Sentinel means a lookout, and the sentinel node is defined as the first lymph node or nodes that are located on the direct draining lymphatic route from the area of a primary tumor.
The lymphatic system was first described in the seventeenth century by Olof Rudbeck who systematically studied the lymphatic vessels collecting extracellular fluids from tissues emptying into lymph nodes. The lymphatic system is more variable than the blood system and is anatomically less well defined. The drainage from tumors seems to vary considerably, making prediction of the draining lymph node difficult without guidance. Tumor cells become metastatic either by invasive growth through basal membranes or by entering into capillaries. The endothelial cells comprising the lymph capillaries are widely fenestrated (fenestration) making easy access to the lymph vessel. There is also an active transport of primarily white blood cells...