Scanning electron microscopic observation of Merkel cells in the lamprey epidermis

The Merkel cell in the epidermis has generally been regarded as a mechanoreceptor which detects tissue deformations with its microvilli, and subsequently releases certain transmitters to nerve endings. In order to analyze the mechanism of mechanoreception, the fine structure of lamprey Merkel cells and their relationships with surrounding tissue were examined by scanning electron microscopy (SEM) after exposure of the cells by NaOH maceration, as well as by transmission electron microscopy (TEM) according to a conventional method. By SEM, lamprey Merkel cells revealed small round cell bodies bearing numerous microvilli on the upper and lower poles in accord with previous TEM reports. Combined SEM and TEM observations showed that the Merkel cell bodies were tightly held in corresponding concavities of other epidermal cells, with some desmosomes connecting the cells with each other. On the other hand, microvilli of the Merkel cells extended freely in intercellular spaces bound with complex microplicae of epidermal cells. The regional difference in mechanical anchorage of the Merkel cells probably leads to transient deflection and subsequent recovery of their microvilli during a given mechanical stimulation, suggesting rapidly adapting mechanoreception by the cells.