40 KERATIN AND KERATINIZATION 



The pattern of 9 + 1 pairs of filaments occurs with remarkable persist- 

 ance throughout the animal and vegetable kingdoms and modified versions 

 of such surface protusions are to be found in such unlikely sites as the 

 rods and cones of the vertebrate eye. 



In electron micrographs of these organelles faint indications of a very 

 fine filamentous (diameter ~ 50 A) cytoplasmic component are common 

 (Fig. 19 (e) ). In ciliated epithelia (Fawcett, 1958), for example, and in 

 association with fine pseudopods in other situations, denser deposits 

 beneath the membranes are to be seen and may indicate a region of firmer 

 gelation of the cytoplasm which helps in maintaining the shape of the 

 surface protusions. Little is known of the detailed composition of such 

 filaments. Nevertheless, it is in such ill-defined fibrous proteins of the 

 cytoplasm that we may find primitive precursor of both keratin and the 

 contractile muscle proteins (Fig. 19 b and c). 



Surface invaginations. These occur as long pleats of the plasma 

 membrane forming double surfaced membranes penetrating far into the 

 cell (Fig. 19 (c) ). They are particularly common in cells involving water 

 transport. Most elaborate examples are noted in the cells of the stomach 

 which secrete hydrochloric acid. 



Temporary invaginations are commonly associated in free living cells 

 with the ingestion of solid material (phagocytosis) and liquids (pinocytosis). 



Specializations of opposed surfaces. Cell contacts. In many tissues of 

 the multicellular organism the component cells are closely opposed and at 

 such surfaces of " contact " a variety of specializations has been observed. 

 Most of these seem to be associated with cell adhesion and in their totality 

 they form the devices by which a cellular tissue is held together. 

 Obviously (as mentioned above) in an external, protective, purely cellular 

 layer, such as the vertebrate epidermis, these devices become of great 

 importance; for while an extracellular cuticle may be effectively con- 

 tinuous and sufficiently strong to retain the enclosed cells, a cellular 

 tissue, however its cells may be hardened by intracellular deposits, will 

 be of little protective value unless the adhesion between the cells them- 

 selves is of an adequate strength. Certain experiments on the growth of 

 keratinizing cells in tissue culture (McLoughlin, 1959) suggest even that 

 the production of strong intercellular adhesion with the formation of a 

 stratified tissue is in itself an important factor in causing the cells to 

 keratinize. 



In electron micrographs of fixed and sectioned tissues it is seen that 

 when two cells are in contact, their dense plasma membranes do not touch 

 or fuse. They remain separated by a space of about 120-200 A, which 

 appears light by contrast with the darker membranes (Fig. 20 and Plate 

 5), but which may sometimes be stained with electron-dense materials. 

 We may suppose that this space is occupied by a cellular exudate or 



