THE GROWTH OF EPIDERMAL STRUCTURES 147 



(Arthur and Shelley, 1959) and its cells grow outwards. Its cells have 

 evidently only a limited possibility of communicating with each other 

 and the rest of the system. The existence of " fleece mosaics " in sheep 

 has recently assumed some importance as a proof of local autonomy. A 

 fleece is said to be mosaic when it comprises two distinct types of wool 

 grown on different areas of the same skin. The areas may be adjacent and 

 it is clear that the follicles concerned, although producing different types of 

 fibre, enjoy the same environment externally and internally, thus demon- 

 strating beyond doubt the over-riding control of local, non-systemic 

 factors on the kind and quantity of fibre formed. It is assumed that 

 mosaics arise from a somatic mutation, i.e. from a mutation occurring in a 

 cell subsequent to the first division of the egg which initiated development. 

 The change will be apparent only in the line of cells issuing from the 

 mutated cell. Thus an area of skin, producing aberrant type of wool, is 

 assumed to be a colony of cells arising from a single cell in which a somatic 

 mutation has occurred. The actual local histological factors have not yet 

 been fully explored, but the existence of the phenomenon proves the 

 genetic control of localized epidermal structures which in turn determine 

 the nature of the product quite independently of systemic factors. 



There is in the epidermis a vertical integration but clearly only a 

 rather limited lateral one. By postulating the same intracellular 

 features, generative mass, differentiated mass (keratin, mucin, etc.) and 

 inhibitor production, the possibilities of control in a simple stratified 

 epithelium may be considered. Division is largely confined to the basal 

 layer and synthesis of specialized products takes place in more distal 

 layers. The inhibitor molecules, produced in the stream of outwardly- 

 moving cells during these later reactions, are largely lost when these cells 

 are shed, and can only feed-back to the germinal layer by back diffusion, 

 and only by crossing the dermoepidermal junction can they reach the 

 general circulation and thus be carried to distant parts of the system. 



Certain possibilities may be made clear by considering a cell which has 

 just been produced by division. Since division has occurred we may 

 suppose that inhibition is minimal. The cell leaves the germinal layer and, 

 at a higher level, begins to differentiate, to synthesize both the differen- 

 tiated product and also the inhibitor. The following conditions may arise : 



(a) Before sufficient inhibitor is produced or diffuses back, the cell in the 

 germinal layer again divides. This would be a condition permitting of 

 uncontrolled, continuous growth. 



(b) Before division can occur again, sufficient inhibitor diffuses back to 

 prevent it. After a further time the concentration falls again (with the 

 decrease in synthetic activity in the differentiated layer and the decay of 

 inhibitor molecules) and division again occurs. We have here a condition 

 of periodic division under the control of the events in the differentiated 



