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endosome A vesicle formed within a cell during forms of endocytosis in which the material to be ingested first binds to receptor sites on the cell surface. These receptors occur in areas of the plasma membrane called coated pits, which are pinched off to become coated vesicles, surrounded by a protein coat (see clathrin) and containing the ingested material, within the cell. The vesicles then shed their coats and merge with other vesicles to form endosomes. The endosomes fuse with vesicles from the Golgi apparatus to form lysosomes.