GENERAL ZOOLOGY 



Fig. 2.7. Animal cells. A, section of fixed and stained cells as seen with the light micro- 

 scope; xl080. B, living cells of a tissue culture as seen with the phase microscope; xl080. 

 (B, courtesy George O. Gey.) 



layers, the inner of which has characteristic folds extending into the matrix of 

 the mitochondrion (Fig. 2.6B). Biochemical studies of mitochondria centri- 

 fuged out of cell homogenates have shown them to contain a number of 

 enzymes necessary for oxidative metabolism in the cell (p. 36). 



Specialized cells of one sort or another contain characteristic products of 

 cellular metabolism. In gland cells there may be secretion granules of protein, 

 as in the pancreas, or of fat, as in the mammary gland. Fat drops occur as 

 storage material in many cells (Figs. 2.6/1 and 2.1 B), and yolk is typically 

 found in egg cells. Glycogen is a characteristic storage product in liver cells 

 (p. 77); pigment granules occur in various kinds of cells. 



A second structure of universal occurrence in the cytosome, identified only 

 with the aid of the extreme magnification of the electron microscope, is called 

 simply the small granular component (Fig. 2.6E). In any cells that are 

 recognized as being highly differentiated in a metabolic sense, such as gland 

 cells and nerve cells, most of the granules lie against the outside of the 

 membranes bounding the cavities of the endoplasmic reticulum and on the 

 outer nuclear membrane. In growing and dividing cells, on the other hand, 

 the characteristic position of these dense granules is in the ground substance. 

 Evidence from a number of sources indicates that these granules contain 

 ribonucleic acid (RNA) and protein (pp. 27 and 28). Ribonucleic acid is 

 responsible for the characteristic staining of certain kinds of cells, but more 

 important is its role as a major factor in cellular synthesis of protein. When 

 cells are homogenized and differentially centrifuged, one fraction contains 

 so-called microsomes. Apparently these are fragments of the membrane of 



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