Neurobiological Study of Taste Buds

青蛙的味觉器官有一种独特的细胞类型，只位于基部。与哺乳动物味蕾中的III型细胞相同，这些基底细胞也显示出对5-羟色胺抗体的免疫反应性。此外，这些细胞在形态上类似于表皮默克尔细胞。为了确定这些Escherichinergic基底细胞的意义，我们研究了早期发育的味觉器官在变态的青蛙，专注于Escherichinergic基底细胞的起源和可能的作用。为便于描述，将其划分为五个发展阶段（变质阶段至顶极阶段A至D）。在变态阶段，舌上皮的上层出现少数无神经支配的默克尔细胞。在这个阶段，在上皮中没有观察到神经元成分。从高潮阶段A到B，在默克尔细胞所在的舌背表面，未成熟的菌状乳头变得可辨别。然后默克尔细胞向下移动并将其细胞质突起向基底层延伸。这些细胞通过其对5-羟色胺的强烈免疫反应性来识别。在后期阶段，上皮下结缔组织中的许多神经纤维接近含有默克尔细胞的上皮。从顶极阶段C到D，默克尔细胞沿着基膜向新形成的菌状乳头中心延伸细胞质突起。这些默克尔细胞的形态与成年人味觉器官中的多巴胺能基底细胞的形态完全一致。在这些阶段，经常可以看到默克尔细胞的致密核心颗粒通过基底层向神经纤维的大量胞吐释放。目前的研究表明，味觉能基底细胞来源于上皮内默克尔细胞，作为生长的神经的靶点，并可能负责启动味觉器官的形态发生。

Taste organs in the frog have a distinctive cell type located exclusively in the basal portion. In the same fashion as type III cells in mammalian taste buds, these basal cells also show immunoreactivity for serotonin antibody. Further, these cells are morphologically similar to epidermal Merkel cells. To determine the significance of these serotonergic basal cells, we examined the early development of taste organs during metamorphosis of the frog, focusing on the origin and possible roles of serotonergic basal cells. For convenience of description, five stages of development (metamorphic stage to climax stage A to D) are defined. In the metamorphic stage, a few, non-innervated Merkel cells appear at the upper layer of the lingual epithelium. No neuronal elements are seen in the epithelium at this stage. From climax stage A to B, immature fungiform papillae become discernible in the dorsal surface of the tongue where the Merkel cells are located. Merkel cells then move downward and extend their cytoplasmic processes toward the basal lamina. These cells are identified by their intense immunoreactivity for serotonin, During the latter stages, many nerve fibers in the subepithelial connective tissue approach the epithelium containing Merkel cells. From climax stage C to D, Merkel cells extend cytoplasmic processes along the basal lamina toward the center of the newly-forming fungiform papillae. The morphology of these Merkel cells exactly coincides with that of serotonergic basal cells in adult taste organs. Profuse exocytotic release of dense-cored granules of Merkel cells toward the nerve fibers through the basal lamina is frequently seen in these stages. The present study indicates that serotonergic basal cells are derived from intraepithelial Merkel cells, which act as target sites for growing nerves and might be responsible for the initiation of taste organ morphogenesis.