On the occurrence of Merkel cells in the epidermis of teleost fishes.

The ultrastructure of a differentiated cell type in the epidermis of two species of teleost fish, Ictalurus melas and Phoxinus phoxinus, is described. This cell type has a synaptic association with nerve fibres, microvillus-like peripheral processes, and membrane-bounded inclusions, which together are the diagnostic features of the Merkel cells of tetrapod vertebrates. Other cytoplasmic features are shared with the epithelial cells. The appearance of the membrane-bounded granules depends on the fixative used; after fixation with glutaraldehyde the granules are of a size and electron-density comparable to that found in tetrapod Merkel cells, but after fixing in osmium tetroxide the granules are inconspicuous.