Hear, Hear, for the Inner Ear

Anthony Graham

Science, December 2000

The inner ear, a complex sensory organ that enables sound to be heard and balance to be maintained, has long been a favorite study tool of biologists. Composed of the fluid-filled cochlea (which transforms sound waves into nerve impulses) and the semi- circular canals (which provide a sense of orientation and balance), the inner ear is formed from a focal thickening of the embryonic ectoderm called the otic placode. The otic placode can be readily identified and isolated during early embryogenesis and its morphogenesis into the inner ear can be easily followed because it generates an elaborately patterned, discernible structure. Indeed, in the pre-molecular era, tissue manipulation experiments, that induced inner ear formation in amphibian embryos, were at the forefront of investigations into key developmental processes. Such experiments identified the mesoderm and the neural tube as the source of inductive signals directing otic development. This process, with its multiple serial cues, was thought to be generally indicative of the inductive processes that underlie the formation of other organs of the vertebrate body. Beyond the identification of the inducing tissues, further advances in our understanding of otic placode induction have been slow, particularly with regard to the molecules that drive this process. Ladher et al. now report that two signaling molecules, FGF-19 and Wnt-8c, work together to initiate inner ear development in chick embryos. Their work not only elucidates two of the key molecules directing the induction of the otic placode, but also clarifies the sequence of events underlying this phenomenon.

Ladher and co-workers identified and characterized a new chick member of the fibroblast growth factor family of signaling molecules, FGF-19, which they found to have a particularly interesting expression pattern in the early embryo. In the developing chick head at about the one-somite stage of embryonic development, the FGF-19 gene is first expressed in the mesoderm underlying the neural plate in the hindbrain. As the neural plate folds up, the ectoderm that lies alongside it also comes into contact with the FGF-19-expressing mesoderm, and it is in this region of ectoderm that the otic placode forms. Subsequently, the expression of FGF-19 is lost from the mesoderm, although it is transiently expressed in the now-closed neural tube. An important facet of the expression pattern of FGF-19 is that, as the authors demonstrate, it co-localizes with otic-inducing activity. If the FGF-19-expressing mesoderm is co-cultured with early embryonic ectoderm in the laboratory, then the expression of a broad range of otic markers is induced in this tissue, including the formation of auditory hair cells, which mark the terminal stages of inner ear differentiation. By contrast, other mesoderm that did not express FGF-19 could not induce otic markers in this ectoderm. Similarly, otic markers could be induced in early ectoderm when it was co-cultured with FGF-19-expressing neuroectoderm and its adjacent mesoderm. Although FGP-19 expression co-localizes with otic inducing activity, Ladher et al. found that the FGF-19 protein alone could not promote the expression of otic markers in either non-otic ectoderm or presumptive otic ectoderm. This protein could, however, elicit an otic response in these tissues if neural tissue was included. Thus, FGF-19 itself can only direct otic development provided another neural-derived signal is also present.

Taking their cue from studies in the frog Xenopus showing that FGFs often work together with another group of signaling molecules (the Wnts), Ladher and colleagues investigated whether Wnt-8c was the second otic inducer. This signaling molecule was already known to be expressed in the area of the neural tube closest to the region where the otic placode forms. On closer scrutiny, they found that Wnt-8c was expressed in the neural tissue overlying the FGF-19-expressmg mesoderm, and that at later stages Wnt-8c and FGF-19 were located in the same area of neural tissue. These authors also showed that FGF-19 could induce the expression of Wnt-8c in early embryonic ectoderm in culture. Thus, Wnt-8c is a prime candidate for the neural signal working with FGF-19 to induce the otic placode. So, they proceeded to analyze the abilities of these two factors, both independently and together, to promote otic development. As before, they found that FGF-19 alone could not promote otic development, but that Wnt-8c, on its own, induced the expression of one otic marker, FGF-3. However, Wnt-8c was unable to direct the robust expression of any other otic marker. In contrast, if ectoderm was treated with both FGF-19 and Wnt-8c, there was strong expression of a gamut of otic markers and the ectoderm began to acquire the morphology of the otic placode. It is noteworthy that the otic induction driven by the combined action of FGF-19 and Wnt-8c is direct, and does not require the prior induction of neural tissue. However, because these two factors did not elicit the formation of cochlear inner hair cells in the cultured ectodermal explants, other signals may be required to promote full inner ear development.

Although previous investigations of otic induction identified the mesoderm and the neural tissue as inducers, the new work provides us with a clearer picture of how the inducer tissues initiate inner ear development. This process begins with the mesoderm. Through production of FGF-19, the mesoderm signals to the overlying neural plate inducing the expression of Wnt-8c in this tissue, and to the ectoderm, which then gives rise to the otic placode. Subsequently, Wnt-8c and FGF-19, emanating from the hindbrain, act together on this ectoderm to induce the otic placode and thus to initiate inner ear development.

The fact that FGF-19 can induce expression of Wnt-8c in presumptive neuro-ectoderm also suggests that FGF-19 plays a part in patterning the neural tube. This is in keeping with previous work on neural patterning that implicated an undefined FGF activity in regionalizing the neural plate into midbrain and hindbrain territories. Hence, it is likely that FGF-19 is acting specifically to pattern the central hindbrain territory alongside which the otic placode forms. It is within this territory that the auditory nuclei of the brainstem arise. Thus, FGF-19 could be pivotal in ensuring the coordinated development of both the inner ear and its neuronal transducing apparatus.

Answer the questions.

1. Why has the inner ear long been a favorite study tool of biologists?

2. What is the history of inner ear studies?

3. What factors direct otic development?

4. Why is FGF-19 pivotal in the development of the inner ear?