Cell structure. Transmission electron micrograph (TEM) of a section through an undifferentiated cell from the intestinal tract, showing rough endoplasmic reticulum (ER, thin dark green lines), nuclei and mitochondria (light green and blue round structures).The function of RER is to assemble amino acids into peptide chains which, with further processing, form proteins. Rough endoplasmic reticulum is present inside most cells and its abundance is proportional to the level of protein synthesis within the cell. Undifferentiated cells like this serve as replacements for cell types both in the glands and villi of the intestine. Magnification: x4000 when printed 10 centimetres wide.