108 KERATIN AND KERATINIZATION 



indicative of protein rather than mucin. The free surfaces of the cells are 

 covered with stubby villi between which appear pockets containing 

 granules in the act of escaping. On leaving the cell (or even before it) the 

 granules commence a transformation. They lose their homogeneous 

 appearance, break into a cluster of coarse granules, then into finer granules 

 which appear to open into masses of short fine filaments (Plate 18 A) whose 

 coalescence produces thick, tapering rodlets and then the horny layer. 



This sequence of changes seems at first sight unlike that found in the 

 formation of other keratins. However, there is a likeness to keratohyalin. 

 In each case an amorphous precursor is produced which accumulates as 

 droplets or granules. In the epidermis the keratohyalin granules transform 

 into fibrils within the cell; in the gizzard on the other hand, the secretion 

 escapes from the cell before the transformation into the fibrous form, 

 which then occurs extracellularly. The problem clearly requires closer 

 study. Since the horny lining is incompletely soluble in keratinolytic 

 solvents and is coloured, the possibility of tanning must be considered. 

 Mucins are produced by the same (or neighbouring) cells but the relation 

 between the two secretions is obscure. 



The organic matrix of tooth enamel may be a keratin (p. 78) and recent 

 micrographs of Watson (1960) show that it is extracellular and that the 

 cytoplasm of the ameloblasts has the basophilio (particle covered) 

 membranes characteristic of secretory cells (see p. 113). 



The synthesis of protein in epidermal systems 



Cytology of Cells which form Protein 



It is desirable to consider at first in greater detail the cytology and bio- 

 chemistry of proteogenic cells. Protein synthesis, in the sense of the 

 initial synthesis of a high molecular weight polypeptide having a specific 

 structure, occurs only within cells; certain other processes which, as we 

 shall see later, are important in the assembly and organization of fibrous 

 structures may, however, occur extracellularly. Epidermal cells as a class 

 are notably active in protein synthesis and, on account of their often simple 

 geometrical arrangement in such organs as skin or hair, they lend them- 

 selves to microscopic study. On the other hand, the number of cells 

 available is not usually large and this does not favour biochemical tech- 

 niques. Thus the wealth of morphological detail is offset by a lack of 

 biochemical information based directly on a study of epidermal cells 

 themselves and we are forced to accept as a working basis the general 

 conclusions of the course of synthesis worked out on more convenient 

 systems, such as the mammalian liver or micro-organisms. It would be 

 impossible to do more than present a sketch of this work which is develop- 

 ing rapidly at the present time. 



