Fig. 3. Middle ear skeletal phenotype of a2-Six2 transgenics. (A) Schematic representation of the second (brown background) and first-arch-derived skeleton. (B) In the absence of Hoxa2, duplicated first arch elements derive from the second arch (brown). (C) Expression of Six2 in E10.5 wild-type embryos. Six2 mRNA is almost excluded from the second (black arrow) and more posterior arches. (D) Six2 is ectopically expressed in the second (white arrow) and more posterior arches and in the somitic mesoderm of a2-Six2 embryos. (E) Middle ear skeleton of an E18.5 wild-type embryo. The stapes is shown in the oval window (^*) and after dissection. (F) Middle ear skeleton of a transgenic littermate. An ectopic cartilage, connected to the styloid process, extends to face the incus (black arrow); ventral view of the dissected styloid process, with the ectopic cartilage delimited by arrows, is shown on the right. The styloid process is thicker (arrowhead) and the manubrium of the malleus is curved (white arrow). The stapes, dissected and shown on the right, is incomplete. Malformation of the tympanic ring was observed only once. (G) Mirror image cartilages in the Hoxa2 mutant. Duplicated elements are marked with an asterisk. (H) Lateral view of a wild-type skull: orange arrow indicates the distal extremity of the lesser horn of the hyoid bone; the end of the styloid process is marked by a white arrow, the greater horn is also indicated. (I) In transgenic embryos, the lesser horn elongates and fuses to the styloid process, generating a continuous structure resembling a Meckel-like cartilage (white arrow). (J) Overexpression of Six2 does not affect Hoxa2 levels. Semi-quantitative RT-PCR on RNA extracted from E10.5 second arches of wild-type and a2-Six2 (tg) embryos, using specific primers for Six2, Hoxa2 and GADPH. g, greater horn; i, incus; l, lesser horn; m, malleus; s, styloid process; st, stapes; t, tympanic ring.