THE KERATINIZATION PROCESS 263 



intercellular space widens (Fig. 98) and bands are deposited (Odland, 

 1958; Mercer, 1958; Horstmann and Knoop, 1958; Pillai, I960). When 

 a cell is finally shed the parting may occur either between the cells, i.e. 

 due to failure of the cement, or in the bulk of the keratin due to its 

 separating into fibrils (Plate 6A). 



The purely mechanical consequences of the changes in the geometry of 

 the surfaces of contact of the cells deserve special emphasis. Not only is 

 the area of contact and therefore the total adhesion vastly increased by the 

 formation of wavy surfaces, but the interpenetrating crests may develop to 

 the point where they constitute veritable " press-studs", which after the 

 hardening of their contents, literally lock the cells together. The enhance- 

 ment of these surface irregularities in the hair may be a consequence 

 simply of the continued addition of filaments to centres near the surfaces 

 of the cells. In the epidermis the numerous desmosomes and their 

 associated fibrils appear to cause the buckling of the membranes (Plate 6A). 



The Fate of the Intracellular Apparatus during 

 Keratinization 



Much of the intracellular apparatus either consists of membranes or is 

 enclosed in membranes. We have mentioned above that there are reasons 

 for supposing that all these membranes have a similar basic constitution 

 and, it is of interest to note that, during keratinization of the cell, a portion 

 of the cellular apparatus undergoes changes in solubility similar to those 

 affecting the external cell membranes and is found along with these among 

 the resistant residues when the keratin is extracted (Figs. 110 and 112). 



The exact fate of the cellular apparatus — nuclei, mitochondria, RNA, 

 etc., in keratinizing cells is obscure (see p. 220). In a structural sense the 

 nuclei can be followed through the keratinization zone and remnants can 

 be demonstrated in the emerging hair. They elongate probably passively 

 with the cell in the upper bulb, and are here Feulgen positive (Fig. 97b); 

 but this reaction fades during keratinization although a "structural residue " 

 is still visible electron microscopically. Spier and Van Caneghem (1957) 

 report increased DNA-ase activity in this zone, and presumably the DNA is 

 depolymerized. Bolliger and Gross (1952 and 1956) found many possible 

 low molecular weight breakdown products (pentoses, uric acid and other 

 purines) of nucleic acid in hair. When keratin is chemically extracted from 

 hair, a portion of the insoluble residue consists of long, thin, chemically- 

 modified remnants of nuclei (Mercer, 1953). 



In the light microscope the nuclei are seen to shrink and grow more 

 dense and are said then to be " pycnotic." Electron micrographs show 

 first a thick gathering of dense material beneath the membrane and some 

 signs that material may be being shed into the cytoplasm. 



Pycnosis is a degenerative condition recognized by an increase in both 



