NCIt definition : An internal membrane structure of the eukaryotic cell. Biochemically similar to the
rough endoplasmic reticulum, but lacks the ribosome binding function. Tends to be
tubular rather than sheet like, may be separate from the rough endoplasmic reticulum
or may be an extension of it. Abundant in cells concerned with lipid metabolism and
proliferates in hepatocytes when animals are challenged with lipophilic drugs.;