STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 53 



9. Nuclear Meynhrane 



The ready penetration of enzymes into isolated nuclei has been stressed by 

 many workers, e.g., Anderson (1953), who have pointed to the apparent 

 porosity of the nuclear envelope. Recent electron micrographs of interphase 

 nuclei (Gall, 1954; Watson, 1955) have clearly revealed the existence of a 

 double nuclear membrane in which circular pores or annuli are formed by 

 continuities between inner and outer membranes (see Fig. 3). Similarly, 

 porous nuclei exist in a wide variety of tissues and the ease of penetration of 

 proteins into nuclei is readily ex^^lained by the physical existence of holes. 

 Nevertheless, the properties of the retention of protein, nucleotide, and K+ 

 by the nuclei of thymus lymphocytes (AJlfrey et at., 1957a; Osawa et at., 1957) 

 raise questions concerning the nature of this particular membrane. 



In addition to this direct contact of nucleus and cytoplasm, involving 

 perhaps one-tenth of the nuclear surface (Watson, 1955), the remaining area 

 is in indirect contact through membrane-enclosed cavities of the endoplasmic 

 reticulum, which, in some cells, at least, appear to be continuous with the 

 outer nuclear membrane (see Fig. 3). It has been suggested that, whereas large 

 molecules may pass through the pores, inorganic ions and small molecules may 

 be exchanged by way of the endoplasmic reticulum. It is thus supposed that 

 there are two distinct pathways of exchange between nucleus and cytoplasm, 

 related to the two structurally distinct points of contact between these parts 

 of the cell. 



C. Cytoplasmic Structures 



1. Historical Notes 



Until about 15 years ago the isolation of cytological structures and their 

 subsequent characterizations were being attempted exclusively by biologists, 

 who also provided much of the chemical data. The anatomists, Bensley and 

 Hoerr (1934a,b), first separated mitochondria and reported on their content 

 of protein and hpid (Bensley, 1942; Lazarow, 1943). Claude attempted the 

 isolation of Rous tumor virus by means of differential centrifugation, which 

 had proved so useful in the isolation of various viruses. In the course of these 

 studies, Claude (1939, 1940) discovered the presence of antigenic submicro- 

 scopic particulates, which he called microsomes, in many normal tissues; 

 indeed, these were found to be extensive contaminations of his virus pre- 

 parations. Simultaneously, Brachet, who was studying the correlation of 

 basophilia and RNA content during embryological development, deduced 

 the existence of a small particle fraction rich in RNA, and with his colla- 

 borators proceeded to the isolation of cytoplasmic particulates in yeast, etc. 

 Although many biochemists had observed the dependence of various en- 

 zymatic activities upon organized cellular structure and had pointed to the 



