SPERM TAIL STRUCTURE AND MOVEMENT MECHANISM 159 



cristae (Figs. 10 and 11), or they may be extensively modified. The 

 most dramatic example of the latter is seen in the opossum where 

 there is a remarkable proliferation of mitochondrial membranes late 

 in spermiogenesis that results in the formation of closely packed con- 

 centric lamellae that completely fill the interior of the organelle. As 

 yet nothing is known about the changes in biochemical activities 

 which may accompany this extreme modification of the internal 

 structure of the mitochondria. 



The mitochondrial sheath presumably participates in sperm loco- 

 motion mainly as a source of energy for the contraction of the longi- 

 tudinal fibrils of the tail. If such is the case, one would expect that 

 the energy requirements of two different kinds of sperm of roughly 

 similar proportions would be much the same. The fact that sea 

 urchin sperm or fish sperm have only a few mitochondria while 

 mammalian sperm may have over a hundred has therefore been 

 puzzling. Comparative electron microscopic studies to date suggest 

 that those spermatozoa which have simple tails consisting only of the 

 nine-plus-two complex of axial fibrils have a short midpiece made 

 up of two to four mitochondria, whereas those spermatozoa which 

 have an additional row of nine coarse fibers outside the regular axial 

 filament complex have a long midpiece with a well-developed mito- 

 chondrial sheath. If the mitochondrial number is correlated with 

 the energy needs of the cell, it would appear that the addition of an 

 outer row of coarse fibers in the mammalian sperm has greatly in- 

 creased the energy requirements for locomotion compared to that 

 of spermatozoa propelled by simple flagella. This observation tends 

 also to support the belief that the outer coarse fibers of mammalian 



Fig. 9. Low-power electron micrograph of bat epididymis. At the 

 lower left is the free surface of the lining epithelium with its long stereo- 

 cilia. In the lumen are a number of spermatozoa sectioned in various 

 planes. Bat sperm have a long thick middle piece. An area like that en- 

 closed by the rectangle is shown at higher magnification in Fig. 10. (Elec- 

 tron micrograph by Arthur Mitchell.) 



Fig. 10. The base of the sperm head is shown at the left and the con- 

 necting piece which is continuous with the dense outer longitudinal fibers 

 of the middle piece. The mitochondria are flattened, very uniform in size, 

 and closely applied to the outer dense fibers. The cristae in the mito- 

 chondria tend to run parallel to the limiting membrane. 



