Joint formation in the middle ear: Lessons from the mouse and guinea pig

The malleus, incus and stapes form an ossicle chain in the mammalian middle ear. These ossicles are articulated by joints that link the chain together. In humans and mice, fusion of the ossicles leads to hearing loss. However, in the adult guinea pig the malleus and incus are normally found as a single complex. In this report, we investigate how the malleus and incus form during mouse and guinea pig development. The murine malleus and incus develop from a single condensation that splits to form the two ossicles. Even before a morphological split, we show that the ossicles have distinct genetic identities and joint markers are expressed. In the guinea pig embryo, joint formation is initiated but no cavitation is observed, resulting in a single complex divided by a thin suture. The malleal-incudo complex in the guinea pig is, therefore, not caused by a defect in joint initiation.