Figure 177 from Chapter 5 (Endoplasmic Reticulum) of 'The Cell, 2nd Ed.' by Don W. Fawcett M.D. This electron micrograph shows the supranuclear region of an odontoblast from a rat incisor, as this cell is actively secreting dentin matrix. The accumulating product of protein synthesis by the rough endoplasmic reticulum is evident as a flocculent material in the lumen. Image by Melvyn Weinstock. A PDF copy of the accompanying chapter is available on the ASCB’s BioEDUCATE website.