A thick compact keratin layer indicates that this is acral skin. Note the proliferation of melanocytes growing in a solid pattern down deep.
At the lesion edge, the melanocytes have obscured the dermal-epidermal junction. There is some pagetoid spread, but it is difficult to determine from this image.
The melanocytes are highly atypical with prominent nucleoli.
There are several subtypes of melanoma, which occur in decreasing frequency as follows: superficial spreading, nodular, lentigo maligna, and acral lentiginous. By definition, the acral type arises in acral sites only (palms, subungual sites, soles). It is the most common melanoma in dark-skinned individuals (Busam).
Acral melanomas can arise de novo, or in a preexisting melanocytic nevus (similar to the superficial spreading type). Histologically, it displays similar features as the lentigo maligna type, with a proliferation of atypical, usually single, melanocytes along the dermal-epidermal junction. Pagetoid spread can be seen as well.
This variant of melanoma is rare and arises later in life and on palmoplantar locations unattributable to sunlight.
In terms of prognosis, it is less favorable than the cutaneous melanoma.
Busam KJ. Dermatopathology: Foundations in Diagnostic Pathology 1st Ed. Philadelphia, PA: Elsevier; 2010: 465-482.
Pollack LA, et al. Melanoma survival in the United States, 1992 to 2005. J Am Acad Dermatol. 2011 Nov;65(5 Suppl 1):S78-86.