Ribosomes. Many of the ribosomes in a cell’s cytoplasm are assembled on the endoplasmic reticulum, the cell’s “shipping and handling” center for membrane protein production. In this electron micrograph, arrows indicate large polyribosomes, or polysomes, which consist of many ribosomes lined up on a single messenger RNA. Mitochondria (M) and microtubules (mt) are also visible, as well as large spiral polyribosomes (S). The interior (lumen) of the rough endoplasmic reticulum often appears expanded (asterisks).