Growth factor induction of lymphangiogenesis via direct and indirect mechanisms. HGF, bFGF, PDGF-B, and IGF-1/2 are ligands for receptors on a wide variety of cell types, such as tumor cells, immune cells, and various mesenchymal cells. All these cells have been reported to produce VEGF-C and/or VEGF-D. Growth factors can further stimulate production of lymphangiogenic factors and may attract cells that produce them. Key to table: + indicates that receptor expression in this cell type is verified in several studies; +/–, receptor expression is reported in a single study; and?, contradicting reports have been published. There are no published reports regarding receptor expression. NOTE: The tumor cell column does not refer to tumor cells in general, but rather to expression of receptors in some tumors.