Simplify the following text for a Grade 13 native English-speaking person.
Original: The three varieties are called rough endoplasmic reticulum , smooth endoplasmic reticulum and sarcoplasmic reticulum .
Simplified: It may be either rough endoplasmic reticulum ( rER ) , so called because it is studded with ribosomes to give it a rough appearance under a microscope , or smooth endoplasmic reticulum ( sER ) .