Figure 175 from Chapter 5 (Endoplasmic Reticulum) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. Electron micrograph of a plasma cell from guinea pig bone marrow. Active synthesis of immunoglobulin by this cell is associated the extensive rough endoplasmic reticulum (RER); the protein produced by the RER accumulates in the lumen of the reticulum, and is revealed by the gray precipitate localized there. A PDF copy of the accompanying chapter is available on the ASCB's BioEDUCATE website.