Melanocytic Lesions Part 3 Visual assessment of melanoma MSLO Melanocytic skin lesion ontology : Digital Medicine GASTROINTESTINAL HEMORRHAGE (2): Melena. Melena, the passage of black, tarry stools, usually implies blood loss greater than 500 ml originating anywhere between the oropharynx and the right hemicolon. Melena occurs most often, SKCIN Melanoma begins in melanocytes the cells that produce the pigment melanin that colours the skin, hair and eyes and tans the skin when it is exposed to UV radiation Figure 2 from Smokers' melanosis in a Nigerian population: