DIFFERENTIATION AND PROTEIN SYNTHESIS 85 



Abercrombie (1947 and 1948) and Moscona (loc. cit.) among many others 

 (De Haan, 1958). That some deterioration in cell adhesion is associated 

 with cancer has been advocated particularly by Coman (1954). 



Certain essential notions concerning the activity of cell surfaces and 

 their behaviour when they are brought into contact are best derived from 

 time lapse films of living cells, such as those made by Weiss, and by 

 Abercrombie and Ambrose. From such observations one gains the 

 impression that many isolated cells behave essentially like unicellular 

 protozoa of the amoeboid type. Their surfaces are constantly thrusting 



Fie. 37. The increase in intimacy of cell contact in passing from an 

 embryonic to an adult condition. A similar change occurs in the differen- 

 tiation of epidermal tissues. 



forth protrusions which bring about the movement of the entire cell. 

 When like cells are brought into contact the motion of the surface ceases 

 at the point of contact, a phenomenon called contact inhibition by 

 Abercrombie (Abercrombie and Heaysman, 1953 and 1954), and the area 

 of contact may spread zipper-like further immobilizing the cells. On the 

 other hand, unlike cells on meeting do not inhibit each other's movements. 

 Thus, as is shown most clearly in the experiments by Moscona and Weiss, 

 in a mixture of like and unlike cells, the like cells, as a result of their 

 specific adhesion following random contact, will sort each other out 

 (Fig. 37). Under other circumstances, a zipper-like spread of contacts 

 may actually mould the cell formation into columnal epithelium (Fig. 44) 

 or an intricate interdigitated condition as seen in the notocord (Waddington, 

 1956). These specific intercellular adhesions seem to be effected by a 

 sticky exudate covering the cell surfaces which we have referred to as an 

 intercellular cement. 



Little is known of the chemical nature of these cements and, further, 

 little concerning the structural devices involved can be learned from light 

 microscopy since the intercellular spacing is of the order of 200 A, far 

 below the resolving power of the light microscope. However, electron 

 micrographs of thin sections have established the existence of the thin 



