Useful notes on the Origin of Endoplasmic Reticulum!
At present the manner of origin of the endoplasmic reticulum is not definitely known. The most concrete hypothesis is that the ER is “budded” off from the nuclear envelope (Wischnitzer, 1974).
Continuities between the nuclear envelope and ER have often been reported. Barer et al., (1959) found evidence that the nuclear envelope reforms from ER-like vesicles after cell division.
Essner and Novikoffs (1962) discovered that both the nuclear envelope and ER membranes catalyze hydrolysis of UDP, GDP and IDP. In rat liver cells both membranes have glucose-6- phosphatase activity. In pancreatic and other cells, ribosomes are attached to the outside membrane of nuclear envelope.
Fawcett’s (1955) observed that, in fasted and refed rat liver cells, new ER seems to appear first near the cell periphery.
Structural difference between ER and nuclear envelope:
The most notable structural difference between ER and nuclear envelope is the presence of numerous conspicuous annuli, about 500 to 1000 A in diameter, in the nuclear envelope, which are not present in ER. But cytoplasmic membranes known as ‘annulate lamellae’ are found in invertebrate and vertebrate oocytes, spermatocytes and some types of somatic cells (Afzelius, 1955, and Swift, 1956).
They have noted a striking similarity of annulate lamellae to the annuli of nuclear envelope. Merriam (1959a) also concluded from his electron microscopic analysis that the annulate lamellae of echinoderm eggs arise by proliferation from nuclear envelope and that 150 A ribosome-like particles are associated with these membranes.
Kessel (1963) has confirmed that the annulate lamellae of Salamander oocytes arise by budding from the nuclear envelope.
Hrubam (1965) has suggested that annulate lamellae represent an intermediate stage in the formation of endoplasmic reticulum. Annulate lamellae lose their pores and ribosomes appear on its surface.
No de nove origin of ER:
The endoplasmic reticulum grows by expansion of pre-existing membranes indicate no de novo origin of ER.