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

Growth of muscles and nerves in the upper eyelid: a morphometrical and immunohistochemical study using term human fetuses

BACKGROUND

There is no information about muscle growth in eyelids with infrequent blinking in fetuses.

METHODS

To examine the muscle and nerve morphology, we morphometrically and immunohistochemically examined sagittal sections of unilateral upper eyelids obtained from 21 term fetuses (approximately 30-42 weeks of gestation) and, for the comparison, those from 10 midterm fetuses (12-15 weeks).

RESULTS

The approximation margin of the upper eyelid always corresponded to the entire free margin in midterm fetuses, whereas it was often (18/21) restricted in the posterior part in term fetuses. Thus, in the latter, the thickness at the approximation site to the lower lid often ranged from 0.8 to 1.6 mm and corresponded to 18-56% of the nearly maximum thickness of the lid. In the lower part of the upper eyelid, a layer of the orbicularis oculi muscles often (14/21) provided posterior flexion at 90-120° to extend posteriorly. Nerve fibers running along the mediolateral axis were rich along the approximation surface at term, but they might not be reported in the upper eyelid of adults.

CONCLUSION

Being different from adult morphologies, the term eyelid was much thicker than the approximation surface and it carried a flexed muscle layer and transversely-running nerve. The infrequent blinking in fetuses seemed to provide a specific condition for the muscle-nerve growth. Plastic and pediatric surgeons should pay attention to a fact that infants' upper eyelid was unlikely to be a mini-version of the adult morphology.

Fetal cervical zygapophysial joint with special reference to the associated synovial tissue: a histological study using near-term human fetuses

Human fetal cervical vertebrae are characterized by the large zygapophysial joint (ZJ) extending posteriorly. During our recent studies on regional differences in the shape, extent, and surrounding tissue of the fetal ZJ, we incidentally found a cervical-specific structure of synovial tissues. This study aimed to provide a detailed evaluation of the synovial structure using sagittal and horizontal sections of 20 near-term fetuses. The cervical ZJ consistently had a large cavity with multiple recesses at the margins and, especially at the anterior end, the recess interdigitated with or were located close to tree-like tributaries of the veins of the external vertebral plexus. In contrast to the flat and thin synovial cell lining of the recess, the venous tributary had cuboidal endothelial cells. No or few elastic fibers were identified around the ZJ. The venous-synovial complex seems to be a transient morphology at and around birth, and it may play a role in the stabilization of the growing cervical ZJ against frequent spontaneous dislocation reported radiologically in infants. The venous-synovial complex in the cervical region should be lost and replaced by elastic fibers in childhood or adolescence. However, the delayed development of the ligament flavum is also likely to occur in the lumbar ZJ in spite of no evidence of a transient venous-synovial structure. The cuboidal venous endothelium may simply represent the high proliferation rate for the growing complex.

Regional differences in zygapophysial joint cavities: A histological study of human fetuses

Human zygapophysial joints (ZJ) have regional differences in shape and orientation during prenatal growth. However, there is limited knowledge of the synovial recess during fetal development. We examined sagittal and horizontal histological sections of the vertebral columns of 30 human fetuses at gestational ages of 8-37 weeks. Fetuses of all gestational ages had subaxial cervical articular processes that were thicker than in the thoracolumbar regions, and as large as the corresponding vertebral bodies. A small or large synovial recess extending beyond the articular cartilage was evident at most regions. The cervical ZJ had large or deep recesses that extended inferiorly in midterm fetuses and posteromedially along the vertebral pedicle and lamina in near-term fetuses. Likewise, the thoracic ZJ had small recesses that extended superiorly in midterm fetuses and medially in near-term fetuses. The lumbar recesses extended laterally beyond the medially shifted articular cartilage of the upper adjacent vertebrae in near-term fetuses and the lumbar articular surface was smallest in the three regions at all stages. At any region, a deep recess appeared before an area expansion of the ZJ cartilage. A drastic change in direction and size of the prenatal recess seemed to occur depending on a possible minute dislocation of the ZJ. In particular, a deep posteromedial recess of the cervical ZJ, which extended far beyond the articular cartilage, might be necessary to maintain high flexibility suitable for the strong flexion posture in utero.

Transient role of the middle ear as a lower jaw support across mammals

Mammals articulate their jaws using a novel joint between the dentary and squamosal bones. In eutherian mammals, this joint forms in the embryo, supporting feeding and vocalisation from birth. In contrast, marsupials and monotremes exhibit extreme altriciality and are born before the bones of the novel mammalian jaw joint form. These mammals need to rely on other mechanisms to allow them to feed. Here, we show that this vital function is carried out by the earlier developing, cartilaginous incus of the middle ear, abutting the cranial base to form a cranio-mandibular articulation. The nature of this articulation varies between monotremes and marsupials, with juvenile monotremes retaining a double articulation, similar to that of the fossil mammaliaform Morganucodon, while marsupials use a versican-rich matrix to stabilise the jaw against the cranial base. These findings provide novel insight into the evolution of mammals and the changing relationship between the jaw and ear.

