CEttS AND TISSUES 45 



regularly in this fashion will grow indefinitely— tissue from a chick heart 

 was grown for over twenty years at the Rockefeller Institute in New 

 York. Such experiments revealed that cells in tissue culture do not grow 

 old, for at the end of the twenty-year period the cells were as vigorous 

 and grew as fast as the original cells. Cells isolated from a sarcoma (a 

 type of cancer) grow with unusual vigor in tissue culture and grow more 

 rapidly in plasma from a healthy person than in plasma from a person 

 with a sarcoma. This observation suggests that the presence of sarcoma 

 cells in the body stimulates certain healthy cells elsewhere to produce 

 some substance which inhibits to some extent the malignant growth. 



Cell morphology may be studied by using a bit of tissue that has 

 been killed quickly with a special "fixative," then sliced with a machine 

 called a microtome, and stained with special dyes. The stained slices, 

 mounted on a glass slide and covered with a glass cover slip, are then 

 ready for examination under the microscope. Since the nucleus, mito- 

 chondria and other specialized parts of the cell are chemically different, 

 they will combine with different dyes and be stained characteristic 

 colors (Fig. 3.4). For observation in the electron microscope a bit of 

 tissue is fixed with osmic acid, mounted in acrylic plastic for cutting 

 in extremely thin sections, and then placed on a fine grid to be inserted 

 into the path of the electron beam. Both light microscopy and electron 

 microscopy have revealed many details about cell structure. 



Some clue as to the location and functioning of enzymes within cells 

 can be obtained by histochemical studies, in which a cell is fixed by 

 methods which do not destroy en/yme activity. Then the proper chemi- 

 cal substrate for the enzyme is provided and, after a specified period of 

 incubation, some substance is added which will form a colored com- 

 pound with one of the products of the reaction mediated by the enzyme. 

 The regions of the cell which have the greatest enzyme activity will 

 have the largest amount of the colored substance (Fig. 3.8). Methods have 

 been worked out which permit the demonstration and localization of a 

 wide variety of enzymes. Such studies have given an interesting insight 

 into the details of cell function. 



Another method of investigating cell function is to measure, by 

 special microchemical analyses, the amounts of chemical used up or 

 produced as a bit of tissue is incubated in a special enclosed glass vessel. 

 In such experiments much has been learned of the roles in cell metab- 

 olism of vitamins, hormones and other chemicals by adding these sub- 

 stances one by one and observing the resulting effects. 



Every living cell, whether it is an individual unicellular animal, or 

 a single component of a multicellular one, must be supplied constantly 

 with nutrients and oxygen. These materials are constantly being metab- 

 olized-used up— as the cell goes about its business of releasing energy 

 from the nutrients to provide for its myriad activities. Some of the sub- 

 stances required by the cell are brought to it and taken in by complex 

 active processes which require the expenditure of energy by the cell, and 

 about which little is known. Other substances are brought to the cell by 

 the simpler, more easily understood physical process of diffusion. To 

 understand this process, so important in many biologic phenomena, we 



