Morphological and morphometric studies with the electron microscope on the Merkel cells and associated nerve terminals of normal and denervated skin

Merkel cells and associated nerve endings of dog snout skin as well as foot and toe pads of rats were studied with the transmission electron microscope. Normal Merkel cells are characterized by the following morphological features: 1) localization restricted to the basal layer of the epidermis, 2) adjoining afferent nerve endings containing many mitochondria, 3) specific small round granules usually accumulated in the cytoplasm on one side of the nucleus facing the junction of the nerve ending, 4) highly lobulated irregular shape of the nucleus, 5) well developed Golgi apparatus with a few immature granules situated on the side of the nucleus opposite the junction at the ending, 6) spine-like processes extending from the cell surface where no nerve endings abut, 7) desmosomes and desmosome-like membrane thickening on the cell surface both abutting the surrounding keratinocytes and the nerve endings. The Merkel cells with nerve endings were observed and measured at various days after denervation. The experiments were carried out on rats whose sciatic nerves were transected. The samples of skin of foot and toe pads of the denervated legs were taken almost every day from 1 to 30 days. Almost no changes were noticed after 24 hrs, but within the next day the nerves retracted and were no longer observed in the epidermis. The Merkel cell granules increased in number in 1--3 days but decreased again to about a half of the value of the control at 30 days. The polarity of granules distribution showing a strong accumulation in the cytoplasm near the nerve ending became less marked after denervation. Some cells sustained severe degeneration in the later stage of experiment. It was remarkable that the activity of Merkel cells might depend on the associated nerves, but they never completely disappeared even after a long period of denervation.