Fractionation of a Microsomal Fraction from Rat Liver or Cultured Cells

The membrane organelles of an organized tissue, such as rat liver, that will always form closed vesicles, irrespective of the mode of homogenization, are the tubular membranes of the smooth and rough endoplasmic reticulum. Almost without exception, these vesicles retain their normal orientation, i.e., the cytoplasmic face is outermost. This is particularly clear in the case of the rough endoplasmic reticulum, for the ribosomes remain attached on the outside face. The microsomal (vesicular) fraction of a homogenate will also contain vesicles derived from any domain of the plasma membrane that, in vivo, is not stabilized by structures, such as tight junctions, desmosomes, or an extensive cytoskeleton. Thus, the blood sinusoidal membrane of hepatocytes and the basolateral membranes of enterocytes both tend to form vesicles, whereas hepatocyte contiguous membranes and enterocyte microvillar membranes will remain intact, as long as the homogenization is relatively mild (see Chapter 6 ).