112 KERATJN AND KERATINIZATION 



The internal structure of the microsome particles is not known. 

 They contain both proteins and RNA but their X-ray diffraction patterns 

 show little resemblance to the patterns given either by RNA or by mixtures 

 of protein and RNA, which can only mean that the RNA is bound into the 

 particles in a form different from what it assumes when free. This may be 

 contrasted with the fact that the structure of isolated DNA is similar to its 

 structure in vivo where it also exists in a DNA-protein complex. The 

 microsomal particles resemble viruses in size, composition and in some 

 respects function (it has been suggested that viruses may be microsomal 

 particles gone wrong!), and the protein moiety dominates the structural 

 picture in some plant viruses. 



After fixation by freeze-drying, which may be considered to introduce a 

 minimum of chemical change and a maximum of retention of material, the 

 particles are less visible among the protein adjacent to the membranes 

 (Hanzon et ai, 1959). This suggests that in their dense, compact form, as 

 seen after osmium fixation, the particles are an artifact. Their original 

 form may be a more diffuse and extended particle, a condition which might 

 render their function as linear templates more understandable. 



Some details of a possible means by which the basophilic material, now 

 identified as the granular, RNA-containing material seen in micrographs, 

 might reach the cytoplasm were revealed by Watson (1954) who drew 

 attention to the existence of small circular markings (diameter 500 A) on 

 the double-layered nuclear envelope. He suggested these markings were 

 openings or " pores " in the double membrane which might permit 

 nucleocytoplasmic interchanges. The particles may pass through the 

 pores and, after associating with the external sheath of the nuclear mem- 

 brane, enter the cytoplasm together with it to form a typical particle- 

 studded membrane (Fig. 49). 



The above description applies to glandular cells, i.e. to cells which 

 produce protein for secretion. The account given above of the cells of the 

 epidermis shows that protein synthesis may be associated with a different 

 type of cytology from that of secretory cells, as was first clearly pointed out 

 by Birbeck and Mercer (1957). The cells of the hair cortex, for example, 

 contain vast numbers of dense particles of the same size and appearance as 

 those noted in the pancreas and elsewhere ; but they are not associated with 

 a membrane system (Fig. 36) (Plate 11). They appear to be scattered in 

 small, often well-defined clusters, throughout the cytoplasmic space. The 

 cytoplasm is uniformly basophilic in these cells due to RNA, and there is 

 no reason to doubt that here, too, the particles contain RNA and participate 

 in the synthesis (see also p. 120). The cytoplasmic distribution of the two 

 kinds of nucleic acid in the hair follicle is admirably demonstrated in 

 Hardy's work illustrated in Fig. 90, p. 220. In the lower bulb, the germinal 

 region, the DNA of the nuclei is obvious; at higher levels where 



