The mature thread-like spermatozoon of Paragonimus ohirai was shown to be composed of 7 distinct segments which have morphologically different characteristics, following staining with silver nitrate. The first segment (1S) is a needle-shaped structure with a perforatorium. The 2S is composed of two filaments loosely paired; 3S consists of a somewhat coarse thread (vestige of the cytoplasmic projection); 4S is composed of two flagella tightly paired; in 5S, two silver-positive filaments are independently seen as in 2S; 6S has a ‘spiral’ structure composed of two flagella; in 7S, two filaments are seen dissociated from each other. The sperm nucleus exists within the ‘spiral’ structure of 6S and this structure may serve to hold and/or protect the nucleus. The technique presented here is useful for gross morphological observations of the long sperm of flatworms.

A study was conducted to evaluate the ability of mammalian oocytes to develop to the pronuclear stage and beyond if injected with freeze-dried sperm nuclei. The rate of development of hamster eggs to the pronuclear stage after microinjection of freeze-dried hamster sperm nuclei was 90%. The pronuclear formation rate of hamster eggs injected with freeze-dried human sperm nuclei was 85%. On the other hand, the rates for eggs injected with untreated sperm nuclei were 84% and 89% respectively. The survival rate of rabbit eggs microinjected with freeze-dried rabbit sperm nuclei was 64%, the fertilisation rate 56% and the cleavage rate 38%. The survival, fertilisation and cleavage rates of eggs injected with non-freeze-dried sperm nuclei were 78%, 45% and 34%, respectively. There was no difference between eggs injected with freeze-dried sperm nuclei and those injected with sperm nuclei that had not been freeze-dried. Of the viable rabbit eggs injected with freeze-dried rabbit sperm nuclei, 6% developed to the 6- to 8-cell stage 62 h after injection.

The aim of this study was to investigate the behaviour of sperm nuclei in the cytoplasm of the 2-cell mouse embryo. To this end, we produced hybrids between anucleate fertilised oocyte fragments and blastomeres of the 2-cell embryos. When sperm nuclei at the stage of decondensation or recondensation were introduced into blastomeres the development of male pronuclei was usually retarded and they never reached the size of the blastomere nuclei. These abortive male pronuclei were unable to initiate transcription but they were capable of synthesising DNA. The majority of sperm nuclei introduced into blastomeres as early male pronuclei developed normally and reached the size of the blastomere nuclei. They synthesised DNA simultaneously with blastomere nuclei and were transcriptionally active. In addition they participated in the cleavage division of hybrid cells. This shows that the very early male pronucleus when transmitted from the oocyte cytoplasm to the blastomere cytoplasm can respond positively to the new cytoplasmic factors, i.e. it undertakes both DNA replication and transcription according to the time schedule characteristic of the second cell cycle.

We have recently presented evidence that the structural integrity of the mouse sperm nuclear matrix may be necessary for the proper unpackaging of sperm DNA for participation in embryogenesis. It is likely that the sperm nuclear matrix contributes to the organisation of the sperm DNA and its disturbance can seriously damage the paternal genome or its expression. In this work, we confirm our previous data and further suggest that even very subtle changes in the sperm nuclear structure may have a significant impact on embryo development. As reported previously, dithiothreitol (DTT) in the presence of an ionic detergent, ATAB, destabilised the nuclear matrix as measured by the halo assay, and oocytes injected with these nuclei failed to develop. We also discovered that omitting the protease inhibitor PMSF from the buffers used to extract spermatozoa prevented sperm injected into oocytes from participating in development. The organisation of DNA into loop domains by the nuclear matrix in these nuclei appeared normal, as measured by the halo assay. Oocytes injected with sperm nuclei that had been washed with ATAB in the presence of phenylmethylsulphonyl fluoride (PMSF) but in the absence of DTT resulted in live births. Neither DTT treatment nor the absence of PMSF would be expected to disrupt the integrity of the paternal DNA. The data therefore suggest that even very subtle alterations in the structural proteins of the nucleus are enough to deprive sperm DNA of the ability to contribute to embryonic development.