Distribution of DNA, nuclear micro-heterogeneities and compaction of the chromatin in rabbit epididymal spermatozoa. Ultrastructural evaluation of the Feulgen-like technique using osmium ammine.

An adaptation of the Feulgen procedure to visualise DNA at the ultrastructural level, using osmium ammine instead of Schiff reagent, was applied to ultrathin sections of rabbit epididymal spermatozoa, known to display increasing chromatin compaction as they progress through the epididymis. In contrast with the somatic cell nuclei of the epididymal epithelium, which display classical staining, the chromatin of spermatozoa is partly or fully destroyed during the hydrolysis step of the technique. The sperm chromatin resistance towards destruction is a function of the initial sperm nuclear compaction and of the duration of hydrolysis prior to Feulgen-like staining. For a given nucleus, the maximum staining intensity is only obtained after an optimal duration of hydrolysis. However, because of this duration and local differences in chromatin compaction, the total DNA of a section is never completely visualized. The most compact parts of the nuclei are not yet stained, when the less compact parts have already been destroyed. This gives rise to a sperm-specific pattern which corresponds to the enhancement of microheterogeneities present in all sperm nuclei and to the local depolymerisation of the nuclear material during hydrolysis. The depolymerised parts of nuclei sit over the sections, and they also bind uranyl acetate, ethidium bromide and anti-protamine antibodies. Therefore, in sperm nuclei, the Feulgen-like staining at the ultrastructural level does not reveal the true DNA distribution. However, the amount of staining can be quantified to evaluate sperm chromatin compaction.