DIFFERENTIATION AND PROTEIN SYNTHESIS 131 



crystals are randomly oriented (Jackson, 1954). As they grow an orientation 

 develops apparently directed by the oriented fibrillar matrix in which the 

 crystal forms. Only the naturally-occurring type of fibril with the 640 A 

 period seems able to initiate crystal deposition (Bachra et al., 1959). 



Epidermal fibrils 



Unfortunately it has not yet proved possible to obtain a soluble 

 precursor of keratin or keratohyalin which will produce fibrils spon- 

 taneously in vitro. Our information concerning fibrogenesis is thus limitep 

 to what can be obtained from the microscopy of the tissues themselves 

 combined with any applications of general principles we can infer from a 

 study of other fibre-forming systems such as those just described. From 

 what has been said it is evident that the formation of fibrous keratin has no 

 exact parallel in other systems. A complicating factor is that essentially 

 the same final system (compare Plates 16 and 17) appears to be arrived at 

 by two different courses : (a) in the epidermis partly through the formation 

 of a non-fibrous intermediate form, keratohyalin and (b) in the hard 

 keratins without the appearance of this intermediate form. The isolation 

 ol keratohyalin, its analysis and its behaviour in vitro would greatly help to 

 clear up this obscurity. Keratohyalin and trichohyalin after accumulating 

 as droplets of isotropic precursor are converted into the fibrous form 

 (Plate 21) in a manner which has about it something akin to crystalli- 

 zation. The orientation of the fibrous form of trichohyalin in the cells of 

 the inner root sheath of the hair follicle is strictly parallel to the axis of the 

 follicle. Here we may suspect that the slight shear affecting the cells of the 

 bulb as they approach the follicular constriction, which orientates the 

 elongated mitochondria and nuclei, also orientates the initial small 

 formations of fibrous trichohyalin and thus directs the subsequent massive 

 transformation. In the epidermis the transformed, fibrous keratohyalin of 

 the stratum lucidum runs approximately parallel to the stratum corneum, 

 at right angles to the prevailing fibrillar orientation in the germinal layer. 

 The cells at this level are already somewhat flattened and probably here 

 too the shear produced during the change in cell shape controls the 

 direction of orientation (Plate 22). 



Many fine filaments are seen attached to desmosomes in the cells of the 

 lower layers, where they seem to provide suitable sites for initiating 

 fibrogenesis, and this attachment may help, by holding on to one end of a 

 tuft of filaments, to orient it when the cell is deformed (Charles and 

 Smiddy, 1957). 



Rather less can be asserted about the origin of orientation in hair, 

 feather, horn and nails. No precursor accumulates and filaments, when 

 they appear, are already oriented. The particulate contents of the cells, 

 long nuclei and mitochondria, seem to be oriented by the flow due to cell 



