The endoplasmic reticulum (ER) is believed to originate from:
Endoplasmic reticulum is extensive network of sheets and tubules arising from outer nuclear membrane, retaining direct luminal continuity and sharing membrane proteins such as nesprins and reticulons that shape curvature. Tomography shows ribosome-studded rough ER extending from nuclear envelope, perinuclear cisterna connecting to peripheral tubules, allowing rapid diffusion of luminal proteins like protein disulfide isomerase and calcium between compartments. Evolutionary model proposes invagination of ancestral nuclear envelope to form reticulum facilitating cotranslational translocation and
Ref: Alberts Ch 12; outer nuclear membrane continuous with rough ER lumen, shared calcium and proteins.
