Barlow L A, Northcutt R G
Department of Neurosciences, Scripps Institution of Oceanography, University of California, San Diego, La Jolla 92093-0201, USA.
Ann N Y Acad Sci. 1998 Nov 30;855:58-69. doi: 10.1111/j.1749-6632.1998.tb10546.x.
Amphibian embryos have long been model organisms for studies of development because of their hardiness and large size, as well as the ease with which they can be experimentally manipulated. These particular advantages have allowed us recently to test the role of innervation in the development of vertebrate taste buds using embryos of an aquatic salamander, the axolotl. The predominant model of taste bud genesis has been one of neural induction, in which ingrowing sensory neurites induce taste bud differentiation in the epithelium that lines the mouth and pharynx. However, when we prevented embryonic sensory neurons from contacting the oropharyngeal epithelium by using transplantation or tissue culture techniques, we found that taste bud differentiation was independent of nerve contact. Additionally, using similar types of experimental manipulations, we have recently shown that taste bud differentiation is not a result of interactions of the oropharyngeal epithelium with craniofacial mesenchyme. Surprisingly, we found that although taste bud genesis occurs very late in embryonic development, it is an intrinsic feature of the presumptive oropharyngeal epithelium extremely early, in fact as early as the completion of gastrulation. These data have prompted us to propose a new model for the development of amphibian taste buds: (i) The presumptive oropharyngeal epithelium is specified by the time gastrulation is complete; (ii) Subsequently, a distributed population of taste bud progenitors is set up within this epithelium via local cell-cell interactions. These progenitor cells give rise to taste buds, which are distributed throughout the mouth and pharynx. How widely applicable this model might be for the genesis of taste buds in other vertebrates remains to be seen. However, since it is likely that the taste system of axolotls more closely resembles the ancestral state from which both the amphibian and mammalian taste systems have evolved, it is possible that many of the same developmental mechanisms that give rise to amphibian taste buds are also used to generate the receptor organs in mammals.
由于两栖动物胚胎具有耐寒性、体型较大以及易于进行实验操作等特点，长期以来它们一直是发育研究的模式生物。这些独特的优势使我们最近能够利用一种水生蝾螈（美西螈）的胚胎来测试神经支配在脊椎动物味蕾发育中的作用。味蕾发生的主要模型一直是神经诱导模型，即向内生长的感觉神经突诱导口腔和咽部内衬上皮中的味蕾分化。然而，当我们通过移植或组织培养技术阻止胚胎感觉神经元与口咽上皮接触时，我们发现味蕾分化与神经接触无关。此外，使用类似类型的实验操作，我们最近表明味蕾分化不是口咽上皮与颅面间充质相互作用的结果。令人惊讶的是，我们发现尽管味蕾发生在胚胎发育的很晚阶段才出现，但它实际上早在原肠胚形成完成时就是推定口咽上皮的一个内在特征。这些数据促使我们提出一个关于两栖动物味蕾发育的新模型：（i）推定口咽上皮在原肠胚形成完成时就已确定；（ii）随后，通过局部细胞间相互作用在该上皮内建立起一群分散的味蕾祖细胞。这些祖细胞产生味蕾，味蕾分布在整个口腔和咽部。这个模型在其他脊椎动物味蕾发生中的适用范围还有待观察。然而，由于美西螈的味觉系统可能更接近两栖动物和哺乳动物味觉系统共同进化而来的祖先状态，产生两栖动物味蕾的许多相同发育机制也可能用于生成哺乳动物的感受器器官。
