains the ossicles (malleus, incus, stapes) that relay mechanical sound vibrations from the outer ear to the inner ear. During the later stages of middle ear development, prior to the onset of hearing, middle ear cavitation occurs, involving clearance of mesenchyme from the middle ear cavity while the immature cartilaginous ossicles attain appropriate size and ossify. Using in situ hybridization, we detected expression of Thra and Thrb genes encoding thyroid hormone receptors α1 and ß (TRα1 and TRß, respectively) in the immature ossicles, surrounding mesenchyme and tympanic membrane in the mouse. Thra(+/PV) mice that express a dominant-negative TRα1 protein exhibited deafness with elevated auditory thresholds and a range of middle ear abnormalities including chronic persistence of mesenchyme in the middle ear into adulthood, markedly enlarged ossicles, and delayed ossification of the ossicles. Congenitally hypothyroid Tshr(-/-) mice and TR-deficient Thra1(-/-);Thrb(-/-) mice displayed similar abnormalities. These findings demonstrate that middle ear maturation is TR dependent and suggest that the middle ear is a sensitive target for thyroid hormone in development.

Tympanic ossicles and pharyngeal arches.

We have performed a study on 11 human embryos regarding the development of the tympanic ossicles and their relationship with the first pharyngeal arch. After performing measurements to date the embryos and foetuses chronologically, we performed a meticulous dissection of the temporal bones. Subsequently, they were fixed in 10% formol, decalcified with 2% nitric acid, embedded in Paraplast, sectioned in 7-mm sequences and stained with Martin's trichrome technique. In the 21- and 24-mm cranium-raquis (CR) length human embryos, we have observed the head of the malleus and the body of the incus close to Meckel's cartilage, in addition to the handle of the malleus, the long limb of the incus and the stapes. Between them there was a mesenchymal band inside the primordium of the tympanic cavity. In the 27-mm CR embryo, the various components of the malleus and incus were fusing, and in the 30-mm CR embryo the union was complete. From our observations, we can conclude that the malleus and the incus are derived from the first and second pharyngeal arches.

[Ontogenic development of the incudostapedial joint]. / Desarrollo ontogénico de la articulación incudoestapedial.

OBJECTIVE: To study the development of the incudostapedial joint in human embryos and foetuses. MATERIAL AND METHOD: 46 temporal bones with specimens between 9 mm and newborns were studied. The preparations were sliced serially and dyed using the Martins trichrome technique. RESULTS: The incudostapedial joint takes on the characteristics of a spheroidal joint at 16 weeks of development. The cartilage covering the articular surfaces is formed by different strata that develop in succession: the superficial stratum at 19 weeks, the transitional between 20 and 23 weeks, and the radial from 24 weeks on. The subchondral bone develops after 29 weeks by the mechanisms of apposition and extension of the periosteal and endosteal bones, but it is not until week 34 that it completely covers the articular surfaces, following constitution of the bone fascicles transmitting the lines of force. The articular capsule is formed from the inter-zone, the surface zone develops the capsular ligament, and the internal surface develops the synovial membrane. CONCLUSIONS: At the time of birth, the incudostapedial joint is completely developed.

Development of the dynamic structure (force lines) of the middle ear ossicles in human foetuses.

