The origin of middle ear ossicles: A narrative and illustrated historical review

The development of the middle ear ossicles, (malleus, incus, and stapes,) has long been linked to the two first visceral (or branchial) arches. Meckel's cartilage, identified by Johann-Friedrich Meckel, is recognized as the first branchial arch cartilage, contributing to the formation of the malleus and incus. In contrast, Reichert's cartilage, associated with the second arch, is tied to the stapes. Despite these historical contributions, there remains significant debate among scientists regarding the exact roles each visceral arch plays in ossicular development, with various theories proposing different origins for these structures. Recent research has highlighted the complexity of this embryonic development, suggesting that the ossicles may arise from a mixture of neural crest cells associated with both branchial arches. Investigations into gene expression, particularly the Hoxa2 gene, have shown that the contributions from the first and second arches to the malleus and incus are more intertwined than previously understood. Some evidence suggests that the malleus and perhaps the incus may incorporate cells from the second arch, while the stapes may also have contributions from both second arch and the otic capsule, thus complicating the classical theories of ossicle development. In conclusion, while the classical understanding of ossicles origins has been rooted in the historical classifications of Meckel's and Reichert's cartilages, modern research indicates a more intricate interplay of cellular contributions from both branchial arches. This nuanced understanding underscores the importance of continued investigation into the embryonic development of the middle ear, as this may shed light not only on human anatomy but also on the evolutionary connections between mammals and other vertebrates. The ongoing exploration of these concepts is crucial for resolving the ambiguities surrounding the ossicular system's formation.