114 KERATIN AND KERATINIZATION 



by : (a) their differences in fine cytology and (b) by the manner in which 

 they dispose of their synthesized protein. In one case the protein simply 

 accumulates within the cell until, perhaps by a kind of mass action or 

 a simple physical exhaustion of space, synthesis is arrested. They appear 

 capable only of a single burst of activity. We shall term such cells retaining 

 cells (Fig. 50). In the other case, the product is discharged from the cell 

 after which another cycle of activity and secretion is initiated. Such cells 

 may continue their synthetic activities for a more or less prolonged period. 

 (Birbeck and Mercer, 1961). 



Secreting cells possess the more complex cytology since, in addition to 

 their often elaborately-developed membrane system, they may possess 

 specialized devices for the temporary accumulation, transfer and for the 

 removal of protein from the cell. Since both types of cell synthesize 

 proteins, the capacity to do this must reside in their common structural 

 feature, the RNA particles of the cytoplasm. The membrane system is 

 therefore secondary and appears to be associated with the prolonged 

 activity and removal of secretion. Obviously for a cell to be able to 

 produce many times its weight of secretion, some elaboration of structure 

 is required to facilitate the entrance, transport and removal of material. An 

 organization analagous to the production line of a factory might be ex- 

 pected, and in fact, such cells are typically polarized. One portion of their 

 surface, the basal region, usually adjacent to the sources of raw material 

 (blood vessels) becomes specialized as an input area; the opposite aspect, 

 the apex, where secretion granules may collect and which usually abuts a 

 storage space or lumen, is the output area. These requirements are the 

 basis of the familiar histological pattern of glandular cells. Internally, as 

 Porter has emphasized (1954), a system of interconnected membranes is 

 admirably suited for the channelled diffusion of metabolites to and from 

 active sites and also for the collection of the products of synthesis. 



According to Porter and Palade the total cytoplasmic membrane system 

 should be regarded as a unit, a definite cell organelle which may assume a 

 variety of forms : flattened interconnected sacs, cisternae, canaliculi and 

 isolated vesicles, for which the name endoplasmic reticulum is proposed. 

 The qualifying adjective " endoplasmic " was suggested by observations 

 on whole cells in tissue culture (Porter, 1954). Since subsequent obser- 

 vations on cells in sections have shown that membranes may spread 

 throughout the cytoplasm, the simpler name " reticulum " may finally be 

 adopted as more accurate. For nomenclature, see Fig. 23, p. 48. 



Such a system of membranes enormously increases the internal surface 

 available as sites for catalysts and is, in fact, precisely the sort of " cyto- 

 skeleton " long demanded by biochemists (Peters, 1937) as a structural 

 support for an organized array of enzymes. The mitochondrion provides a 

 similar, even more compact, bundle of membrane-supported sites. 



