KERATIN AND MOLECULAR BIOLOGY 47 



to Golgi clusters. They may be imitated in vitro using preparations of 

 phospholipids extracted from cells (Stoeckenius, 1959; Mercer, 1960). 

 The myelin sheath of vertebrate nerve and the stacks of plates in the 

 retinal receptors are special cases of such structures (Sjostrand, 1956). 



The similar morphology (see p. 37) of all these membranes and the 

 observations on the polymorphic possibilities of phospholipid membranes 

 in vitro, has led to the growing opinion that all intracellular biological 

 membranes have a common molecular basis in consisting of biomolecular 

 leaflets of phospholipids covered with layers of protein, as was proposed 

 many years ago for the plasma membrane itself and described above. 

 The different appearances and functions are thought to be determined 

 by the absorption on their surfaces of various macromolecules. Several 

 systems of nomenclature have already been proposed and, since some 

 confusion is possible, these will be outlined here. Sjostrand distinguishes 

 three types of membranes : (a) a-cytomembranes or membranes associated 

 with dense particles, (b) jS-cytomembranes, smooth surface membranes 

 found in the Golgi region and (c) y-cytomembranes, smooth flattened 

 invaginations of the cell membrane. Porter and Palade also recognize 

 these types, but prefer to regard all cytoplasmic membranes as portions of a 

 single membrane system which may become locally specialized for certain 

 functions, e.g. for protein synthesis by becoming associated with RNA 

 particles. The system of particle-studded (a-cytomembranes) is also iden- 

 tifiable with the basophilic ergastoplasm of Gamier (1897) (see Haguenau, 

 1958) and Bernhard and his associates (Bernhard et al., 1951 and 1954) are 

 inclined to refer to the entire membrane system as ergastoplasm. These 

 several proposals are set out in Fig. 23. 



Some authors would go further in an attempt to unify the membrane 

 systems of cells under a single concept, by considering even the external 

 plasma membrane of the cell as part of this system. This view is implicit 

 in Ben Geren's views on the origin of the myelin sheath of nerve fibres 

 as an elaborate involution of the Schwann cell membrane and by 

 the work of Robertson (1959). In some cells, such as amoeba, the 

 formation of many vacuoles by invagination of the external membrane 

 is obvious, and all membranes retain the same fine structure (Mercer, 

 1959). A common molecular framework forming the basis of bio- 

 logical membranes is also envisaged by the Danielli and Harvey theory 

 of membrane structure already referred to on p. 38. 



A certain lability of membrane structure is indicated by the profusion 

 of forms assumed by the cytoplasmic membranes in vivo and also by the 

 experimentally-produced breakdown of the reticulum and its re-formation 

 as smaller microsome vesicles (p. Ill) (Plate 10B). Bacterial membranes, 

 which are membranes of a widely- different origin, also possess a similar 

 property of reforming smaller vesicles on breaking up. 



