Model of benign metastasis. In this model, a melanocytic stem cell (1) undergoes an initiating event that transforms it into an immature nevus progenitor cell (2). This cell (2) may remain quiescent until local environmental factors stimulate it to proliferate into a nevus (3). Alternatively, this loosely adherent cell could enter the lymphatic system (4). Upon encountering a lymph node, the progenitor cell could either implant in the node and proliferate into a nodal nevus (5) or pass through without being sequestered (6) to eventually reach the circulatory system (7). The progenitor cell would continue to circulate (8) until a transforming event, like a mutation, or environmental conditions signals the cell to implant in the skin (9). The implanted nevus progenitor cell would remain quiescent in the skin until local environmental factors stimulate it to proliferate into a nevus (10).
