Whitear M
Department of Biology, University College London, United Kingdom.
Arch Histol Cytol. 1989;52 Suppl:415-22. doi: 10.1679/aohc.52.suppl_415.
In lower vertebrates, Merkel cells are widely distributed in the epidermis. Dense-cored specific granules similar to those in mammalian Merkel cells are found in amphibians, dipnoans and lampreys, but only in some species of teleosts, others having few and small-cored vesicles. The characteristic microvilli are found in all these groups, but their number and disposition vary. In amphibians a Merkel cell is associated with a single nerve fibre, known to be mechanoreceptive, which forms reciprocal synapses at a varicosity against the cell and continues on in the epidermis. In lampreys most of the specific granules are grouped around spur-like processes of a nerve fibre which has other branches in the epidermis. In teleosts, one or more nerve fibres wind around a Merkel cell; synaptic modifications are variable. Fluorescence histochemistry shows the presence of quinacrine, neuron-specific enolase, met-enkephalin and serotonin in Merkel cells of some species that have been investigated, but experimental results from amphibians suggest that the receptive element may be the nerve fibre. The Merkel cell can act as a target for growing nerve fibres, but other functions, especially connected with the synapses, most of which are morphologically afferent, remain unknown. Merkel cells do not require trophic maintenance from the nerve. The basal cells of taste buds in teleosts and amphibians have been compared to Merkel cells, but are not identical, although in frogs the basal cells have dense-cored vesicles. In teleosts, both cell types qualify as paraneurons, but there are considerable differences in their cytology especially in the form of the synaptic specializations.
在低等脊椎动物中，默克尔细胞广泛分布于表皮。在两栖动物、肺鱼和七鳃鳗中发现了与哺乳动物默克尔细胞中相似的有致密核心的特殊颗粒，但仅在某些硬骨鱼物种中存在，其他物种则有少量且核心较小的囊泡。在所有这些类群中都发现了特征性微绒毛，但其数量和分布各不相同。在两栖动物中，一个默克尔细胞与一根已知为机械感受器的单一神经纤维相关联，该神经纤维在细胞的曲张处形成相互突触并在表皮中继续延伸。在七鳃鳗中，大多数特殊颗粒围绕着一根在表皮中有其他分支的神经纤维的刺状突起聚集。在硬骨鱼中，一根或多根神经纤维缠绕在一个默克尔细胞周围；突触修饰各不相同。荧光组织化学显示，在一些已被研究的物种的默克尔细胞中存在奎纳克林、神经元特异性烯醇化酶、甲硫氨酸脑啡肽和5-羟色胺，但来自两栖动物的实验结果表明，感受元件可能是神经纤维。默克尔细胞可以作为生长中的神经纤维的靶标，但其他功能，尤其是与突触相关的功能，其中大多数在形态上是传入性的，仍然未知。默克尔细胞不需要来自神经的营养维持。硬骨鱼和两栖动物味蕾的基底细胞已与默克尔细胞进行了比较，但并不相同，尽管在青蛙中基底细胞有致密核心的囊泡。在硬骨鱼中，这两种细胞类型都可归类为副神经元，但它们的细胞学有很大差异，尤其是在突触特化的形式上。
