A network of membrane-bound tubes and sacs that extends throughout the cytoplasm, serving as a highway for transporting materials and a site for protein and lipid synthesis. It exists in two forms: rough ER (with ribosomes) and smooth ER (without ribosomes).
From Greek 'endon' (within), 'plasma' (formed thing), and Latin 'reticulum' (little net). The term reflects its appearance as a net-like structure within the cell's cytoplasm, first described in detail by electron microscopy in the 1940s.
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