Retention of bulla mesenchyme, particularly when it is associated with the mastoid air cells, is also a risk factor for inflammation of the middle ear (otitis media) in humans (Jaisinghani et al., 1999) and, in the Cdh11 knockout (KO) mouse, retention of mesenchyme is associated with hearing deficits, reduced bulla size and otitis media (Kiyama et al., 2018). This evidence concerns the gene CDH11 and otitis media.