derived spermatocytes they are more nearly spherical, their cristae are 

 irregularly oriented, and the average number per cell is greater. The 

 mitochondria in spermatids appear empty owing to the fact that the 

 cristae are folded over and flattened against the inner limiting mem- 

 brane. In mature spermatids of the opossum, the large spherical mito- 

 chondria are filled with many layers of concentrically arranged cristae 

 (Figure 3-8). During the same developmental sequence, not only does 



Figure 3-8. Electron Micrograph of Mitochondria trom Opossum Sper- 

 matid, showing the concentric disposition of the cristae. The mitochondria 

 in early germ cells of the opposum testis are typical in their structure, with 

 cristae oriented perpendicular to the limiting membrane. These cytoplasmic 

 organelles change in the course of spermatid differentiation to give rise to 

 morphological types of the kind shown above. (From Selby, C. C, 1959. 

 "Electron Microscopy: Techniques and Applications in Cytology," in "Ana- 

 lytical Cytology," R. C. Mellors (Ed.), 2nd ed., McGraw-Hill Book Co., 

 Inc., New York, N. Y., Fig. 4-lOA, p. 300. Electron micrograph by Dr. 

 D. W. Fawcett, Harvard Medical School.) 



the submicroscopic appearance of the mitochondria change, but also 

 there are marked changes in their particular location in the cell. In the 

 young spermatid, the mitochondria aggregate in large numbers adjacent 

 to the cell membrane, but in later stages of differentiation they collect 

 around the base of the flagellum where they elongate to form the mito- 

 chondrial sheath of the middle piece. 



That the mitochondria play a major role in cell respiration has been 

 known for some time. Biochemical studies over the past twenty years, 

 especially on mitochondrial-rich fractions of the cell, have provided a 



32 / CHAPTER 3 



