Is the Responding Tissue Unique The Concept of Competence

There are two distinct formal possibilities concerning the properties of the ectoderm that gives rise to the otic placode. On one hand, this ectoderm could be uniquely able to give rise to the otic placode from an early age. Alternatively, many regions of embryonic ectoderm could, in principle, give rise the otic pla-code if they received appropriate inducing signals. These two possibilities can be tested experimentally by transplanting different populations of ectoderm to the site where the otic placode normally forms, and testing whether such foreign populations can form an otic placode in this new location. Populations of foreign tissue that can respond in this way are said to be competent to give rise to the otic placode. A less rigorous variation on this experiment is to surgically

Figure 2.1. Schematic diagram indicating the time course and parameters of otic placode induction in the chick embryo, as described in Groves and Bronner-Fraser (2000). The figure shows how much of the embryonic ectoderm is initially competent to form the otic placode, with this competence decreasing over time. Local inductive signals specify the otic placode, which gradually becomes committed to a placode fate.

Figure 2.1. Schematic diagram indicating the time course and parameters of otic placode induction in the chick embryo, as described in Groves and Bronner-Fraser (2000). The figure shows how much of the embryonic ectoderm is initially competent to form the otic placode, with this competence decreasing over time. Local inductive signals specify the otic placode, which gradually becomes committed to a placode fate.

ablate the otic placode at different ages, and to determine whether the surrounding tissue is competent to regenerate the ablated placode.

These sorts of experiments have been carried out in amphibians and chick embryos for the past 75 years (Kaan 1926; Yntema 1933; Jacobson 1963a,b; Gallagher et al. 1996; Groves and Bronner-Fraser 2000). Although the specific details of these experiments are not relevant here, a common theme is strikingly clear. Much of the early embryonic ectoderm is competent to form the otic placode, provided both the host and donor embryos are sufficiently young. However, these diverse populations of ectoderm lose competence to form the otic placode when taken from progressively older embryos, or when grafted into much older hosts (Groves and Bronner-Fraser 2000). This illustrates a general principle of embryonic developmentā€”that the initially plastic and multipotent cells of the young embryo become progressively restricted in their fates as development proceeds. At present, the molecular basis of competence is not well understood, nor do we have a molecular picture of how competence is lost from cell populations with time. Nevertheless, these experiments suggest that it is the properties of the environment around the presumptive otic placode that direct ectoderm to a placodal fate, rather than the presumptive placodal ectoderm possessing some unique propensity for ear formation.

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