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.

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.

This individual presents high levels of numerical abnormalities in germ cells, caused by two different nondisjunction mechanisms during meiosis I. To our knowledge, this work represents the first time that PSSC has been demonstrated in human male germ cells.

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CenM-FISH is a useful tool to study the meiotic chromosomal disorders and mechanisms leading to chromosomally abnormal spermatozoa.

Selecting the best embryo for transfer is key to success in assisted reproduction. The use of algorithms or artificial intelligence can already predict blastulation or implantation with good results. However, ploidy predictions still rely on invasive techniques. Embryologists are still essential, and improving their evaluation tools can enhance clinical outcomes. This study analyzed 374 blastocysts from preimplantation genetic testing cycles. Embryos were cultured in time-lapse incubators and tested for aneuploidies; images were then studied for morphokinetic parameters. We present a new parameter, “st2, start of t2”, detected at the beginning of the first cell cleavage, as strongly implicated in ploidy status. We describe specific cytoplasmic movement patterns associated with ploidy status. Aneuploid embryos also present slower developmental rates (t3, t5, tSB, tB, cc3, and t5-t2). Our analysis demonstrates a positive correlation among them for euploid embryos, while aneuploids present non-sequential behaviors. A logistic regression study confirmed the implications of the described parameters, showing a ROC value of 0.69 for ploidy prediction (95% confidence interval (CI), 0.62 to 0.76). Our results show that optimizing the relevant indicators to select the most suitable blastocyst, such as by including st2, could reduce the time until the pregnancy of a euploid baby while avoiding invasive and expensive methods.