The Merkel cell in the labial ridge epidermis of the anuran tadpole. II. Electron microscope observations on the appearance and differentiation of the Merkel cell.

The appearance and cytodifferentiation of the Merkel cell were studied electron microscopically using the developing labial ridges of Rana japonica. The most primitive Merkel cell was identified with specific cored granules with an average diameter of about 90 nm. Such a primitive Merkel cell was found in the basal layer of the primitive labial ridge epithelium. As the primitive labial ridge continued to mature, the primitive Merkel cell accumulated an increased number of the specific granules, developed digital shaped cytoplasmic processes and came into contact with nerve fibers. When the labial ridge was sufficiently developed the cell showed typical morphological features of the mature Merkel cell except for a small accumulation of tonofilament-like structures and glycogen particles. It was also observed that the position of the Merkel cell in the epithelium changed from the basel layer to the third basal layer during development. On the other hand, it was difficult to find the Merkel cell in the mesenchymal layer of the developing labial ridges. From the results, it was suggested that the cytodifferentiation of the Merkel cell in the labial ridge of an anuran tadpole occurs within the epithelium.