Neurite complexes with Merkel cells in larval tentacles of Xenopus laevis

Summary

Electron microscopy of larval tentacles of Xenopus laevis tadpoles revealed that they are richly innervated structures containing a high concentration of neurite complexes with epidermal Merkel cells. Myelinated sensory nerve fibres supplying each tentacle are abundant. Prior to penetrating the epidermis, they gradually shed their myelin sheaths and Schwann-cell coverings, and subsequently end as naked axon terminals within the epidermis. Many of these intra-epidermal neurites terminate in intimate synaptic contact with granulated Merkel cells. Groups of dense-cored vesicles characterize the Merkel cell, while clusters of clear synaptic vesicles occupy the adjacent sensory nerve terminals. At sites of synaptic contact, the two cells often exhibit an increased thickening of their apposing membranes. Such Merkel cell — sensory neurite complexes occur as isolated units and are profusely scattered throughout the extent of each tentacle. Their ultrastructure is, in most respects, similar to that described previously in specialized regions of other vertebrate and amphibian species. It is suggested that they represent the morphological basis for tactile sensitivity and impart a mechanoreceptive function to the larval tentacle prior to metamorphosis. The larval tentacle in this anuran species may represent a new and convenient research model for future developmental and experimental studies of trophic interactions between sensory nerve fibres and Merkel cells.