8 The Biochemistry of Semen 



spermatozoa determined in this manner ranged from 1-240 to 1-334; 

 there was a negative correlation between the mean specific gravity 

 and the percentage of unripe spermatozoa, that is those which 

 still possessed the kinoplasmic droplet; in each experiment, the 

 concentration of unripe spermatozoa was significantly higher in 

 the 'floating', than in the sedimenting, fraction, the specific gravity 

 of the unripe sperm cells being less than that of the ripe ones. 



To some extent, the specific gravity of spermatozoa may be 

 accounted for by the high concentration of deoxyribonucleoprotein 

 in the sperm nucleus, but in a large measure it is also due to the state 

 of 'dehydration' which is characteristic of the sperm protoplasm 

 and its protein constituents. Hand in hand with the high specific 

 gravity goes a remarkably high refractive index and light-reflection 

 power of the spermatozoa. In general, the refractive index of most 

 living animal cells lies between 1-350 and 1-367, corresponding to a 

 10-20% concentration of solids; but in human spermatozoa exam- 

 ined by the immersion method, Barer, Ross and Tkaczyk (1953) 

 obtained values corresponding to a content of almost 50% solids. 

 Nephelometric measurements of light reflection carried out with 

 bull semen samples containing a varying percentage of 'unripe' 

 spermatozoa, showed that the capacity of the sperm cell to reflect 

 light increases with ripening (Lindahl, Kihlstrom and Strom, 1952); 

 there appears to be a close relationship between the light-reflecting 

 power of sperm and the characteristic 'luminosity' of the surface 

 of spermatozoa under dark-field illumination, which, in all prob- 

 ability, is due to the 'waterlessness' of the lipid capsule surrounding 

 the ripe sperm cell. 



Sperm transport in the female reproductive tract and ' capacitation' 



There is evidence that the process of sperm ripening is not halted 

 at ejaculation but proceeds in the female reproductive tract, where 

 the sperm cell undergoes a definite change, called capacitation, 

 before it becomes capable of penetrating the egg surface (Austin, 

 1951; Chang, 1951; Austin and Braden, 1952; Thibault, 1952). It is 

 quite likely that the success which some early investigators had in 

 achieving fertilization with artificially inseminated epididymal sper- 

 matozoa, was due to the continuation of sperm ripening processes 

 in the female reproductive tract. 



