38 GENERAL CONCEPTS 



Figure 3.4. Tissue sections of human adrenal gland stained to show cellular details; 

 left, magnified 600X; right, magnified 1500X (courtesy of Dr. Kurt Benirschke). 



One or two small, dark-staining spherical bodies, called centrioles, 

 are found in the cytoplasm near the nucleus of animal cells. The cen- 

 triole plays a role in cell division in determining the location of the 

 spindle fibers on which the chromosomes move (p. 42). It would appear, 

 however, that centrioles are not essential for cell division, for plant cells 

 are able to divide without them. 



The cytoplasm may contain droplets of fat, and crystals or granules 

 of protein or glycogen which are simply stored for future use. In addi- 

 tion, it contains the metabolically active cell organelles, mitochondria, 

 microsomes and Golgi bodies. Microsomes are too small to be seen with 

 an ordinary microscope and are invisible whether or not the cell has been 

 stained. By centrifuging cells at high speed it can be shown that mito- 

 chondria are heavier, and the Golgi bodies are lighter, than the ground 

 substance of protoplasm. The Golgi bodies are usually concentrated in 

 the part of the cytoplasm near the centrioles and appear to have a role 

 in the production of secretions. They may have the appearance of 

 granules, rods, threads or canals. The mitochondria are organized groups 

 of enzymes by means of which carbohydrates, fatty acids and amino acids 

 are metabolized to carbon dioxide and water with the release of most 

 of the energy required by the cell for survival. 



The cytoplasm of certain cells, chiefly those of lower animals, con- 

 tains vacuoles, cavities filled with fluid and separated from the rest of 

 the cytoplasm by a vacuolar membrane. Most protozoa, and the endo- 



