Abstract
The morphology of melanoma in the progressive and regressive periods was studied in 3 White × Duroc pig litters. Tumorous cells in the progressive period (19 to 22 days after birth) were mostly polygonal with high mitotic activity. In the regressive period (90 days after birth) spindle like melanocytes without mitotic figures were surrounded by adipose tissue. The tumorous cells showed a positive reaction with the melanoma HNB45 antibody and a negative reaction with the CD3 marker in the tumorous mass. Four types of cells were observed by electron microscopy: macrophages, large granular lymphocytes, melanocytes and melanophages. Melanocytes and melanophages, mainly in regressive period, showed various stages of apoptosis.