Morphology of cultured human epidermal melanocytes observed by atomic force microscopy

The objective of this study was to image the surface structure of cultured human epidermal melanocytes using atomic force microscopy (AFM). Epidermis obtained from human foreskins was treated with 0.5% dispase. Cell suspensions of the epidermis were prepared and seeded in six-well plates, in which sheets of mica had been placed. Samples for AFM were fixed on mica and scanning AFM images were captured by contacting and tapping modes operated under normal atmospheric pressure and temperature. Human epidermal melanocytes exhibited rounded, oval, triangular or quadrangular perikarya from which eight to 10 thick dendrites arose. These dendrites first bifurcated near the soma and then divided profusely into daughter branches, which spread out in all directions. We observed string-like long thin projections, growth cones and shorter thicker projections, which arose from the dendritic shafts, in which groups of melanosomes were arrayed. In addition to such structures, the most striking feature was the presence of filopodia arising from the melanocyte dendrite tips and the melanocyte cell body, many of which contained melanosomes. The termini of dendrites formed unbranched terminal protrusions (approximately 1,500-2,000 nm wide) consisting of two to three melanosomes wrapped in an arc, with their filopodia extending outwards. The tips of these structures also appeared to be squeezed and finally pinched off by the melanocyte to form a pouch filled with numerous melanosomes. We conclude that secondary and tertiary branches and subordinate branches might take part in transferring melanosomes into keratinocytes in addition to the transfer through the tips of the dendritic shafts. The melanin granules were expelled by exocytosis.