The incudopetrosal joint of the human middle ear: a transient morphology in fetuses

In spite of the amount of research on fetal development of the human middle ear and ear ossicles, there has been no report showing a joint between the short limb of incus and the otic capsule or petrous part of the temporal bone. According to observations of serial histological sections from 65 embryos and fetuses at 7-17 weeks of development, the incudopetrosal joint exhibited a developmental sequence similar to the other joints of ossicles, with an appearance of an interzone followed by a trilaminar configuration at 7-12 weeks, a joint cavitation at 13-15 weeks and development of intraarticular and capsular ligaments at 16-17 weeks. These processes occurred at the same time or slightly later than any other joint. Thus, the joint development might coordinate with vibrating ossicles in utero. The growing short limb of incus appeared to accelerate an expansion of the epitympanic recess of the tympanic cavity. Additional observations of five late-stage fetuses demonstrated the incudopetrosal joint located in the fossa incudis joint changing to syndesmosis. Consequently, a real joint with a cavity existed transiently between the human neurocranium and the first pharyngeal arch derivative (i.e. incus) in contrast to the tympanostapedial joint or syndesmosis between the neurocranium and the second arch derivative. The newly described joint might have an effect on the widely accepted primary jaw concept: the mammalian jaw should thus have been created within the first pharyngeal arch, although the connection with neurocranium by the stapes is of a different origin.

Multiphoton imaging for morphometry of the sandwich-beam structure of the human stapedial annular ligament

BACKGROUND

The annular ligament of the human stapes constitutes a compliant connection between the stapes footplate and the peripheral cochlear wall at the oval window. The cross section of the human annular ligament is characterized by a three-layered structure, which resembles a sandwich-shaped composite structure. As accurate and precise descriptions of the middle-ear behavior are constrained by lack of information on the complex geometry of the annular ligament, this study aims to obtain comprehensive geometrical data of the annular ligament via multiphoton imaging.

METHODS

The region of interest containing the stapes and annular ligament was harvested from a fresh-frozen human temporal bone of a 46-years old female. Multiphoton imaging of the unstained sample was performed by detecting the second-harmonic generation of collagen and the autofluorescence of elastin, which are constituents of the annular ligament. The multiphoton scans were conducted on the middle-ear side and cochlear side of the annular ligament to obtain accurate images of the face layers on both sides. The face layers of the annular ligament were manually segmented on both multiphoton scans, and then registered to high-resolution μCT images.

RESULTS

Multiphoton scans of the annular ligament revealed 1) relatively large thickness of the core layer compared to the face layers, 2) asymmetric geometry of the face layers between the middle-ear side and cochlear side, and variation of their thickness and width along the footplate boundary, 3) divergent relative alignment of the two face layers, and 4) different fiber composition of the face layers along the boundary with a collagen-reinforcement near the anterior pole on the middle-ear side.

CONCLUSION AND OUTLOOK

Multiphoton microscopy is a feasible approach to obtain the detailed three-dimensional features of the human stapedial annular ligament along its full boundary. The detailed description of the sandwich-shaped structures of the annular ligament is expected to contribute to modeling of the human middle ear for precise simulation of middle-ear behavior. Further, established methodology in this study may be applicable to imaging of other middle-ear structures.

Inner ear ossification and mineralization kinetics in human embryonic development - microtomographic and histomorphological study

Little is known about middle and inner ear development during the second and third parts of human fetal life. Using ultra-high resolution Microcomputed Tomography coupled with bone histology, we performed the first quantitative middle and inner ear ossification/mineralization evaluation of fetuses between 17 and 39 weeks of gestational age. We show distinct ossification paces between ossicles, with a belated development of the stapes. A complete cochlear bony covering is observed within the time-frame of the onset of hearing, whereas distinct time courses of ossification for semicircular canal envelopes are observed in relation to the start of vestibular functions. The study evidences a spatio-temporal relationship between middle and inner ear structure development and the onset of hearing and balance, critical senses for the fetal adaptation to birth.

The expression of tenascin-C and tenascin-W in human ossicles

The ossicles of the middle ear (the malleus, incus and stapes) transmit forces resulting from vibrations of the tympanic membrane to the cochlea where they are coded as sound. Hearing loss can result from diseases such as rheumatoid arthritis (RA) that affect the joints between the ossicles or degenerative processes like otosclerosis that lead to ankylosis of the footplate of the stapes in the oval window of the cochlea. In this study, immunohistochemistry was used to determine if the extracellular matrix glycoproteins tenascin-C or tenascin-W are expressed in the incudomalleolar and incudostapedial joints of ossicles dissected from human cadavers. Tenascin-C, which is expressed during inflammatory conditions including RA, was seen in the articular cartilage of the incudomalleolar joints and the head of the stapes. Tenascin-W, in contrast, was enriched in the annular ligament that anchors the footplate of the stapes into the oval window of the cochlea.