ELECTRON MICROSCOPE EXAMINATION OF SUBCELLULAR FRACTIONS III. Quantitative Analysis of the Microsomal Fraction Isolated from Rat Liver

Morphometric studies of the cellular membranes have been carried out on normal rat liver by Loud (29) and by Weibel et al . (47) . Such measurements in situ offer the advantage of allowing easy cytological identification of the structures, but they do not permit direct correlation between morphological and biochemical data on the same material . Such a correlation can be more directly established when the same subcellular fractions are submitted to the morphometric and the biochemical analysis . We report here the results of a quantitative morphological analysis of rat liver microsomes and of microsomal subfractions isolated by density equilibration in a sucrose gradient. This study parallels a biochemical analysis, the results of which have been briefly reported before Rat liver microsomes and microsomal subfractions isolated by density equilibration were submitted to a quantitative morphological and biochemical analysis . The total area of the endoplasmic reticulum was estimated at 7 .3 m2 per g of liver . The microsome fraction contained 2.8 mg of phospholipids and 6 .7 mg of proteins per m2 of membrane area . After correction for ribosomal and intracisternal proteins, the latter value was lowered to 4.7 mg of membrane protein per m 2. More than half of the microsomal vesicles carried ribosomes . After density equilibration of the microsomes, the distribution pattern of ribosomes followed closely that of RNA. The ribosome load of the microsomal vesicles increased steadily along the density gradient, indicating the existence of a continuous spectrum of microsomal entities ranging from entirely ribosome-free vesicles to vesicles heavily coated with ribosomes .