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In the next stage of tumor progression, a solid tumor invades nearby tissues by breaching the basement membrane. The basement membrane, or basal lamina, is a sheet of proteins and other substances to which epithelial cells adhere and that forms a barrier between tissues. Once tumors are able to break through this membrane, cancerous cells not only invade surrounding tissue substances but also enter the bloodstream—often via a lymphatic vessel, which discharges its contents into the blood. Tumor cells that have invaded a lymphatic vessel often become trapped in lymph nodes, whereas cells that gain access to blood vessels are ...(100 of 22022 words)