How developed is the endoplasmic reticulum in rapidly dividing cells?

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. In meristematic cells and immature dividing cells, the endoplasmic reticulum is absent or poorly formed. Endoplasmic reticulum membranes are thinner than most other cell membranes.