Fig. 2. Analysis of the structural elements of the middle ear in E18.5 Bapx1^-/- mice. Alizarin Red and Alcian Blue staining of dissected middle ears from E18.5 wild-type (A,B), heterozygous (C,D) and homozygous mutant (E,F) embryos. (A,C,E) Lateral view. (B,D,F) Frontal view. At E18.5, the tympanic ring and gonium are evident as discrete ossified elements located below Meckel's cartilage and adjacent to the developing middle ear ossicles (A,B). Embryos heterozygous for the Bapx1 mutation have no overt defects associated with the tympanic ring but do display variable hypoplasia of the gonium (C,D). All homozygous-null embryos lack a gonium and display hypoplasia of the anterior part of the tympanic ring (E,F). Arrowheads in B-D indicate the extremities of the gonium. G, gonium; M, malleus; Me, Meckel's cartilage; Ty, tympanic ring.