[Frontiers in Bioscience 16, 1675-1692, January 1, 2011]

Can you hear me now? Understanding vertebrate middle ear development

Susan Caroline Chapman

Clemson University, Biological Sciences, 132 Long Hall, Clemson, SC, 29634, USA

TABLE OF CONTENTS

1. Abstract
2. Introduction
3. Morphology
3.1. Amphibian morphology
3.2. Reptile morphology
3.3. Avian morphology
3.4. Mammalian morphology
4. Neural crest origin of the skeletal elements
5. Signal mechanisms patterning neural crest contributions
6. Mesenchymal condensation
7. Chondrogenesis and osteogenesis
8. Skeletal elements: ossicles
8.1. Columella
8.2. Mouse ossicles - malleus
8.3. Incus
8.4. Stapes
8.5. Tympanic ring and gonial bone
8.6. Otic capsule
9. Other components of the middle ear
9.1. Tympanic membrane and external auditory meatus
9.2. Middle ear cavity and Eustachian tube
9.3. Muscles and Nerves
9.4. Joint formation
10. Conclusions
11. Acknowledgements
12. References

1. ABSTRACT

The middle ear is a composite organ formed from all three germ layers and the neural crest. It provides the link between the outside world and the inner ear, where sound is transduced and routed to the brain for processing. Extensive classical and modern studies have described the complex morphology and origin of the middle ear. Non-mammalian vertebrates have a single ossicle, the columella. Mammals have three functionally equivalent ossicles, designated the malleus, incus and stapes. In this review, I focus on the role of genes known to function in the middle ear. Genetic studies are beginning to unravel the induction and patterning of the multiple middle ear elements including the tympanum, skeletal elements, the air-filled cavity, and the insertion point into the inner ear oval window. Future studies that elucidate the integrated spatio-temporal signaling mechanisms required to pattern the middle ear organ system are needed. The longer-term translational benefits of understanding normal and abnormal ear development will have a direct impact on human health outcomes.