14 The Biochemistry of Semen 



run uninterruptedly through the whole length of the middle-piece 

 and tail. These fibrils probably represent the main contractile ele- 

 ment of the sperm cell, responsible for the whip-like lashing of the 

 tail. In most species investigated so far by means of the electron 

 microscope, eleven fibrils have been identified; two of these, which 

 occupy the central position, are sensitive to the action of water and 

 digestive proteolytic enzymes, whereas the remaining nine fibrils 

 which form an 'outer cylinder' around the 'central pair', are remark- 

 ably resistant to the action of plasmolysing and digestive agents, 

 and even prolonged proteolysis with pepsin or trypsin fails to disrupt 

 them; these fibrils also resist effectively attempts at solubilization by 

 means of salt solutions, acids and weak bases. 



The finer structure of the individual fibrils is still a matter of active 

 investigation. In the case of mammalian spermatozoa, doubling of 

 fibrils has been observed, at any rate in the middle-piece, and in 

 addition to the outer cylinder of nine fibrils, another, so-called 

 inner cylinder has been described, consisting of nine, much thinner 

 fibrils. The precise chemical nature of the fibrillar protein is un- 

 known. A certain resemblance to muscular contraction prompted 

 Engelhardt (1946) to ascribe to the contractile substance of sperma- 

 tozoa myosin-like properties, and to sperm adenosine-triphos- 

 phatase the role of 'spermosin'. However, this c^aim remains at 

 present unsubstantiated since it was not accompanied by satisfac- 

 tory evidence that the spermatozoa used for the experiments, were 

 really free from phosphatases, especially the powerful adenosine- 

 triphosphatase, of seminal plasma. 



In the middle-piece (or midpiece) which in the human sperma- 

 tozoon is about the length of the sperm-head though only one-tenth 

 as wide, the axial filament is surrounded by the 'broad helix', also 

 called 'spiral body' or 'mitochondrial sheath'. This lipid-rich struc- 

 ture, which is believed to be derived from mitochondria, has the 

 shape of a broad paired thread, wound helicoidally round the 

 'outer cylinder' of sperm fibrils. It is here that the cytochrome- 

 cytochrome oxidase system of spermatozoa is believed to be con- 

 centrated. The junction between the middle-piece and tail is marked 

 by the presence of a ring centriole. 



The tail or 'flagellum' in the human spermatozoon is about ten 

 times the length of the middle-piece and lacks the 'broad helix' 



