In our opinion, the present state of knowledge does not justify setting up 

 any very sophisticated terminology for purposes of classification. 



The endoplasmic reticulum tends to show a characteristic morphologi- 

 cal pattern depending on the kind of cell in which it is found. Likewise, 

 variations in structural complexity and disposition of the endoplasmic 

 reticulum during cell differentiation have been noted (Fawcett, 1959). 

 In the developing guinea pig spermatogonia, the endoplasmic reticulum 

 consists of only a few tubular and vesicular elements. In the spermato- 

 cytes, it is a centrally disposed network of extensive cisternae which are 

 peripherally arranged in layers parallel to each other and to the cell 

 surface. Most of the granules are distributed throughout the cytoplasm 

 rather than being extensively associated with the membrane system. 

 During development of spermatids, cisternae disappear, leaving a reticu- 

 lum consisting of a sparse network of tubular elements. By the end of 

 development, there is virtually no trace of the reticulum and the Palade 

 granules are randomly disposed in the cytoplasm. 



The functional significance of the membrane system in general is not 

 known. The cisternae with their associated granules seem to be involved 

 in protein synthesis, but whether this is a function of the membrane 

 system per se or of the granules alone is, so far, not clear. Porter has 

 suggested the following functions: (1) to provide an internal membrane 

 surface for the orderly distribution of enzymes, (2) to divide the cyto- 

 plasm into compartments so as to facilitate segregation of metabolites, 

 and (3) to transmit impulses in structures such as muscle and nerve 

 cells. Except for the association with protein synthesis, there is no very 

 good experimental evidence for the other functions mentioned. 



MICROSOMES 



The microsomes are small, particulate components of the cytoplasm 

 which were first isolated from liver homogenates by differential centrifu- 

 gation by Claude (1941). It now seems clear that this fraction (Figure 

 3-26 (a) and (b)) consists of membrane fragments of the endoplasmic 

 reticulum with their associated small granules of Palade, or ribosomes 

 (Palade and Siekevitz, 1956). Biochemical studies have shown that both 

 the microsomal fraction and the small granules of Palade have a high 

 RNA content. The concept that RNA is implicated in protein synthesis 

 was first postulated by Brachet (1941) and Caspersson (1941). This 

 is based largely on the observation that cells, such as liver cells and 

 meristematic cells in growing roots, which synthesize large amounts of 

 protein are also relatively rich in RNA. On the other hand, muscle and 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 55 



