Superficial spreading melanoma, light micrograph. Superficial spreading melanoma (SSM) is the most common subtype accounting for almost 70% of all cutaneous melanomas. This light photomicrograph shows both the radial phase as well as the vertical growth phase. The radial phase of SSM shows proliferation of atypical melanocytes arranged singly or in clusters throughout all levels of epidermis. The tumour cells are epithelioid with abundant cytoplasm, pleomorphic nuclei, prominent nucleoli, frequent mitoses, and dusty melanin pigmentation. The vertical (invasive) growth phase consists of cohesive nests, clusters, and nodules invading the dermis. The nests are morphologically different than those in the intraepidermal component with great pleomorphism, apoptosis and frequent mitoses.