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.

Anti-apoptotic gene Bcl2 is required for stapes development and hearing.

In this paper we describe novel and specific roles for the apoptotic regulators Bcl2 and Bim in hearing and stapes development. Bcl2 is anti-apoptotic while Bim is pro-apoptotic. Characterization of the auditory systems of mice deficient for these molecules revealed that Bcl2⁻/⁻ mice suffered severe hearing loss. This was conductive in nature and did not affect sensory cells of the inner ear, with cochlear hair cells and neurons present and functional. Bcl2⁻/⁻ mice were found to have a malformed, often monocrural, porous stapes (the small stirrup-shaped bone of the middle ear), but a normally shaped malleus and incus. The deformed stapes was discontinuous with the incus and sometimes fused to the temporal bones. The defect was completely rescued in Bcl2⁻/⁻Bim⁻/⁻ mice and partially rescued in Bcl2⁻/⁻Bim⁺/⁻ mice, which displayed high-frequency hearing loss and thickening of the stapes anterior crus. The Bcl2⁻/⁻ defect arose in utero before or during the cartilage stage of stapes development. These results implicate Bcl2 and Bim in regulating survival of second pharyngeal arch or neural crest cells that give rise to the stapes during embryonic development.

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.

Middle-ear development. VI: Structural maturation of the rat conducting apparatus.

BACKGROUND: The contribution of middle-ear development to the overall development of hearing has not been explored in great detail. This presentation describes the maturation of conductive elements in the rat middle ear, and provides the basis on which future studies of middle-ear functional development will follow. METHODS: The middle-ear apparatus was examined at nine different ages (between 1 and 80 days postpartum) in Long Evans rats. At each age elements of the conducting apparatus was observed with either light or scanning electron microscopy (SEM), and quantitative measurements were made from video enhanced photomicrographs. Tympanic membrane area and cone depth, the length of the malleus and incus arms, ossicular weight, stapes foot plate and oval window areas, and bulla volume were all measured. Development of the area and lever ratios were derived from these measurements. The data were fitted to exponential equations and the time in days required to reach 90% of the adult level determined. RESULTS: The pars tensa achieved 90% of total area by 17 days. The oval window achieved the 90% criterion by 13 days, while the area ratio was within 10% of its adult size by 8 days. The ossicles took between 26 and 34 days, while bulla volume took 59 days to reach the 90% level. CONCLUSIONS: Middle-ear growth was very orderly and systematic in the data reported. When maturation of the area ratio was considered against development of the endocochlear potential or the round window compound action potential, it was clear that the growth of this important aspect of the middle ear preceded the onset of cochlear function.

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.

Middle-ear development: II. Morphometric changes in the conducting apparatus of the chick.

The ontogeny of various middle-ear structures was examined in 11 groups of chicks between 10 days embryonic and adult. Measurements of the tympanic membrane surface area and height, columella length, and that of the columella footplate, annular ligament, and oval window area were obtained using video micrographs and computer digitization techniques. The oval window matures first at 53 days post-hatching, whereas the columella achieves adult size at 74 days. The tympanic membrane surface area is the last middle-ear variable studied to reach adult size (79 days post-hatch). The columella increases its length from 0.63 mm (10 days embryonic) to 2.73 mm in the adult. The tympanic membrane area expands by 280% whereas the columellar footplate area increases by 11x. As a result, the pressure amplification of the middle ear due to the tympanic membrane/columellar footplate area ratio improves by over 400%. These data further contribute to our understanding of the functional development of the middle ear.

Postnatal development of the middle ear: areal ratios in kittens.

Developmental changes in tympanic membrane to stapedial footplate areal ratios were investigated in kittens from birth through the twentieth postnatal day. Measurements from 45 kittens (85 temporal bones) were obtained from the pars tensa region of the tympanic membrane and the stapedial footplate region bounded by the annular ligament using standard macrophotographic and computer digitization techniques. Tympanic membrane and stapedial footplate areas were approximately 70% and 80%, respectively, of their adult size at birth and relevant areas increased exponentially during early postnatal life. Adultlike dimensions were achieved by the end of the first postnatal week. Tympanic membrane:stapedial footplate areal ratios increased from approximately 25:1 to 30:1 during the first postnatal week, before the developmental period during which adultlike auditory thresholds and response amplitudes are acquired. Thus, the areal ratio influences on impedance-matching characteristics of the middle ear do not appear to significantly affect the postnatal development of auditory function in cats.

A topographic quantitative analysis of the post-natal bone formation in the auditory ossicles of the dog.

The amount and distribution of the post-natal bone deposition in the auditory ossicles and in the left tibia of dogs of varying ages were studied by means of alizarin labelling. The relative amount of fluorescent new-formed bone was expressed as a percentage ratio NB/(NB+PB) of new bone (NB) on the pre-existing bone (PB). The result was that the post-natal bone deposition (1) was larger in the tibia than in the incus, malleus and stapes; (2) significantly decreased with age both in the ossicles and in the tibia; (3) in the stapes it stopped at 3 months, while it was present in the tibia, incus and malleus even at 12 months. In the ossicles the post-natal bone deposition takes place both on the periosteal surfaces of the ossicles and on the internal surfaces of the haversian systems. The first process produces an appositional growth that stops in all three ossicles within the 1st month of post-natal life, the second one produces an internal growth that continues until the age of three months in the stapes, while in the incus and malleus it occurs in small amounts, even in the 12th month of life. In the ossicles all the new-formed bone tissue, periosteal and osteonic, is built up by primary bone (addition bone). In the tibia from 50 days of age the primary bone is gradually replaced by secondary haversian systems as a consequence of remodelling processes.