C. R. AUSTIN AND M. W. H. BISHOP 81 



cent by retaining spermatozoa in the epididymis for 20 to 25 

 days. The observations of SaHsbury, Bratton, and Foote (1952) 

 suggest that the incidence of embryonic mortahty in cattle in- 

 creases with the age of the spermatozoa. 



It is now well known that mammalian spermatozoa in vitro, 

 unlike sea urchin spermatozoa, metabolize nutrients in the sur- 

 rounding medium to maintain their physiological functions. Un- 

 der anaerobic conditions they maintain their motility by the gly- 

 colytic breakdown of fructose (present in the seminal plasma of 

 most species), or glucose if available (for example, in the secre- 

 tions of the female tract ) ; in the presence of molecular oxygen a 

 number of extracellular substrates can be metabolized. Even with- 

 out exogenous oxidizable material mammalian spermatozoa can 

 respire actively and maintain their motility for an appreciable 

 length of time. Lardy and Phillips (1941, 1945) have attributed 

 this to oxidation of intracellular phospholipids, and Rothschild 

 and Cleland (1952) have found evidence that sea urchin sper- 

 matozoa obtain energy for motility by a similar mechanism. 

 Mammalian spermatozoa are known to contain the enzymes in- 

 volved in the Embden-Meyerhof glycolytic system and the Krebs 

 cycle as well as a complete cytochrome system (see Mann, 1954; 

 Tyler, 1955); these systems are evidently located within the 

 spermatozoon tail since headless spermatozoa can swim ac- 

 tively (Redenz, 1924, 1925, 1926; Moench and Holt, 1929; Han- 

 cock and Rollinson, 1949; Bishop, 1955). 



Within the female genital tract spermatozoa probably obtain 

 energy for motility through both the glycolytic and respiratory 

 systems, but while the importance of the former is widely appre- 

 ciated that of the latter has received little recognition. Campbell 

 ( 1932 ) has shown that the oxygen tension of the rabbit uterus is 

 comparable to that in many other tissues of the body (20-45 mm. 

 Hg), and in view of the known affinity of the cytochrome system 

 for oxygen this should be ample for aerobic metabolism. Oxida- 

 tive metabolism is ten to fourteen times more productive of en- 

 ergy than glycolytic metabolism (Fruton and Simmonds, 1953), 

 and there is evidence that the motility of mammalian spermatozoa 

 is more closely related to oxygen uptake than to fructolysis 



