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 lipid synthesis near nucleus. Plasma membrane origin inconsistent with different lipid composition and lack of ribosomes, Golgi origin reverses secretory flow since COPII carries from ER to Golgi not vice versa, peroxisomes actually bud from ER subdomains. Experimental photobleaching of GFP-KDEL present in both outer nuclear membrane and ER shows rapid recovery across compartments within seconds, confirming single lumen. Shared calcium pool couples nuclear signaling to cytosolic release via IP3 receptors localized in both membranes. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.
Ref: Alberts Ch 12; outer nuclear membrane continuous with rough ER lumen, shared calcium and proteins.