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型细胞一样，这些基底细胞对血清素抗体也表现出免疫反应性。此外，这些细胞在形态上与表皮默克尔细胞相似。为了确定这些血清素能基底细胞的意义，我们研究了青蛙蜕变过程中味觉器官的早期发育，重点研究了血清素能基底细胞的起源和可能的作用。为便于描述，将其发育分为5个阶段（变质阶段至高潮阶段A至高潮阶段D）。在变质期，少数无神经支配的默克尔细胞出现在舌上皮的上层。这一阶段上皮内未见神经元成分。从高潮期A到高潮期B，在默克尔细胞所在的舌背表面可以看到未成熟的真菌状乳头。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。这些细胞对血清素具有强烈的免疫反应性，在晚期，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， Merkel细胞沿着基底层向新形成的真菌状乳头的中心扩展细胞质突。这些默克尔细胞的形态与成人味觉器官中血清素能基底细胞的形态完全一致。在这两个阶段，经常可以看到大量致密核的默克尔细胞颗粒通过基底层向神经纤维释放。目前的研究表明，5 -羟色胺能基底细胞来源于上皮内的默克尔细胞，作为神经生长的靶点，可能负责味觉器官形态发生的起始。

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.