DIFFERENTIATION AND PROTEIN SYNTHESIS 101 



The synthesis of fibrous keratin commences in the cells of the cortex in 

 the middle and upper bulb. Small wispy bundles of filaments are already 

 visible electron-microscopically in the cells of the middle bulb (Plate 12B). 

 There is no obvious accumulation of non-fibrous precursor as occurs in the 

 sheath cells; however, if follicles are fixed for 12 hr in buffered formal- 

 dehyde and stained with phosphotungstic acid, a procedure designed to 

 retain more completely the contents of the cells, the cytoplasm is seen to be 

 packed with a structureless protein which may suggest the existence of a 

 precursor in a soluble form. 



Synthesis is delayed in the hair cuticle until above the bulb, when 

 droplets of an amorphous keratin separate in peculiar patterns packed 

 against the outer wall of the tilted cuticle cells (Fig. 43) (Plate 20). 



The characteristic products of the hair follicle, fibrous keratin, cuticular 

 keratin and fibrous trichohyalin, are thus formed in distinctly different 

 ways which will be more fully discussed in a subsequent section (p. 223). 

 Returning for a moment to consider the cells of the epidermis, it would 

 seem that there the two methods of forming fibrils are actually to be found in 

 the same cell, although at different times, for filaments are built up directly 

 in the basal and prickle-cell layers and from keratohyalin in the stratum 

 granulosum. The conversion of the granular layer into the clear bi- 

 refringent stratum lucidum involves a transformation of the isotropic 

 keratohyalin granules into a fibrous form apparently analogous to that 

 found in the inner root sheath cells. The fibrils of both origins seem to 

 fuse into a common formation in the stratum corneum. This problem is 

 further discussed in Chapter 6. 



The Feather Follicle 



A brief account of the feather follicle has already been given (p. 69). 

 The feather grows out from a germinal matrix (Fig. 48) at the bottom of 

 the follicular shaft consisting of an ectodermal wall and a relatively large 

 mesodermal core called the feather pulp (Figs. 45, 46 and 47). The feather 

 cylinder itself comprises three layers: an external one forming a protective 

 sheath to the developing feather, a thicker middle layer from which the 

 feather itself is derived and an inner layer next to the pulp. The external 

 and internal layers may be likened to the root sheaths of the growing hair. 

 All three layers are produced by the proliferation of the generalized cells 

 of the matrix, called the collar, at the base of the follicle (Fig. 48). 



In its growing phase the feather is a hollow, pointed cylinder set like a 

 cap on the core of mesoderm; it simply elongates owing to the addition of 

 cells by division at its lower end, the " collar ". Some details of this process 

 including barb formation are illustrated in the simplified drawing Fig. 45. 

 Here at (a) a portion of the growing cylinder is seen from its ventral side. 

 R is the rhachis found on the dorsal side, the barbs B, greatly reduced in 



