Distribution of aneuploidy in human gametes: comparison between human sperm and oocytes

Assisted reproductive technologies: genetic and nursing implications

The convergence of the fields of clinical genetics and assisted reproductive technologies is providing couples at risk for transmitting a genetic disorder to their children with new reproductive alternatives. The ability to test the preimplantation embryo for genetic anomalies, sort for X- and Y-bearing sperm, and improve genetic screening of gamete donors and couples at risk for a genetic disorder, are examples of these alternatives. The scope of nursing practice will be affected by the integration of these technologies into the health care offered to consumers. Opportunities also will exist for nurses to assist in the redefinition of health and illness that will be the serendipitous outcome of these scientific advances.

Numerical chromosome abnormalities in spermatozoa of fertile and infertile men detected by fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) with single-color chromosome-specific probes was used to study the rates of disomy for chromosome 1, 16, X, and Y in sperm of fertile and infertile subjects. Diploidy rates were studied using a two-color cocktail of probes for chromosomes 17 and 18 in the same sperm samples. Two-color methodology was not available at the outset of the study. A total of 450,580 spermatozoa were studied from 21 subjects (9 fertile, 12 infertile). Significant differences were observed in the disomy rates between chromosomes with the highest frequency observed for chromosome 16 (0.17%) and the lowest for the Y chromosome (0.10%). No differences were observed between fertile and infertile subjects for either diploidy or disomy. Total disomy rates for chromosomes 1, 16, X and Y ranged from 0.34% to 0.84% among infertile subjects, and 0.32% to 0.61% among fertile subjects. Our data suggest that generalized aneuploidy in sperm is not a major contributor to unexplained infertility.

Incidence of chromosome 18 disomy in human sperm nuclei as detected by nonisotopic in situ hybridization

Nonradioactive in situ hybridization with an alpha-satellite DNA probe specific for chromosome 18 was performed on human interphase sperm nuclei to detect the frequency of sperm cells disomic for chromosome 18. A total of 16,127 sperm heads from eight healthy donors, aged 23-57 years, was investigated, and a minimum of 2000 sperm nuclei per proband was analyzed. The disomy rate ranged from 0.25% to 0.5%, with an average of 0.36%. This frequency does not differ significantly from that determined for other chromosomes.

Functional competence of abnormal spermatozoa

The processes of abnormal sperm penetration and incorporation into human oocytes during IVF and after sperm microinjection, assessed by TEM, are reviewed. A spectrum of morphologically abnormal sperm with head, neck and midpiece defects penetrate the egg vestments of oocytes (1-3 h after insemination with sperm from normal donors) in both unfertilized and normally fertilized oocytes. Sperm with aberrant head shapes, acrosomal and nuclear defects penetrate the outer zona pellucida but are rarely encountered in the inner zona and perivitelline space, showing that the zona prevents abnormal sperm penetration. Grossly abnormal sperm are, however, incorporated into the ooplasm of zona-denuded oocytes. Microinjection of poor-quality sperm from male-factor patients into the perivitelline space or directly into the ooplasm of oocytes also reveals a variety of structural defects conforming to those observed in washed sperm pellets, highlighting the difficulties of selection of 'normal' sperm for microinjection. Abnormal sperm have been seen to interact and fuse with the oocyte in the perivitelline space, and to be incorporated into the ooplasm. Those with nuclear and neck (centriolar) defects are of particular significance, as they might contribute to aberrant development. The functional competence of immotile, round-headed and epididymal sperm is also briefly discussed.

Trisomy in humans: incidence, origin and etiology

Molecular studies conducted over the past year have demonstrated the importance of aberrant genetic recombination in the etiology of several human trisomies, and have begun to shed light on the basis of the association between advancing maternal age and trisomy. Preliminary studies of gametes using fluorescence in situ hybridization indicate that this will be a useful approach in the analysis of human non-disjunction.

