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Updated: Feb 4, 2026
Dissection of the Auditory Bulla in Postnatal Mice: Isolation of the Middle Ear Bones and Histological Analysis
Published on: January 4, 2017
Hox-A2 protein expression in mouse embryo middle ear ossicles
S Louryan1, Marie Lejong1, Myriam Choa-Duterre1
1Laboratoire d'anatomie, biomécanique et organogenèse (Dir: Prof. S. Louryan), faculté de médecine, université Libre de Bruxelles, route de Lennik, 808 (CP 619), B1070 Bruxelles, Belgium.
Abstract:
The origin of the mammalian middle ear ossicles from mandibular and hyoid pharyngeal arches remains controversial and discussed. Two adverse theories are proposed. The first claims that malleus and incus derive from the Meckel's cartilage of the mandibular arch, and stapes from Reichert's cartilage of the hyoid arch. The second postulates that handle of malleus and long process of the incus are derived from the second arch as well as the stapes. Contradictory analyses support alternatively each theory without any experimental evidence. In order to bring new data, we analyzed by immunohistochemistry the expression of Hox-A2 protein in ossicular anlagen in E11 to 13 mouse embryos. HOXA2 gene is known to be expressed in second arch cells and to be absent from mandibular arch derivatives. Surprisingly, Hox-A2 protein was present in all ossicular primordia, as well in Reichert's cartilage. Meckel's cartilage was free of staining. Unlabeled cells were also present in ossicular blastemata. These results suggest that ossicular condensations could arise from mixed cell populations originated in both mandibular and hyoid pharyngeal arches. However, we cannot exclude that diffuse Hox-A2 immunoreactivity could correspond to a secondary expression in craniofacial mesenchyme independently from the branchial origin of cells.
Insights
The origin of mammalian middle ear ossicles is debated. This study found Hox-A2 protein in all ossicular primordia, suggesting mixed cell origins from mandibular and hyoid pharyngeal arches.
Area of Science:
- Developmental biology
- Craniofacial development
- Evolutionary developmental biology
Background:
- The precise embryonic origin of mammalian middle ear ossicles (malleus, incus, stapes) from pharyngeal arches is a long-standing debate.
- Two main theories exist: one proposing derivation solely from the mandibular arch (malleus, incus) and hyoid arch (stapes), the other suggesting a mixed origin for malleus and incus from both arches.
Purpose of the Study:
- To investigate the cellular origins of mammalian middle ear ossicles.
- To provide experimental evidence clarifying the contribution of mandibular and hyoid pharyngeal arches to ossicle development.
Main Methods:
- Immunohistochemistry was used to detect Hox-A2 protein expression in ossicular anlagen of mouse embryos (E11-E13).
- Hox-A2 expression patterns were analyzed in relation to Meckel's cartilage (mandibular arch derivative) and Reichert's cartilage (hyoid arch derivative).
Main Results:
- Hox-A2 protein was unexpectedly detected in all developing middle ear ossicles and Reichert's cartilage.
- Meckel's cartilage showed no Hox-A2 staining, but unlabeled cells were present in ossicular blastemata.
- These findings challenge existing theories by indicating a potential mixed cellular contribution.
Conclusions:
- Ossicular condensations likely arise from a combination of cell populations from both the mandibular and hyoid pharyngeal arches.
- While Hox-A2 expression suggests a hyoid contribution, the presence of unlabeled cells indicates a complex, potentially mixed, developmental origin for middle ear ossicles.
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