Unformatted text preview: verview decreasing patient survival . At early stages, melanoma is constrained within the epidermis. At this stage it is thought to have a low metastatic potential; with complete surgical removal, 5‐year survival rates are high. As the tumour continues to develop, cells penetrate the basal membrane, and continue to expand in the dermis. The risk of metastasis gradually increases as the tumour invades deeper into the dermis. Once the melanoma reaches the subcutaneous fat tissue, the risk of metastasis is high and systemic metastases are likely to occur. Cells that are shed from the primary lesion infiltrate the circulatory and lymphatic system, and migrate to new sites where they adhere to the walls of the capillary and invade a new organ. It has been reported that 3‐4 x 106 cells are shed daily, it is not known however how many of these cells have metastatic potential . At the secondary site, micrometastases can survive for several years before they become proliferative, stimulate angiogenesis, and begin to form a metastatic tumour . Advanced stage melanomas are described by the presence of regional or distant metastases. Melanoma origin and progression The precise origin of melanoma is still uncertain and the exact mechanism of its development and progression remain to be elucidated. Several possibilities exist which explain melanoma origin, from epidermal melanocytes, or alternatively from melanocytic stem cells, or even from dermis‐derived stem cells, suggested to give rise to epidermal melanocytes and potentially also to melanoma . The
likelihood of melanomas arising from bulge melanocyte stem cells or even follicular matrix melanocytes is low since most of the primary melanomas are observed at the epidermal‐dermal junction. A more likely origin would be from epidermal melanocytic cells, since the existence of interfollicular melanocyte stem cells have not yet been confirmed . Large numbers of melanomas, around 40%, are observed to arise from pre‐
existing precursor lesions . A linear step‐wise model has been suggested to
explain melanoma progression from a melanocytic naevus through to a dysplastic naevus, followed by radial and vertical growth phases, via clonal evolution and...
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- Spring '14
- pH, ........., Melanocyte