Transmission electron micrograph (TEM) of the ultrastructure of the organelles involved with protein synthesis. The assembly of peptides and polypeptides occurs in the rough endoplasmic reticulum (rER) and is seen as fine particulates in the membranous cisternae. Exported from the rER, vesicles are transported to the Golgi membranes which process the contents further. Exiting the Golgi, vesicles containing proteins are aggregated to form immature secretory vesicles that adopt a circular shape when they are termed mature zymogen granules. These show a degree of density indicating compaction/concentration of proteins. Proteins may be destined for export as secretory products of the cell. Magnification: x15,000 when printed at a width of 10cm.