High susceptibility for diploidy in ovulated oocytes from XO mice

Analysis of colchicine-induced chromosomal aberrations in embryos obtained from XO mice

A high incidence of chromosomal anomalies has been observed in the offspring of our XO mouse colony. However, the reason(s) are unknown. We hypothesized that XO dams might be more susceptible to certain chromosomal mutagens. Therefore, we assessed whether the oocytes of XO mice are more susceptible to colchicine. Pregnant XO and XX mice were intraperitoneally (i.p.) injected with colchicine, 0.2 mg/kg, 6 h after mating (between resumption of meiosis II and extrusion of the second polar body). Contrary to our expectation, the incidence of chromosomal anomalies induced by colchicine exposure did not differ between XX and XO dams. These findings suggest that the chromosomal stability of the oocytes from XO mice may not be affected by colchicine exposure at the stage of extrusion of the second polar body. The effects of chromosomal imbalance inherent in XO mice on chemical susceptibilities should be further investigated.

Teratogen susceptibility of XO mothers and XO embryos in mice

We examined whether the chromosomal imbalance inherent in an XO constitution in mice is more susceptible to teratogenic influence of biotin deficiency using a newly established mouse colony with pure X monosomy. We hypothesized that XO mothers or XO embryos might be more susceptible to certain teratogens. Contrary to our expectation, the incidence of external malformations induced by biotin deficiency did not differ either between XX dams and XO dams or between XX fetuses and XO fetuses.

Effect of growth hormone dosing on ultimate weight and length of XO mice

Mice with the karyotype 39, XO and XX mice with the same genetic background were injected for a six month period with daily subcutaneous doses of 0.5 IU biosynthetic human growth hormone (hGH) or with placebo. The XO mice dosed with hGH obtained a significantly higher body weight and greater final length than the XO mice dosed with placebo (39.2 g versus 29.0 g and 21.1 cm versus 19.2 cm respectively). The body weight remained constant after cessation of dosing. The XX mice became significantly longer but not heavier as a result of hGH dosing. The placebo dosed XO mice became significantly longer than the placebo XX mice but their body weights were comparable. Dosing with hGH caused a significant increase in growth rate in both XO and XX mice, and the ultimate body weight was reached within a significantly shorter time period. In the XO mice the hGH injections increased significantly the length and thickness of the femoral bones and the length of the tibia bones. Stimulated endogenous plasma growth hormone levels were significantly higher in XX mice compared to XO mice. Plasma IGF-I levels were significantly higher in XO compared to XX mice, but the levels were not affected by hGH dosing. XO mice may be useful in future metabolic and hormonal studies.

Effect of exogenous hormones on the ovulation of primary and secondary oocytes in LT/Sv strain mice

LT/Sv strain mice ovulate both primary and secondary oocytes. These are fertilizable and give rise to digynic triploid and normal diploid conceptuses, respectively. A previous study [Kaufman and Speirs, 1987] had indicated that just over 20% of embryos recovered on the 10th day of gestation from spontaneously ovulating females had a triploid chromosome constitution. This value was considerably lower than might have been expected by extrapolation from earlier studies in which LT/Sv mice had been given exogenous gonadotrophins. In the present study, therefore, cytogenetic analysis of fertilized eggs was performed at the first cleavage mitosis in (1) spontaneously ovulating females mated to F1 hybrid males, and (2) superovulated females mated to similar males. Additional females from group (1) were autopsied on the 10th day of gestation, and the ploidy of embryos isolated at this stage of gestation was determined. Exposure to exogenous gonadotrophins significantly increased the proportion of eggs that were ovulated as primary oocytes (34.4%), compared to the situation observed following spontaneous ovulation (24.4%). All the triploids encountered in both series were of the digynic type and characteristically (for LT/Sv mice) had an oocyte-derived set with 40 chromosomes present, and a sperm-derived set containing 20 chromosomes. Similar numbers of eggs were recovered from spontaneously ovulating females on the 1st and 10th days of gestation, and the incidence of triploidy observed on the 10th day was 22.1%. The influence of exogenous hormones in increasing the "spontaneous" level of triploidy in LT/Sv and in other strains of mice is briefly reviewed.

