chondriales, which generally extend into the interior or matrix of the 

 organelle perpendicular to its long axis (Figure 3-3). Whether the cristae 

 are invaginations of the inner membrane or separate structures is still 

 debatable. The matrix into which the cristae extend is a homogeneous, 

 amorphous material. Electron-dense granules, the function of which is 

 by no means clear, are sometimes encountered apparently embedded in 

 the matrix. A number of three-dimensional models of the mitochondrion 

 have been proposed. One such model is shown in Figure 3-4. 



Mitochondria 



outer 



limiting 



membranes 



t ransverse 



membranes 



(cristae) 



inner 



limiting 



membranes 



Figure 3-4. Diagrammatic Drawing to Illustrate the Three-dimensional 

 Structure of the Mitochondrion. (From Ham, A. W., 1957. "Histology," 

 3rd ed., J. B. Lippincott Co.. Philadelphia, Pa., Fig. 58, p. 79.) 



While the structural organization described above is the most typical, 

 many variations have been noted. The significance of these in terms of 

 function is not known, but it seems highly probable that they ultimately 

 will be shown to be associated with alterations in specific function. For 

 example, in certain kinds of cells, both plant and animal, the cristae 

 extend parallel to the long axis rather than perpendicular to it (Figure 

 3-5). In still other cases, as in the interstitial cells of the opossum testis, 

 the cristae are irregular in both spacing and orientation (Figure 3-6). 

 In secretory cells of the adrenal cortex, the cristae are frequently either 

 filamentous or saccular in form. Cristae of similar morphology have also 



STRUCTURE AND FUNCTION OF CYTOPLASMIC ORGANELLES / 29 



