Development of the Human Incus With Special Reference to the Detachment From the Chondrocranium to be Transferred into the Middle Ear.

The mammalian middle ear represents one of the most fundamental features defining this class of vertebrates. However, the origin and the developmental process of the incus in the human remains controversial. The present study seeks to demonstrate all the steps of development and integration of the incus within the middle ear. We examined histological sections of 55 human embryos and fetuses at 6 to 13 weeks of development. At 6 weeks of development (16 Carnegie Stage), the incus anlage was found at the cranial end of the first pharyngeal arch. At this stage, each of the three anlagen of the ossicles in the middle ear were independent in different locations. At Carnegie Stage 17 a homogeneous interzone clearly defined the incus and malleus anlagen. The cranial end of the incus was located very close to the otic capsule. At 7 and 8 weeks was characterized by the short limb of the incus connecting with the otic capsule. At 9 weeks was characterized by an initial disconnection of the incus from the otic capsule. At 13 weeks, a cavity appeared between the otic capsule and incus. Our results provide significant evidence that the human incus developed from the first pharyngeal arch but independently from Meckel's cartilage. Also, during development, the incus was connected with the otic capsule, and then it was detached definitively. The development of the incus in humans provides evidence that this ossicle is homologous to the quadrate. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc.

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.

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.

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.