R.R. Angell

First-Meiotic-Division Nondisjunction in Human Oocytes

Reject oocytes from in vitro-fertilization patients are currently the only practical source of human oocyte material available for meiotic studies in women. Two hundred clearly analyzable second meiotic (MII) metaphase oocytes from 116 patients were examined for evidence of first meiotic (MI) division errors. The chromosome results, in which 67% of oocytes had a normal 23,X chromosome complement but none had an extra whole chromosome, cast doubt on the relevance, to human oocytes, of those theories of nondisjunction that propose that both chromosomes of the bivalent fail to disjoin at MI so that both move to one pole and result in an additional whole chromosome at MII metaphase. The only class of abnormality found in the MII oocytes had single chromatids (half-chromosomes) replacing whole chromosomes. Analysis of the chromosomally abnormal oocytes revealed an extremely close correlation with data on trisomies in spontaneous abortions, with respect to chromosome distribution, frequency, and maternal age, and indicated the likelihood of the chromatid abnormalities being the MI-division nondisjunction products that lead to trisomy formation after fertilization. The most likely derivation of the abnormalities is through a from of misdivision process usually associated with univalents, in which the centromeres divide precociously at MI, instead of MII, division. In the light of recent data that show that altered recombination patterns of the affected chromosomes are a key feature of most MI-division trisomies, the oocyte data imply that the vulnerable meiotic configurations arising from altered recombination patterns are processed as functional univalents in older women. Preliminary evidence from MI-metaphase oocytes supports this view.

Predivision in human oocytes at meiosis. I, A mechanism for trisomy formation in man

SummaryCytogenetic preparations from oocytes remaining unfertilised after in vitro fertilisation revealed single chromatids (as opposed to whole chromosomes) in 4 out of 38 meiosis II metaphases. In one oocyte, a single chromatid was present in addition to the normal 23,X complement, and in three oocytes, two identical but separate chromatids replaced one whole chromosome within the complement. The data indicate that the chromatids have arisen as a result of premature division of the centromeres at meiosis I (“predivision”). None of the oocytes were hyperhaploid with an extra whole chromosome. These findings are at variance with the general belief that trisomy in man is largely a consequence of nondisjunction of whole bivalents at meiosis I and they suggest that predivision resulting in separate chromatids may be a significant mechanism for human trisomy.

Chromosome studies in human in vitro fertilization

SummaryThe chromosome constitution of 22 human preimplantation embryos from donor oocytes fertilized in vitro by donor sperm was studied to assess the contribution of lethal chromosome anomalies to the high failure rate of implantation of in vitro fertilized embryos after embryo transfer in infertile women. Evidence was found of nondisjunction, resulting in trisomy, monosomy, and nullosomy; structural abnormalities; haploidy; and triploidy. Despite the lethality of their chromosome complements, these embryos could not be distinguished morphologically from those with normal chromosomes.

The recovery of preovulatory oocytes using a fixed schedule of ovulation induction and follicle aspiration

Thirty‐two unteer women, scheduled for laparoscopy, were subjected to a fixed regimen of ovulation induction and ovarian follicle aspiration, following manipulation of the previous menstrual cycle with oral contraceptive pills or norethisterone. This allowed the time of oocyte recovery to be planned several weeks in advance. The recovery rate of mature oocytes (loosely dispersed cumulus) in the patients who had their cycles adjusted with norethisterone was comparable to that in a group of women treated in an embryo‐transfer programme who were being individually monitored, although there was evidence from cleavage rates and concentration of sex steroids in follicular fluid that further follicular maturation could have occurred. It is suggested that a modification of this schedule could provide an acceptable recovery rate of pre‐ovulatory oocytes for research purposes, and possibly by simplifying the monitoring technique could be alicable to a wider range of patients seeking embryo transfer.

Consequences of polyspermy in man

Polyspermy occurs with a frequency of about 12% under some hormonal regimens currently being used for in vitro fertilization and embryo transfer. Oocytes with three pronuclei usually show normal cleavage and are morphologically indistinguishable from embryos produced by normal fertilization. Chromosome studies and DNA measurements on dispermic embryos show that about one-half are triploid, the others having either a normal diploid chromosome complement or a severely depleted chromosome complement. The evidence suggests that these errors must arise at the first mitotic division of the oocyte. Aberrant spindle formation is implicated.

Meiosis I in human oocytes

Oocytes aspirated from women undergoing laparoscopic sterilisation were matured in vitro from the germinal vesicle stage. They showed that germinal vesicle breakdown spanned a period of about 12 h; the oocytes were at diakinesis by 12 h, and at metaphase I by 16 h where they remained for a further 10-12 h. Anaphase I took place after about 26 h. The meiotic preparations lacked the clarity of chromosome definition seen in mouse oocytes and accurate chiasma counts could not be made. However univalents at first meiotic metaphase could be detected without ambiguity and were found in women significantly older than those whose metaphase I oocytes contained bivalents only.

A comparison of fixed regimens for obtaining human cleaving oocytes for research purposes

Summary. A fixed schedule for ovarian stimulation and follicular aspiration, previously used in our department for research purposes, was modified in an attempt to increase the recovery and cleavage rates of the oocytes. Three different clomiphene regimens were used to stimulate the ovaries of normal volunteer women requesting laparoscopic sterilization (50 mg and 150 mg daily for 5 days, and 50 mg daily for 10 days). Oocytes were recovered from 83% of the aspirated follicles, i.e. 1·6 oocytes/patient: 65% of the oocytes cleaved after in‐vitro fertilization and, on average, 1·0 cleaving egg was obtained per patient. There were no differences in the recovery and cleavage rates between the three clomiphene regimens. It is concluded that a sufficient number of cleaving embryos for research purposes can be generated with the present fixed regimen, which offers little inconvenience to the volunteers as the day of egg recovery can be predicted some time in advance.