Different levels of Hoxa2 are required for particular developmental processes.

Hoxa2 is required for a variety of developmental processes in the branchial arches and in the hindbrain. We have created a Hoxa2 allele that is about 45% as active in transcription as its wild-type counterpart. This allele, together with the Hoxa2 null and wild-type alleles, allowed the generation of embryos developing in the presence of different levels of Hoxa2 activity. Analysis of these embryos indicates that in general the hindbrain is more resistant to Hoxa2 deficiencies than the second branchial arch. Also, within the second arch, proximo-caudal areas are more sensitive than the rostro-distal. In the hindbrain, basic segmentation and patterning processes seem to occur normally at Hoxa2 levels as low as 20% of the normal. In addition, specific neuronal markers along the dorso-ventral axis of the hindbrain seem differentially affected by reduced Hoxa2 levels. These results provide new clues to understand the role of Hoxa2 in the different embryonic areas where it is required.

Assembling a functional tympanic membrane: signals from the external acoustic meatus coordinate development of the malleal manubrium.

In terrestrial mammals, hearing starts with the perception of acoustic pressure by the tympanic membrane. Vibrations in this membrane are then transduced into the inner ear by the ossicle chain of the middle ear, composed of the malleus, incus and stapes. The proper connection of the ossicle chain with the tympanic membrane, provided by the insertion of the manubrium of the malleus into the eardrum, is essential for the functionality of the hearing apparatus. We describe here the mechanisms regulating the development of the manubrium and its integration into the tympanic membrane. We show that the external acoustic meatus (EAM), which eventually forms the outer epithelium of the tympanic membrane, plays an essential role in this developmental process. Histological and expression analyses indicate that the manubrium develops close to the EAM with a similar temporal sequence. In addition, when the middle ear ossicles are allowed to develop in vitro under conditions that do not support further EAM development, the manubrium develops only up to the stage of its induction at the time of explantation. Moreover, genetically or teratogenically derived alterations in the EAM also have an effect on manubrial development. Finally, we show that the EAM is the source of two quite opposite activities, one that induces chondrogenesis and another that represses it. The combination of these two activities results in the proper positioning of the manubrium.