ough, insoluble protein found in tissues such as hair, nails, and epidermis of the skin. ??? ??? ??? Melanin the dark pigment synthesized by melanocytes responsible for skin color. ??? ??? ??? Melanocytes Cells that produce melanin. ??? ??? ??? Mucous membrane membrane that forms the linings of body cavities open to the exterior (digestive, respiratory, urinary, and reproductive tracts) ??? ??? ??