In rapidly dividing cells, the endoplasmic reticulum is?

Concept: The endoplasmic reticulum (ER) is a vast, dynamic structure in the cell that performs numerous functions such as calcium storage, protein synthesis, and lipid metabolism.

Except for adult erythrocytes, the endoplasmic reticulum (ER) is a system of membrane-lined channels found in all eukaryotic cells. It accounts for more than 30 to 60% of total cell membranes. In meristematic cells, immature dividing cells, and so forth, the endoplasmic reticulum is absent or poorly formed. Endoplasmic reticulum membranes are thinner than most other cell membranes.