SITES WITHIN THE CELL 45 



many various cells ; it is especially developed in secretory cells like liver or 

 pancreas. This structure was described as a system of double lamellae 

 (Sjostrand and Hanzon, 1954; Bernhard^f a/., 1954)orof elongated vacuoles 

 forming a reticulum (Porter, 1953). Palade and Porter (1954) observed that 

 the membranes of the reticulum are lined with small granules of a rather 

 uniform size (100-150 A in diameter). The number of these granules 

 seems to be correlated with the basophilia, i.e. the RNA content of the 

 tissue. 



By electronmicroscopy, Slautterback (1953) observed that isolated mouse 

 liver microsomes contain essentially two types of structures : rather large 

 formations of about 0-1 /i, and smaller dense particles associated with the 

 larger ones. Better pictures of microsomal fractions isolated from rat liver 

 showed that they were made of more or less swollen vesicles of various 

 sizes (Kuff et al, 1956) corresponding probably to the larger of Slautter- 

 back's particles. Littlefield et al. (1955) examined microsomal preparations 

 with the electron microscope before and after deoxycholate treatment. 

 Although the pictures they obtained were rather poor, it was clear that 

 deoxycholate treatment destroys or dissolves the larger components and 

 leaves the smaller particles. Parallel centrifugation experiments indicated 

 that the smaller particles were the ribonucleoproteins which had been 

 shown to be the most active in protein synthesis. Excellent systematic 

 studies by Palade and Siekevitz (1956), in which centrifugation, electron 

 microscopy and biochemical methods were combined, definitely estab- 

 lished that the so-called microsomal fraction of liver results from the dis- 

 ruption and dispersion of the ergastoplasm or endoplasmic reticulum into 

 the homogenization medium. The trabeculae and cisternae observed on 

 sections of osmic acid fixed tissue are found in the homogenates as vesicles 

 in the microsomal fraction. These vesicles react like osmometers to changes 

 of sucrose or salt concentration, and they dissolve in deoxycholate. The 

 Palade granules which line the lamellae of the reticulum do not dissolve ; 

 these are the ribonucleoprotein particles, most of which are normally 

 attached to the lamellae in vivo. It is possible, however, that the mem- 

 branes of the microsomal vesicles also contain some RNA (Moule et al., 

 1960). 



In guinea pig pancreas, the picture is similar, except that many free 

 ribonucleoprotein particles are observed between the lamellae, outside the 

 cisternae. Very large dense granules are seen within these cavities; these 

 are secretory granules at different stages of formation, i.e. packages of 

 zymogens which are the main proteins produced and excreted by the 

 pancreas (Siekevitz and Palade, 1958). In kinetic experiments in which 

 labelled amino acids are injected to the animal, the ribonucleoprotein 

 particles are the first to contain labelled polypeptides and also labelled 

 zymogens. Siekevitz and Palade (1958) have very good evidence that in 



