THE CYTOPLASM 201 



The mitochondria are the most prominent cytoplasmic structures of 

 the liver cell that can be seen in the light microscope. Simultaneous counts 

 of free nuclei and mitochondria in liver homogenates have indicated that 

 the number of mitochondria per cell is of the order of 400.^^ These ubiquitous 

 cell orgenelles are characterized generally by a filamentous, rod-like, or 

 granular form and by certain well-defined staining properties.^* -^^ Cyto- 

 logical observations have, in fact, led to the belief that the mitochondria 

 of various cells and even of the cells of various species are structurally and 

 functionally similar — a view that has received strong support from recent 

 electron microscopic studies of the internal structure of mitochondria.^^ -^^ 

 Thus, by the use of extremely thin tissue sections, Palade^'' has shown 

 that the mitochondria of a wide variety of cells uniformly possess a surface 

 membrane 7 to 8 m^ in thickness, a system of parallel regularly spaced 

 ridges protruding from the inside surface of the membrane towards the 

 interior, and an internal matrix containing occasional minute granules, 

 but othermse structureless. Similar findings have been made independently 

 by Sjostrand and Rhodin.** Other structures of microscopically visible 

 dimensions within the liver cell cytoplasm are small spherical granules, 

 0.5 to 1 /i in diameter, located near the periphery of the cell, and larger 

 lipid droplets, 2 to 3 /x in diameter, located in the interior. Both of these 

 components are demonstrable in fresh preparations by their affinity for 

 neutral red;^^ the former are considered to be secretory granules, presum- 

 ably containing bile. The unsettled status of the Golgi apparatus in liver 

 will be discussed later. In the cells of tissues other than liver, a number of 

 additional, specialized cytoplasmic structures can, of course, be seen. 

 These include pigment granules, other types of secretory granules, inclusion 

 bodies, and the various structures related to the myofibrils of muscle cells. 



One of the main contributions of electron microscopy to cytology has 

 been the disclosure of the structural details of the optically empty ground 

 substance or hyaloplasm of the cell. Thus, in all cells examined, a rather 

 large proportion of the cytoplasmic volume has been found to contain a 

 complicated reticular network that is too small to be seen in the light 



i^ G. H. Hogeboom, W. C. Schneider, and M. J. Striebich, J. Biol. Chem. 196, 111 

 (1952); cf. C. Allard, R. Mathieu, G. de Lamirande, and A. Cantero, Cancer Re- 

 search 12, 407 (1952). 



1^ E. V. Cowdry, in "General Cytology" (Cowdry, ed.), p. 113. Univ. of Chicago 

 Press, Chicago, 1924. 



1^ G. H. Bourne, in "Cytology and Cell Physiology" (Bourne, ed.), 2nd ed., p. 

 313. Clarendon Press, Oxford, 1951. 



"G. E. Palade, Anat. Record 114, 427 (1952); J. Histochem. and Cytochem. 1, 188 

 (1953). 



18 F. S. Sjostrand and J. Rhodin, Nature 171, 30 (1953). 



'9 G. H. Hogeboom, W. C. Schneider, and G. E. Palade, J. Biol. Chem. 172, 619 

 (1948). 



