Fine structures of Merkel cells and associated nerve fibers in the epidermis of certain mammalian species.

Abstract

In specimens taken from albino and black mice, the dog and a human fetus, the Merkel cell, a tactile receptor in the epidermis, and its associated nerve fibers were studied with the electron microscope.1. The Merkel cell, situated in the stratum basale or the deep part of the stratum spinosum of the epidermis, is lens-like or elliptical in shape orienting its long axis either perpendicular or parallel to the plane of the dermoepidermal junction. The nucleus is always deeply indented with its long axis parallel to that of the cell.2. The Merkel cell contains many specific granules 700-1000A in diameter. The electron density of these granules is variable and some of them look like empty vesicles. The specific granules are produced in the Golgi apparatus and greatly accumulate in the cytoplasmic area facing the nerve ending.3. The specific granules of the Merkel cell may contain a transmitter substance which may be released from the cell through the mechanism of diacrine secretion to stimulate the nerve ending to initiate the impulse of tactile sensation.4. In the deeply pigmented snout skin of the dog the Merkel cells contain melanosomes, which may have been taken up by the Merkel cell as do the keratinocytes.5. The Merkel cell is provided with desmosomes on the cell surface adjoining the keratinocytes but not the nerve fibers. The fine structure of the desmosome on the Merkel cell is not different from those appearing between the keratinocytes. The presence of desmosomes suggests that the Merkel cells may differentiate from undifferentiated basal epidermal cells as a result of innervation by afferent nerve fibers.6. Nerve fibers and endings associated with the Merkel cell contain a large number of mitochondria. Though they are in a close contact with the Merkel cell, there is no plasma membrane specialization such as desmosomes. A few synaptic vesicles in these afferent endings are considered to be possibly related to the induction of differentiation of the epithelial cell into the sensory cell.