46 KERATIN AND KERATINIZATION 



(c) Small dense particles. Since the electron microscope can resolve 

 macromolecules of diameters as small as 20-30 A, the " ground sub- 

 stance " of cells often presents a fine particulate appearance. Conspicuous 

 among these fine particles on account of its size (100-200 A) and density 

 is a particle shown by Palade (1955) to contain ribonucleic acid (RNA). 

 These particles, ribosomes, may be free or associated with membranes to 

 form an important cell organelle, the basophilic reticulum described below. 



Membrane systems 



Membraneous systems are particularly well preserved and " stained " 

 by fixation in osmium solutions and in permanganate and are therefore 

 conspicuous in electron micrographs. Several organelles recognizable in 

 the light microscope are now known to be based on a skeleton of 

 membranes. The mitochondrion described above as a particulate may be 

 regarded as an example. Others less delimited in their extent are 

 described below. 



Systems of particle covered membranes (ergastoplasm, basophilic reticulum). 

 The cytoplasm of many cells (see p. 108 et seq.) contains more-or- 

 less elaborately developed systems of membranes whose surfaces are 

 covered with small dense particles identical with those described above 

 (Fig. 22 (b))-. The membranes exhibit an intricate complexity of profiles 

 when examined in sections, which appear to be views of a geometrically 

 complex and intricately-interconnected reticulum of surfaces. This in effect 

 divides the cytoplasm into two parts: that inside the vesicular system 

 and that outside. 



Systems of smooth surfaced membranes. The membranes here are 

 similar to those just described, but their surfaces are not associated with 

 particles. Some are simple " empty " vesicles of uncertain import. Two 

 rather more defined formations are usually described. The first, often 

 well developed in cells associated with the transfer of water, consists of 

 simple pairs of closely opposed parallel membranes which have been 

 traced back and shown to be deep pleats of the plasma membranes of the 

 cell (Fig. 22 (c)). The second is a variable yet always characteristic 

 stack of double membranes (flattened sacs) (Fig. 22 (d) and (e)) which 

 appears in all cells. It is usually assumed to correspond to the Golgi 

 complex of light microscopy, an organelle of uncertain function (p. 110) 

 (Grasse, 1956; Haguenau and Bernhard, 1955; Baker, 1955; Dalton and 

 Felix, 1956). Other rather more regular stacks of flattened sacs associated 

 with the nuclear membrane have also been described. 



Concentric membranes, " whorls," myelinic forms. These formations may 

 vary from very perfectly-formed concentric shells of membranes with an 

 intermembrane spacing of the order of 40 A, through more open, con- 

 centric-shell formations, to stacks of parallel membranes closely related 



