Origins of Inner Ear Sensory Organs Revealed by Fate Map and Time-Lapse Analyses

Abstract

The inner ear develops from a simple ectodermal thickening called the otic placode into a labyrinth of chambers which house sensory organs that sense sound and are used to maintain balance. Although the morphology and function of the sensory organs are well characterized, their origins and lineage relationships are virtually unknown. In this study, we generated a fate map of Xenopus laevis inner ear at otic placode and otocyst stages to determine the developmental origins of the sensory organs. Our lineage analysis shows that all regions of the otic placode and otocyst can give rise to the sensory organs of the inner ear, though there were differences between labeled quadrants in the range of derivatives formed. A given region often gives rise to cells in multiple sensory organs, including cells that apparently dispersed from anterior to posterior poles and vice versa. These results suggest that a single sensory organ arises from cells in different parts of the placode or otocyst and that cell mixing plays a large role in ear development. Time-lapse videomicroscopy provides further evidence that cells from opposite regions of the inner ear mix during the development of the inner ear, and this mixing begins at placode stages. Lastly, bone morphogenetic protein 4 (BMP-4), a member of the transforming growth factor beta (TGF-beta) family, is expressed in all sensory organs of the frog inner ear, as it is in the developing chicken ear. Inner ear fate maps provide a context for interpreting gene expression patterns and embryological manipulations.

Ablation studies on the developing inner ear reveal a propensity for mirror duplications

“…Fate map studies in frog indicate that cell mixing is occurring during the stages we did our ablation, yet our half ablation results are also consistent with a compartment model of inner ear development in that the remaining half often could only regenerate tissues with its own identity (Brigande et al, 2000;Kil and Collazo, 2001). These two results may seem to contradict, but our ablations and the fate map experiments address different questions that are not comparable.…”

“…2; Table 1). The stage the ears were scored, typically 48, was a tadpole stage where the semicircular canals are present and five of the eventual eight sensory organs have developed (Kil and Collazo, 2001;Bever et al, 2003). The ear was scored as completely regenerated if it appeared identical to nonmanipulated control ears.…”

“…To better visualize the otic placode (stages 21-27) and otocyst (stages 28 -31), the pigmented epidermis was peeled away using quartz glass micropipettes pulled into fine needles using a P-2000 micropipette puller (Sutter Instrument Co.). Removing this epidermis does not disturb ear development (Kil and Collazo, 2001); it heals quickly, and this most superficial epidermis does not contribute any tissue to the inner ear at placode or otocyst stages (Hausen and Riebesell, 1991;arrows in Fig. 7C,D).…”

“…Labeling of hair cells was done as previously described (Kil and Collazo, 2001). Briefly, inner ears of stage 47-49 tadpoles, or any structure that had regenerated in its place, were injected with a solution of the styryl fluorescent dye 4-Di-2-ASP or FM1-43 (Molecular Probes) to visualize hair cells (Meyers et al, 2003).…”

OC29 is preferentially expressed in the presumptive sensory organ region of the otocyst

“…However, as there is no fate map of the rodent otocyst as in other species, we have to be cautious with our speculation (Brigande et al, 2000a;Kil and Collazo, 2001).…”