New insights on the origin and relevance of aneuploidy in human spermatozoa.

In humans, the most common chromosomal abnormality is aneuploidy. Because the majority of aneuploid conceptuses die during the early stages of embryonic development, an accurate estimate of the frequency of aneuploidy at conception can only be assessed by directly studying the gametes. The vast majority of aneuploidies arise de novo as a result of sporadic chromosome missegregation in paternal or maternal meiosis. In this review, we present the basic current knowledge about the incidence of aneuploidy in human spermatozoa in the general population and in patient populations where elevated levels of sperm aneuploidy are observed. These include infertile patients, patients with abnormal somatic karyotypes, and individuals exposed to certain environmental/lifestyle hazards. The clinical impact of increased levels of aneuploidy is discussed. We then focus on the non-disjunction mechanisms that cause aneuploidy during spermatogenesis and the factors that predispose to non-disjunction in male germ cells followed by an analysis of the sex differences in the incidence of gamete aneuploidy. Recent meiotic studies using multiplex-FISH on three fertile men have revealed that the frequency of conservative aneuploidy of metaphase II spermatocytes is similar to that observed in non-inseminated oocytes of young women. These findings suggest that the differences in the incidence of aneuploidy between spermatozoa and oocytes are not due to differences in chromosome segregation errors but rather to more effective checkpoint mechanisms in spermatogenesis than in oogenesis.

Meiotic non-disjunction mechanisms in human fertile males.

BACKGROUND: In humans, little is known about the mechanisms of non-disjunction working in male meiosis, although considerable attention has been given to these mechanisms in female meiosis. The present study explores the origin of meiotic non-disjunction during human spermatogenesis and the chromosomes most commonly involved in this process. METHODS: We used Multiplex fluorescence in situ hybridization to carry out meiotic analyses in metaphase I (MI) and metaphase II (MII) spermatocytes from three fertile donors. Testicular biopsy was obtained during a vasectomy procedure. RESULTS: We examined a total of 317 MI and 248 MII spermatocytes. The frequency of numerical chromosome abnormalities at MII (14.5%) was 5.5 times higher than at MI (2.5%). We observed 88 (27.7%) spermatocytes I with chromosome bivalents with a low chiasma count, usually small chromosomes displaying two separated univalents. Chromosomes X, Y and 21 were the most commonly found as achiasmate chromosomes at MI and the most frequently involved in disomy at MII. Hyperploidy frequency in spermatocytes II (disomy) was significantly higher (P< 0.001) than that found in spermatocytes I (trisomy). CONCLUSIONS: Achiasmate non-disjunction and premature separation of sister chromatids appear to be the two main non-disjunction mechanisms during the first meiotic division in human spermatogenesis, and both mechanisms contribute equally to the genesis of aneuploidy. The elevated frequencies of disomy detected in spermatocytes II are significantly higher than those previously described in human spermatozoa, suggesting the existence of a postmeiotic checkpoint monitoring numerical abnormalities.

Centromere-specific multicolour fluorescence in situ hybridization on human spermatocyte I and II metaphases.

BACKGROUND: Most meiotic studies in metaphase spermatocytes have been carried out with classic cytogenetic techniques. The aim of this work was to adjust the centromere-specific multicolour fluorescence in situ hybridization (cenM-FISH) procedure to spermatocyte metaphases I and II in order to improve the identification of meiotic chromosome abnormalities. METHODS: A total of 168 spermatocytes I and 66 spermatocytes II from two fertile males have been studied using cenM-FISH. RESULTS: The mean frequency of meiotic abnormalities (synaptic, numerical and structural errors) found in metaphases I and II was 22.1 and 3.0%, respectively. The cenM-FISH technique has not only enabled the individual identification of chromosomes involved in meiotic disorders, but also increased the number of analysable cells, principally at metaphase II stage. CONCLUSIONS: CenM-FISH is a useful tool to study the meiotic chromosomal disorders and mechanisms leading to chromosomally abnormal spermatozoa.