Ovulation and fertilization of primary and secondary oocytes in LT/Sv strain mice

In this study, the chromosome constitution of both unfertilized oocytes and fertilized eggs isolated from the oviducts of LT/Sv strain mice were analyzed. Air-dried chromosome preparations from unfertilized oocytes revealed that about one-third of those examined were ovulated as primary oocytes. These were arrested at metaphase of the first meiotic division and exhibited the characteristic "tetrad" chromosome configuration. The remaining two-thirds of the unfertilized oocytes were ovulated at metaphase of the second meiotic division. The fertilized eggs were isolated from the oviducts of LT/Sv females previously mated to (C57BL x CBA) F1 hybrid males. Analysis of the fertilized eggs at metaphase of their first cleavage mitosis revealed that about one-third of the eggs examined were digynic triploids, whereas the remaining two-thirds had the normal diploid chromosome constitution. In the triploids, the 40 female chromosomes present (mouse, n = 20) were derived from a single diploid pronucleus formed after the extrusion of a first polar body, and following the monospermic fertilization of primary oocytes. The female pronuclear-derived chromosomes invariably exhibited "homologous pairing," and these were associated at their centromeres. The ovulation, penetration, and subsequent fertilization of primary oocytes is an extremely unusual phenomenon in mammals and only appears to occur on a regular basis in LT/Sv mice. The premature "cytoplasmic maturation" of these oocytes is of interest, as they clearly have the same developmental capacity as secondary oocytes. The significance of these observations in relation to folliculogenesis and litter size in LT/Sv mice is discussed.

Inheritance of a meiosis I error expressed in mouse oocytes and modulated by a maternal factor

NMRI/Han mice ovulate significant numbers of diploid oocytes after gonadotrophin stimulation, most of them arrested at metaphase I. In contrast, females from other mouse strains ovulate only oocytes having completed meiosis I. We investigated the heritability of the trait Dipl I by analysing F1hybrid females from crosses between the sensitive NMRI/Han stock and two mouse strains (C3H/HeHan and BALB/c) considered as non-sensitive, and females from backcrosses to NMRI/Han males (only crosses with C3H/HeHan). The results show (1) that the trait Dipl I is inheritable; (2) that an X-chromosomal (location proximally from the X–Y pairing region) or autosomal recessive mode of inheritance is excluded; and (3) that the expression of the trait, measured as frequency of diploidy, is modulated by a maternally transmitted factor.

Chromosome segregation at meiosis I in female T(2;4)1Gö/+ mice: no evidence for a decreased crossover frequency with maternal age

The influence of age and hormones on chromosome segregation at meiosis I was studied in female mice heterozygous for the T(2;4)1Gö translocation. Females of two age groups (18-22 and 40-56 weeks old) were stimulated for ovulation with different doses of gonadotropins (1.5 IU PMS/1.0 IU HCG or 10 IU PMS/10 IU HCG). Analysis of metaphase II oocytes revealed the highest level of hyperhaploidy (1.8%) and presegregation (4.4%) in the young females receiving the low dose. Presegregation preferentially affected the small 4(2) marker chromosome. There was no significant interference of the tetravalent with disjunction of the nontranslocated normal bivalents. Moreover, no remarkable difference in the mode of segregation (adjacent I, II or alternate) was observed. Recombination within the interstitial pairing segments of the chromosomes involved in the translocation allowed us to calculate cross-over frequencies in ovulated oocytes. For both the large 2(4) and the small 4(2) marker chromosomes, this frequency was higher in old than in young T(2;4)1Gö/+ females. Our data do not support the production line hypothesis of Henderson and Edwards (1968) which claims that chiasma frequency in oocytes decreases with maternal age.