Toluidine blue staining reveals changes in chromatin stabilization of mouse spermatozoa during epididymal maturation and penetration of ova.

Sperm samples, recovered from various regions of the reproductive tract of male and female mice, were air dried, fixed for 2 min in acetic alcohol and stained with toluidine blue. Testicular sperm heads were deeply stained, while in the samples from the successive regions of caput epididymidis the proportion of stained heads gradually decreased and many of them were stained in their distal part only. Spermatozoa from the corpus and cauda epididymidis, vas deferens, uterus, oviduct and from the perivitelline space were colourless (except for one type of misshapen head). Sperm heads that had penetrated the vitellus became stained distally. Vas deferens sperm heads were fully stained after treatment with dithiothreitol for 30 min. We suggest that the inability to stain with toluidine blue is characteristic of sperm chromatin stabilized by disulphide bonds.