Earlier studies evaluating the condensation of chromatin during spermiogenesis revealed that deoxyribonucleoprotein (DNP) becomes more resistant to thermal denaturation as the round spermatid elongates and assumes the shape of a mature spermatozoon (Ringertz, Gledhill & Darżynkiewicz, 1970). Unfortunately, only heterogeneously aged populations of cells, represented by spermatids in loosely defined categories and at differing intervals before release from the germinal epithelium, could be studied. Homogeneous preparations of spermatozoa leaving the testis were unavailable. Because the `testicular spermatozoa' group evaluated by Ringertz et al. (1970) was a mixture of spermatids and spermatozoa, it was not possible to determine if final stabilization of germ-cell DNP occurs during the latter phases of spermiogenesis or in the caput epididymidis. From the data for two bulls, Gledhill (1972) suggested that the thermal denaturation profile for spermatozoa removed from the caput epididymidis was different from that for cells obtained from the ductus deferens. He suggested, therefore,
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