The endoplasmic reticulum is a network of folded membranes that form channels. They extend from the cell membrane through the cytoplasm to the nuclear membrane. The ER was discovered in 1945 by Albert Claude in Belgium and Keith Porter at Rockefeller Institute. It was first noticed in the cytoplasm of chick embryo cells.
All eukaryotic cells have endoplasmic reticulum except the red blood cells of mammals.
The membranes of the ER are concentrated in the inner region of the cell, the endoplasmic region. The membranes surround and inner cavity called the lumen. The lumen is basically a compartment for storage of substances that must be kept separate from the cytoplasm. The main function of the ER is to transport materials through the cell. It also serves as the site of biochemical reactions.