Melanosome macrocomplex: an ultrastructural component of patterned and nonpatterned seborrheic keratoses.

An ultrastructural study of five seborrheic keratoses arising in a symmetrical, patterned distribution following the lines of cleavage on the backs of two patients showed the presence of melanosome macrocomplexes 2 to 5 microns in size. Identical findings were seen in fifteen pigmented seborrheic keratoses randomly distributed on the skin of five patients. The membrane-bound macrocomplexes were clusters of closely packed, discrete melanosomes. Present in melanocytes, keratinocytes, Langerhans cells, and dermal macrophages, they were considered to result from sequestration of melanosomes in cell phagosomes. No homogeneous inclusions typical of melanin macroglobules ("giant melanosomes") were found in any of the specimens, supporting the view that neither patterned nor nonpatterned seborrheic keratoses are nevoid in nature.