Reactivation of the Feulgen reaction of ram spermatozoa by dithiothreitol

Role of mammalian sperm nuclear structure in fertilization and embryo development

Although intact spermatozoon is successfully collected from infertile patients, repeated implantation failure or pregnancy loss are often experienced. Sperm nuclear defects have been thought to be one of the most important reasons for repeated assisted reproductive technology failure. In comparison with other mammalians, characteristic heterogeneity has been found in each mature human sperm nuclei, therefore it is necessary to investigate the significance between fertilization failure, developmental disability and structural abnormality of human sperm nuclei. Furthermore, if close relationships between the heterogeneity of human ejaculated sperm nuclei and DNA fragmentation are defined by analyzing sperm nucleoproteins, it would be clearly shown that impaired sperm chromatin leads to failure of embryo development in vitro or in vivo, so called late paternal effect on embryo development. It will be necessary in the near future to study the strategy for more novel methodology than those previously reported in terms of sperm selection. The present report reviews the roles of mammalian sperm nuclear structure, especially in humans, in fertilization and embryo development after the insemination procedure. (Reprod Med Biol 2006; 5: 161-168).

Suppression of endogenous estrogen during development affects porcine epididymal sperm maturation

Estrogen plays an important role in male reproduction, critical for sustained fertility in some species. Reducing estrogen's interaction with its receptor(s) in monkey and mouse models is associated with reduced sperm motility and, in some cases, documented elimination of sperm fertilizing ability, suggesting that normal epididymal function may be estrogen dependent. The objective of these experiments was to evaluate the effects of reduced endogenous estrogen on development of epididymal function in the pig, a species in which males have very high levels of endogenous estrogen. Letrozole, a potent inhibitor of estrogen synthesis, was administered to neonatal boars from 1 week of age and markedly suppressed estrogen production. Epididymal function assessed as acquisition of sperm fertilizing ability (in vitro fertilization of zona-free oocytes) was reduced in Letrozole-treated animals at 24 and 28 weeks of age (23% and 30% fertilization, respectively compared with 37% and 54% in vehicle controls) but had recovered by 32 weeks of age. Cauda epididymal sperm numbers were reduced in treated animals (35% of control values at 20 weeks of age) but appeared to be recovering at 32 weeks of age. Reduction of endogenous estrogen had no effect on other aspects of epididymal function (percentage of motile sperm, sperm motion parameters, sperm head morphometrics, or ability of sperm to undergo an acrosome reaction). Reducing endogenous estrogen during postnatal development appears to have transient effects on porcine epididymal function. These transient effects suggest that the pig, with its high endogenous estrogen, may respond differently than other species to reduced estrogen synthesis.

Effects of antioxidants on human sperm preparation techniques

The effect of two different sperm preparation techniques, Percoll gradient centrifugation and swim-up from a washed pellet were tested on the functional competence of the selected spermatozoa. Percoll gradient centrifugation brought about an improvement in sperm motility parameters such as curvilinear velocity and straight-line velocity, an increase in the rates of hyperactivation and the acrosome reaction and an increase in the percentage of motile spermatozoa after 24 h of incubation compared to the centrifugation, swim-up procedure. The effects of antioxidants such as dithiothreitol (DTT) or reduced glutathione (GSH), and reactive oxygen species-scavenging enzymes such as catalase or superoxide dismutase (SOD) during the stage of centrifugation before the swim-up procedure were also studied. Though all of these agents prevented the fall in sperm motility after 24 h incubation, only DTT and SOD improved the rates of both hyperactivation and the acrosome reaction. GSH also improved the acrosome reaction, whereas catalase was without significant effect on the rates of hyperactivation or the acrosome reaction. These results indicate that Percoll gradient centrifugation selects spermatozoa with better functional competence than does centrifugation swim-up. The damage caused by the centrifugation can be prevented by the addition of antioxidants, suggesting that the differences noted with the Percoll gradient method was due to better protection against peroxidative damage due to the centrifugation of unselected spermatozoa. However, the use of DTT is limited by virtue of the fact that this sulphydryl reducing agent leads to destabilization of the sperm chromatin. In contrast, GSH and SOD could have therapeutic potential.

Metachromatic staining of human sperm nuclei after reduction of disulphide bonds

Intra- and interprotamine cross-linking by disulphide bonds are the main factors responsible for the highly compact and stable structure of chromatin in mammalian spermatozoa. Unfixed or methanol fixed smears of human sperm and sperm suspensions from fertile donors and oligospermic patients were subjected to a reductive cleavage of disulphide bonds by using 2-mercaptoethanol (ME) or dithiothreitol (DTT). Untreated (control) and ME or DTT treated samples were stained with toluidine blue (TB) and examined in light microscopy; spectral characteristics of TB stained sperm nuclei were also analyzed. Untreated smears from fertile donors showed an orthochromatic (pale blue) staining of most sperm heads, while a variable proportion of metachromatic nuclei was found in spermatozoa from patients with oligospermia. After treatment with DTT followed by TB staining, fixed and unfixed smears showed metachromatic sperm heads. ME treatment only induced a scarce colour shift, whereas a striking metachromatic reaction and variable nuclear swelling were observed in DTT treated sperm suspensions. These results indicate that after cleavage of disulphide bonds, phosphate groups from chromatin DNA are unmasked and available for TB binding and metachromatic staining.

