Human Embryo (week 8, Carnegie stage 22) - Inner Ear Development
Stage 22 embryo (week 8) showing the embryo near the end of the embryonic period.
A. Lateral view of the whole embryo with 5 mm scale bar. Note the well developed external ear with simplified adult structure and narrower meatal opening. The grey bar through the head indicates the plane of cross-section for (B) and (C).
B. Cross-section of the head at the plane of the skull base and oral cavity to the top. The otic capsule is well formed by this stage containing all the membranous labyrinth structures. It is still a cartilaginous structure ventral to the brainstem and lying behind the oral cavity. The tongue occupies the floor of the oral cavity with the unfused palatal shelves lying lateral and the auditory tubes clearly shown on the posterior wall. The external ear is visible on the right hand side of the head with a band of cartilage (dark stain) within the auricle.
C. The gray box indicates this region: detail of inner and middle ear development. The middle ear cavity has not yet formed and the ossicles (malleus shown) are embedded in mesenchyme that is being lost. The tensor tympani muscle is differentiating in the adjacent mesenchyme. The inner ear membranous labyrinth has formed its adult external structure. The section through the turns of the cochlear duct shows the internal cochlea structure is still underdeveloped; in contrast, the balance region is more developed.
Image Source: UNSW Embryology
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