A dramatic remodeling of sperm chromatin occurs during mammalian spermiogenesis. Nuclear elongation and chromatin condensation are concomitant with modifications in the basic protein complement associated with DNA. A number of biochemical events accompany the displacement of histones and the appearance of protamines in elongating spermatids. The mRNAs of transition proteins and protamines are transcribed and stored in the cytoplasm of spermatids until days later when they are translated. The intrinsic regulation of the expression of the transition protein and protamine genes occurs at three levels: transcription, translation, and posttranslation. The aim of this review is to cover most of the morphological, biochemical, and functional events which concern nuclear protein transitions during spermiogenesis and which are thereby involved in the nuclear status of ejaculated sperm cells.

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A retrospective study of 49 men with proven fertility and 396 suspected infertile men was conducted with the primary objective of investigating the relationship between the nuclear maturity of sperm and male fertility. Acidic aniline blue staining was used to detect chromatin defects of sperm nuclei related to their nucleoprotein content as associated with DNA. The discriminant value of the percentage of unstained nuclei (= percentage of mature heads, MH) and of other semen characteristics, was analysed by a stepwise (forward) linear regression model. Semen characteristics that discriminated significantly between the two groups of subjects were, in descending order: (1) the percentage of normal sperm, (2) the percentage of amorphous heads, (3) the percentage of tapered heads, (4) semen volume, and (5) the percentage of MH. The discriminant value of each of the significant characteristics was studied by means of ROC-curves. MH had the best ROC-curve profile; its cut-off value was found to be equal to 70% (74.5 +/- 2.6% for the donor group versus 53.0 +/- 1.1% for the patient group). A simple infertility score (SIS) including MH, was built according to the cut-off values inferred from the ROC-curves. SIS allowed an overall satisfactory separation of the two groups (less than or equal to 4 = fertile, 5-6 = uncertainty zone, greater than 6 = infertile). Our results indicate that the addition of the evaluation of sperm head maturity to routine semen analysis improves the assessment of fertility in men.

The profound architectural changes that transform spermatids into spermatozoa result in a high degree of DNA packaging within the sperm head. However, the mature sperm chromatin that harbors imprinted genes exhibits a dual nucleoprotamine/nucleohistone structure with DNase-sensitive regions, which could be implicated in the establishment of efficient epigenetic information in the developing embryo. Despite its apparent transcriptionally inert state, the sperm nucleus contains diverse RNA populations, mRNAs, antisense and miRNAs, that have been transcribed throughout spermatogenesis. There is also an endogenous reverse transcriptase that may be activated under certain circumstances. It is now commonly accepted that sperm can deliver some RNAs to the ovocyte at fertilization. This review presents potential links between male-specific genomic imprinting, chromatin organization, and the presence of diverse RNA populations within the sperm nucleus and discusses the functional significance of these RNAs in the spermatozoon itself and in the early embryo following fertilization. Some recent data are provided, supporting the view that analyzing the profile of spermatozoal RNAs could be useful for assessment of male fertility.