564 GROWTH IN TISSUE CULTURE 6 



Stability or change, though it is to be hoped that in the future these may come 

 vmder experimental control. 



Bloom (1937) and Willmer (1954) conclude that, although cellular multipli- 

 cation and cellular differentiation are distinct processes, each usually takes place 

 only in the absence of the other. Histodifferentiation, or the process by which 

 cells become organized into tissues, as distinct from cellular differentiation, may 

 also take place in growing cultures. Fell (1951) has reviewed this topic, to which 

 she has been a major contributor for more than 20 years. From the pioneer work of 

 Champy (1914), Rienhoff (1922), Drew (1923a, b) and others on the interre- 

 lations of epithelium and mesenchyme has developed the more precise studies of 

 recent years, notably of Fell and Mellanby (1952, 1953) and of Grobstein (1953a, 

 b, c; 1955). Great possibilities are beginning to be seen and exploited in manipu- 

 lating the environment to control or alter the direction of histogenetic activity 

 in vitro. Spratt (1948), for example, made a study of the effects of some specific 

 nutrients on the development of early chick embryo in vitro, and demonstrated the 

 overriding importance of an energy source (carbohydrate) for morphogenesis and 

 growth. Differences between cell types in quantitative and qualitative substrate 

 utilization and especially in ability to incorporate the carbon skeleton of glucose 

 into various amino acids, have been studied and discussed by Markert (1955). 

 Limb bone rudiments from the chick, explanted by the classical technique of 

 Fell and Robison (1929), grow and differentiate in a manner closely similar to 

 normal development in vivo. When exposed to an excess of vitamin A, they undergo 

 degenerative changes quite similar to those seen in hypervitaminotic animals, 

 suggesting that the effect of vitamin A in vivo may be directly on the bones. 

 Perhaps even more striking is the effect of vitamin A on tissue (containing both 

 dermis and epidermis) from the body wall of the 6-7 day chick embryo. In a 

 normal medium, feather germs and melanocytes develop and keratinization of the 

 epidermal epithelium occurs (Fell and Mellanby, 1953). With excess of vitamin 

 A, the epithelium assumes the character of a secretory membrane. Ciliated cells 

 also develop, and the skin tissue in this medium takes on a remarkable histological 

 similarity to the normal mucus-secreting ciliated membrane of the nasal cavity. 

 On withdrawing the excess vitamin A, the explants (after a brief acceleration of 

 differentiation of the secretory layer) resume their differentiation into squamous 

 epithelium. It was shown by Weiss and James (1955) that only brief and inter- 

 mittent exposures to vitamin A were necessary to swing the differentiation in the 

 direction of mucus-secreting epithelium. Vitamin A appears to exert upon cultures 

 of chick fibroblasts a purely proliferation-stimulating effect, as judged by a signifi- 

 cant increase in mitotic rate (Lasnitzki, 1955). 



The variables growth, survival and differentiation have been studied from the 

 viewpoint of their responses to nutritional factors by Wolff and his school in 

 cultures of syrinx and gonads. Certain individual amino acids favour all three 

 variables in cultures of chick embryo syrinx, whereas other amino acids, which 

 stimulate growth, diminish the proportion of differentiation and survivals (Em. 

 Wolff, 1955). Similarly, Stenger-Haffen (1955) found that certain amino acids 

 specifically encourage differentiation of chick embryo gonads. Wilde (1955) has 

 studied the functions of phenylalanine in the chemical processes involved in the 



