THE ENDOPLASMIC RETICULUM 131 



teristic for these to lie parallel in groups or stacks of several units. Some- 

 times these vesicles or cisternae may possess dimensions of several microns 

 in the plane of the lamellae and have a thickness not greater than 50 m/i. 

 Such large cisternae are further characterized by having large populations 

 of small particles (150 to 200 A) attached to their outer surfaces. This is 

 the surface contiguous with the matrix phase of the cytoplasm and, indeed, 

 the particle may properly be regarded as a component of the cytoplasmic 

 matrix. In pancreas cells, such cisternae with particles occupy about three- 

 fourths of the volume of the cytoplasm. In liver cells they are less 

 prominent and occur in small arrays scattered throughout the cell (Fig. i). 

 In both these cells and in others as well, such arrays are observed to 

 coincide with the more intensely basophilic parts of the cytoplasm 

 (ergastoplasm) in which, histochemical tests have shown, there are high 

 concentrations of RNA [8, 9]. This form of the ER has a widespread 

 occurrence among cells and is now regarded as present in one or another 

 minor variation in all cells known to be engaged in the synthesis of a 

 protein for secretion (or export) from the cell. These facts of morphology, 

 together with much older observations on the behaviour of the ergasto- 

 plasm during phases of secretion and recovery, would convince one, even 

 without other evidence, that this form of the ER is somehow functional in 

 protein synthesis [10, 1 1]. The investigation of this topic has not, however, 

 stopped with microscopic cytological and histochemical studies, and much 

 more convincing evidence of this functional association is now available. 



The microsome 



This body, the subject of this session of the current symposium, was 

 identified and characterized first as a submicroscopic unit or particle of 

 the cell that could be consistentlv separated out (from a variety of animal 

 cells) by following a certain schedule of centrifugal fractionation [12, 13]. 

 The microsome fraction was further characterized by Claude as being rich 

 in phospholipids and RNA, and as being related in some manner to the 



Fig. I. This depicts the fine structure of a normal liver cell of the laboratory 

 rat. The nucleus {N) is at the ritjht; the marginal part of a blood sinusoid (s) is at 

 the left. Mitochondria are numert)us and readily identified by their characteristic 

 morphology. The endoplasmic reticulum is represented by two distinct forms. One, 

 shown in the centre of the field (er), consists of large, thin, lamellar vesicles or 

 cisternae with small (150 A) dense particles attached to their surfaces. In thin 

 sections these appear as rough, line-limited profiles, associated in groups which 

 coincide with the basophilic component of the light microscope image. The other 

 form is particle-free or smooth and is constructed of fine tubular elements ca. 

 50 m/^t in diameter (ers). These comprise in liver cells what appears as a loose tri- 

 dimensional lattice. The Golgi component is represented at G. 