OBJECTIVES: To study the ontogenic development of the organisation of the human middle ear ossicles structure. MATERIAL AND METHODS: 46 human temporal bones of ages varying from 32 days post-conception to newborns. RESULTS: The development of the structural organisation of the malleus begins at 16 weeks via two cortical fascicles situated in the neck; at 21 weeks they extend towards the head, at 23 weeks to the lateral process and at 24 weeks to the handle. In the handle, the force lines are transmitted via three cardinal fascicles, two of them via the cortical fascicle and one via the centre, which starts after 29 weeks' development and is consolidated after 31 weeks. In the incus the force lines start at 16 weeks via two cortical fascicles situated in the long process, which progressively extend in a rostro-caudal direction between 17 and 20 weeks. At 21 weeks they occupy the whole extension of the long process and at 22 weeks the fusion of both cortical fascicles begins. From 30 weeks onwards it is strengthened by the crossing of bone trabeculae from one cortical to another. Two fascicles come out of the incus body, surrounding the medullary cavity and going in the direction of the short process. In the beginning, the stapes have two cortical fascicles in their crura. The remodelling process makes the internal cortical fascicle disappear and after 31 weeks all the force lines run through the external cortical fascicle. The tympanic membrane of the stapes footplate undergoes a remodelling process and after 28 weeks bony trabeculae are deposited. In newborns (40 weeks), the ossicles' structure is cavitary and has not been completed. The fan-shaped trabecular fascicle, which starts in the articular facets of the malleus and the incus, still has to develop.

Development of the stapes and associated structures in human embryos.

The objective of this study was to clarify the development of the stapes in humans and its relationship with the cartilage of the second branchial arch. The study was carried out in 25 human embryos between 6 and 28 mm crown-rump length. The stapes develops at the cranial end of the second branchial arch through an independent anlage of the cartilage of this arch. Between the stapedial anlage and the cranial end of the Reichert's cartilage there is a formation called the interhyale, the internal segment of which gives rise to the tendon of the stapedial muscle. The stapedial anlage is a unique formation with two distinct parts: the superior part that will comprise the base and the inferior part that will be crossed by the stapedial artery during embryonic development and will constitute the limbs and the head of the stapes. According to the results, the otic capsule is not involved in formation of the base of the stapes.

Descriptive growth model of the height of stapes in the fetus: a histopathological study of the temporal bone.

Temporal bone histological findings can be evaluated from several points of view. The most basic consists of a description of the characteristics and abnormalities of particular temporal bones. The second one is the measurement of various structures in a larger set of temporal bones and the monitoring of these structures over time. The height of stapes was measured in a set of 40 temporal bones from 27 fetuses, and the growth of stapes from the 13th to 36th weeks of pregnancy was determined. A computer-assisted nonlinear regression analysis of diagnostics enabling simultaneous examination of data (influential points, i.e., outliers and leverages) was carried out, a growth curve model proposed and a mathematical method with Ratkowski criteria for estimation applied to find the best descriptive model of the height of stapes versus time y= f(x) growth curve; the results of 13 growth models were examined. It was found that the maximum growth of the height of stapes was between the 13th and the 24th weeks of pregnancy. The average height of stapes was 1.05 mm in the 13th week and 2.6 mm in the 24th week. Later, after the 25th week, the growth of the height of stapes was slower, and the average height in the 30th week was 3.0 mm.

Ectopic stapes: a case report with embryologic correlations.

A case report of unilateral congenital stapes misplacement revealed by computed tomography is presented. In addition to this malformation, the malleus was synostosed to the middle ear roof. This kind of stapes ectopia has not been described previously. We have analyzed the malformative pattern in the light of normal and teratologic development of the stapes. In a teratologic model in which retinoic acid is administered to pregnant mice, we have observed an ectopic stapes primordium independent of the otic capsule. We discuss the possible pathogenesis of this abnormality in terms of the genetic events of middle ear development, which can be perturbed by retinoic acid administration.

Fetal development of the human tympanic ossicular chain articulations.

OBJECTIVES: To obtain further knowledge on the morphogenesis of the articulations in the tympanic ossicular chain in humans. MATERIAL AND METHODS: In 25 temporal bones of human fetuses the structural development of incudomallear, incudostapedial and stapediovestibular articulations was studied. The chronological ages were between the 7th week (21 mm) and the 29th week (270 mm). RESULTS AND DISCUSSION: Incudomallear articulation showed diarthrosis and sellar joint characteristics. It showed a homogenous interzone in the 7th week of development, a three-layered interzone in the 8th week, the first cavitation signs in the 9th week and the presence of an articular cavity in the 10th week. The presence of a hyaline cartilage covering articular surfaces was observed starting in the 20th week of development. Incudostapedial articulation showed typical characteristics of a diarthrosis and spheroidal joint with a homogenous interzone at the 7th week, showing similar characteristics for 12 weeks, and completed its cavitation at the 16th week. We observed hyaline cartilage on articular surfaces from 29 weeks. Stapediovestibular articulation showed typical characteristics of syndesmosis. The annular ligament primordium derived from cartilage differentiation, both from stapedial footplate and from the surrounding otic capsule, into mesenchyme and its subsequent transformation into fibrous tissue, reaching definitive characteristics from the 12th week.

