Collagen is synthesized on ribosomes as pre-pro-collagen. From there it enters the endoplasmic reticulum where the signal sequence is removed from the terminal end and glycosylation and hydroxylation of proline and lysine residues occur, resulting in the formation of procollagen. From endoplasmic reticulum it is secreted extracellularly through the golgi apparatus. Here the extracellular enzymes cleaves the extension peptides and finally the triple helical collagen moleculs assemble into collagen fibers. Ref: Harper's Principles of Internal Medicine, 25th Edition, Page 697 and 29th Edition, Chapter 48 ; Lipppincott Biochemistry, 2nd Edition, Page 40 ; Vasudevan Biochemistry, 4th Edition, Page 240