Chromatin arrangement in mouse sperm nuclei: an ultrastructural cytochemical study

The arrangement of mouse sperm nuclei chromatin and, in particular, of DNA has been studied by electron microscopic cytochemistry. It had been previously shown that, after a Feulgen-type reaction using an osmium ammine complex (OAC), the OAC-stained DNA was distributed in a spotted pattern in the nucleus (Biggiogera: Basic Appl Histochem 30:501-504, 1986). The present chapter shows that this pattern is characteristic of mouse spermatozoa from testis to vas deferens, with the exception of some testicular spermatozoa, in which DNA was homogeneously stained. DNase digestion of thin-sectioned nuclei resulted in a distribution of residual material complementary to the pattern of the unstained zones after the OAC reaction. These findings are discussed considering the role of -S-S- crosslinks, characteristics of this extremely condensed chromatin, in limiting the availability of DNA to acid hydrolysis.

Lead-induced changes in the stabilization of the mouse sperm chromatin

Long-term exposure of male mice to inorganic lead was previously found to reduce their fertility. In the present study chromatin stability in spermatozoa from such mice was investigated by means of quantitative cytochemical analysis. Sperm head sizes were determined and the capacity for nuclear chromatin decondensation (NCD) was evaluated after exposure to a solution of sodium dodecyl sulfate/dithiothreitol. A decreased uptake of propidium iodide (PI), a DNA intercalating dye, was found in spermatozoa from the vas deferens of the lead-exposed mice. However, after thermal denaturation of the DNA, the spermatozoa showed a higher uptake of PI in comparison to those of the controls. After reductive cleavage of S-S bonds with DTT and staining with a thiol-specific reagent (monobromobimane), significantly fewer reactive disulfide bonds were also observed in the spermatozoa. Furthermore, a significant delay in the capacity for NCD was noted. These findings indicate that exposure to lead increases the stabilization of the sperm chromatin, which in turn probably affects the decondensation of the nucleus, thereby interfering with the fertility of the mice.

Structure and function of the epididymis

Testicular spermatozoa are functionally immature in that they cannot fertilize ova. It was first demonstrated by Young that spermatozoa undergo certain changes as they migrate through the epididymis. He proposed that spermatozoa ripen during epididymal transit. It is now known that specific maturational changes occur in spermatozoa during epididymal transit which result in their developing the ability to fertilize ova. Concomitant with this functional maturity are changes in spermatozoal morphology, motility, chemistry, permeability, density and metabolism. It is apparent that in some way not understood these changes are necessary for sperm to achieve the ability to complete the fertilization process. When these mechanisms are understood, we may be able to effectively treat conditions such as necrospermia or abnormally low sperm motility. Furthermore, with the development of the hamster-egg penetration test a "new" type of male infertility has become evident in recent years; the inability of otherwise normal sperm to penetrate an ovum. It is during epididymal transit that this ability is normally acquired. Thus, any insight into how sperm attain the capacity to penetrate an ovum could lead to an effective treatment of patients whose sperm do not have this ability. In addition, the epididymis holds significant promise as the site of action for a male contraceptive. Thus, it is the purpose of this review to describe the structure and function of the mammalian epididymis with particular emphasis on the factors regulating sperm maturation.

Nuclear decondensation of mammalian spermatozoa: changes during maturation and in vitro storage

Swelling of the spermatozoan nucleus and decondensation of the chromatim occur soon after penetration of spermatozoa into the egg cytoplasm. This decondensation was duplicated in vitro by incubating pre-ejaculatory ram, rabbit and bovine spermatozoa and also stored post-ejaculatory bovine spermatozoa in 1% sodium dodecyl sulfate and 2.0 mM dithiothreitol. Spermatozoa obtained from the testis and epididymal caput, corpus and cauda showed a progressive resistance to nuclear decondensation, while no change was evident in the decondensation time of spermatozoa obtained from the epididymal cauda, vas deferens and ejaculated semen. There was also a significant increase in decondensation time after the spermatozoa had been stored in vitro at 25 degrees C. This increased resistance to nuclear decondensation in the in vitro stored spermatozoa, reflecting an increase in cross-linking within the spermatozoan histones by formation of disulfide bonds, may account for part of the increased embryonic mortality observed when spermatozoa are stored in vitro prior to insemination.