Aneuploidy detection in human sperm nuclei using fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) was performed on human interphase sperm nuclei to determine the utility of this technique for aneuploidy detection. Repetitive DNA sequences specific for chromosomes 1, 12 and X were biotinylated and hybridized with mature sperm, which had been treated with cetyltrimethylammonium bromide and dithiothreitol to render them accessible to the probes. Detection of bound probe was accomplished with fluoresceinated avidin and antiavidin. For each of the chromosomes studied, chromosome number was determined by counting the fluorescent signals, representing hybridized regions, within the sperm nuclei. The frequencies for disomy, that is for nuclei containing two signals, for chromosomes 1, 12 and X were 0.06%, 0.04% and 0.03%, respectively. The congruence of these results with those determined by the cross-species hamster oocyte-human sperm assay, and the high efficiency of hybridization indicate that FISH is a sensitive and reliable tool for aneuploidy detection in human sperm.

The sex ratio of normal and manipulated human sperm quantitated by the polymerase chain reaction

OBJECTIVE

To establish the primary sex ratio, the relative abundance of X and Y chromosome-bearing sperm, in unselected sperm and in sperm selected by swim-up or Sephadex filtration (SpermPrep column; Fertility Technologies, Inc., Natick, MA). This was done to evaluate the possibility that these semen manipulations change the primary sex ratio.

DESIGN

Ninety-eight unmanipulated semen samples were analyzed for sex chromosome content using quantitative polymerase chain reaction (PCR). A smaller number of samples was analyzed before and after either swim-up or Sephadex filtration.

RESULTS

The mean percentage of all sex chromosomes identified as a Y chromosome in unmanipulated semen samples ranged from 41.9% to 56.7%, with a mean average of 50.3%. There was no significant change in sex chromosome composition after either swim-up (n = 17) or column filtration (n = 20).

CONCLUSIONS

The chromosome compositions of semen samples from a large number of men have equal numbers of X and Y. Swim-up and SpermPrep filtration do not appear to alter the primary sex ratio.

Maternal age effect: the enigma of Down syndrome and other trisomic conditions

Aneuploidy is the most frequently observed chromosome abnormality in human liveborn, abortuses and oocytes. The only etiological factor that has been established is advanced maternal age for the occurrence of trisomies, particularly trisomy 21 which causes Down syndrome. The maternal age effect remains an enigma. Recent molecular data bearing on this question are reviewed as are the hypotheses that have been proposed linking nondisjunction and maternal age. Rationale is presented for a compromised microcirculation hypothesis that explains the cause of nondisjunction and why its occurrence changes with maternal age from menarche to menopause. It takes into account two facts: (1) 95% of Down syndrome children receive their extra chromosome from their mother, and in 80% or more of these the nondisjunction occurred in the first meiotic division, which is completed in the ovary. (2) The ovarian follicle containing the primary oocyte has no internal circulation. The hypothesis proposes that aneuploid oocytes arise from a concatenation of events. It begins with hormonal imbalance that causes a less-than-optimal microvasculature to develop around the maturing and mature follicles. The resulting decrease in the size of the perifollicular capillary bed reduces the volume of blood flow through the area, leading to an oxygen deficit and a concomitant increase inside the follicle of carbon dioxide and anaerobic products, such as lactic acid. This in turn causes a decrease in the intracellular pH of the oocyte that diminishes the size of the spindle, with consequent displacement and nondisjunction of a chromosome. The compromised microcirculation hypothesis explains the occurrence of aneuploidy in primary and secondary oocytes, sperm precursor cells, tumor and embryonic cells. It also explains why women of all reproductive ages may have a Down syndrome child.

Reduced recombination and paternal age effect in Klinefelter syndrome

The parental origin of the additional sex chromosome was studied in 47 cases with an XXY sex chromosome constitution. In 23 cases (49%), the error occurred during the first paternal meiotic division. Maternal origin of the additional chromosome was found in the remaining 24 cases (51%). Centromeric homo- versus heterozygosity could be determined in 18 out of the 24 maternally derived cases. According to the centromeric status and recombination rate, the nondisjunction was attributable in 9 cases (50%) to an error at the first maternal meiotic division, in 7 cases (39%) to an error at the second maternal meiotic division and in 2 cases (11%) to a nullo-chiasmata nondisjunction at meiosis II or to postzygotic mitotic error. No recombination, and in particular none in the pericentromeric region, was found in any of the 9 cases due to nondisjunction at the first maternal meiotic division. Significantly increased paternal age was found in the paternally derived cases. Maternal age was significantly higher in the maternally derived cases due to a meiotic I error compared with those due to a meiotic II error. There were no significant clinical differences between patients with respect to the origin of the additional X chromosome.