Isolated congenital stapes ankylosis: an embryologic survey and literature review.

OBJECTIVE: Isolated congenital stapes ankylosis is rare but is a definite entity. Both small series and case reports have been published in various languages. The aim of this study was to review the world literature regarding isolated congenital stapes ankylosis and to critically evaluate the embryonic development of the stapes to explain the possible pathologic development of this ankylosis. DATA SOURCES: All the publications in the English, German, and French literature regarding congenital stapes anomalies were reviewed, and original research articles on the embryonic development of the stapes and related structures were extensively and critically reviewed. STUDY SELECTION: Of the many varieties of congenital stapes anomalies described in the literature, only the isolated congenital stapes fixation due to footplate or suprastructure fixations were selected in this study. DATA SYNTHESIS: After extensive and critical review of the embryonic development of the stapes, the complex and confusing embryonic development is explained in a simplified way with schematic illustrations for easy understanding. The possible theories of congenital stapes ankylosis are explained on an embryologic basis and supplemented with schematic illustrations. CONCLUSION: Based on the development of the stapes an attempt has been made to explain the possible theories for the basis of suprastructure fixation. Theories of congenital fixation of footplate also discussed.

On the role played by ontogenetic remodeling and functional transformation in the evolution of terrestrial hearing.

Using examples from the octavolateral system, evidence is reviewed suggesting a relationship between regressive events, such as loss of one function, or loss of one sensory subsystem, and progressive evolutionary changes in topologically associated systems. While none of the neuronal examples in the evolutionary reorganization of the otic region are as clear-cut as the initial example of non-neuronal reorganization on which the correlation of regressive with progressive changes is based (the functional transformation of the hyomandibular bone into the stapes), the general principle that a chance correlation of two insignificant events may lead to a novel function may be valid for more aspects of the evolution of the ear, in particular the auditory system, than is currently appreciated. It is suggested that regressive events may not only be an evolutionary dead end but that they may provide, through the relaxation of constraints imposed on the respective structure, a source for innovations. However, transformations of functionally uncoupled structures into a novel adaptive function will occur only when topologically adjacent structures require these transformations to improve their own function.

Desarrollo del oído medio humano entre la 5a y la 17a semana de gestación / Human middle ear development between the 5th and the 17th week of pregnancy

Se realizó un estudio de la embriogénesis del oído medio humano entre la 5a y la 17a semana de gestación, utilizando 50 individuos (9 embriones y 41 fetos), los cuales fueron fijados, realizándose técnicas de tinción corrientes (hematoxilina-eosina), histoquímicas, tricrómicas e inmunohistoquímicas (anticuerpos monoclonales antilaminina, anticolágeno IV, antineurofilamentos). Seis fetos fueron procesados para microscopía electrónica de barrido, intentando obtener la mayor información posible del material humano. El análisis de los resultados reveló que el desarrollo del oído medio humano es mas tardío que el de los animales, distinguimos un crecimiento interesante de la tuba auditiva y como esta recubre a los huesecillos formando sus mesos, describimos la fijación de la platina del estribo a la pared medial de la caja timpánica en un período específico del desarrollo y la posterior formación, por muerte celular programada de la ventana oval y un nuevo pericondrio, hecho llamativo en la embriogénesis. En los embriones y fetos estudiados se describe la formación de nudos epidérmicos en la membrana timpánica (en su capa epitelial), dentro del desarrollo normal de esta estructura. Este hallazgo, no ha sido comunicado en la literatura. Con el conocimiento actual y el avance en la tecnología, se puede utilizar técnicas mas avanzadas de tinciones histológicas, inmunohistoquímicas y el uso de microscopio de barrido, para esclarecer las interrogantes del desarrollo del oído medio humano, agregando nueva información a lo tradicionalmente conocido

Ossification patterns of the tympanic facial canal in the human fetus and neonate.

Dehiscences in the bony facial canal are comparatively common in the human adult. The highest incidence occurs in the tympanic segment of the facial nerve near the region of the oval window. Thirty-three fetal temporal bones, ranging from 16 to 40 weeks' gestation, and four from 1, 2, 4 and 12 weeks' postpartum neonates, were studied to evaluate the normal patterns of ossification of the fallopian canal of the tympanic facial nerve segment in the human. The tympanic facial nerve segment elongates three-fold during this period (from 1 mm to 3 mm). The ossification starts at 21 weeks' gestation anteriorly from apical otic ossification centers and at 26 weeks from canalicular ossification centers near the stapedius muscle. The ossification proceeds in an anterior-to-posterior direction as two periosteal shelves of bone surround the facial nerve. The superior periosteal bony ledge contributes 75% of the circumference of the fallopian canal. The anterior ossification center forms over 83% of the fallopian canal length. The two centers fuse post partum near the region of the oval window. The anatomic location of the facial nerve, nerve branching, and neural vasculature precede ossification. In 80% of the paired temporal bones, this ossification pattern appears to be symmetrical. The patterns and incidence of bony dehiscences within the tympanic fallopian canal segment can be explained by these observations. This study demonstrates that fallopian canal dehiscences are not congenital anomalies, but variations of normal developmental anatomic processes.

[Some structural characteristics of articulation of the incus and stapes in man]. / Nekotorye osobennosti stroeniia nakovalnestremennogo sochleneniia cheloveka.

Structural characteristics of human incudostapedial articulation (ISA) have been studied on a series of histological sections from temporal bone decalcified pyramids in three planes and by means of macro-micropreparation of tympanal structural elements from 96 human fetuses at various terms of gestation. It was established that the surface of the lenticular process has a spherical form. Its length surpasses its width. The articular surface on the head of the stapes presents a dome-shaped lacuna. The capsule is usually thicker in its posterior part than in the anterior one. It can endure deviation of the long lenticular process up to 3 mm without rupture. When the deviation overruns 3.5 mm, the articular bursa and round ligament of the base of the stapes break. This property of the capsule should be kept in mind when operating on the stapes.

Developmental anatomy of the mouse stapediovestibular and temporomandibular joints.

The genesis, development and growth of the mouse stapediovestibular joint (SVJ), which contains the annular ligament, and the temporomandibular joint (TMJ) were examined in an attempt to study the stress-bearing articular tissue that is thought to be derived from embryonic mesenchyme; the findings were also compared with those in the ossicular joints. The following conclusions were obtained: 1) The articular cartilage of the mandibular condylar process, stapedial foot plate and otic capsule is derived from fetal fibrous articular tissue. 2) The fetal TMJ developed into a typical double diarthroses containing an articular disc. 3) The fetal syndesmodial SVJ differentiated into the annular ligament containing characteristic palisade-like fibroblasts and hammock-like fibers; no interzone formation or synovial cavities were observed in the completed and mature syndesmodial SVJ. 4) Like the TMJ, the major elastic system fibers in the annular ligament were also mechanical-resistant elaunin. 5) Elastogenesis was closely related to functional and mechanical factors in the auditory ossicular chain, TMJ and annular ligament; the mature incudomalleal and incudostapedial joints contained mainly mature elastic fibers, but the mature SVJ and immature TMJ contained mainly pre-elastic elaunin fibers. 6) Stress elastosis, turnover of the fibrillar component and age changes in cellular and fibrous components were not evident in either the mature SVJ or the young functional TMJ.