Trisomic Embryos Research Articles

The development potential of ethanol-induced monosomic and trisomic conceptuses in the mouse.

The development potential of fertilized embryos isolated from female mice previously given a single dose of either a dilute solution of ethanol or distilled water (controls) by mouth was studied. Exposure to ethanol occurred at various times during the cycle leading to ovulation and shortly after fertilization. The chromosome constitution of all preimplantation embryos isolated from these females was determined either at the first cleavage mitosis or at the morula stage. The incidence of aneuploidy in the ethanol-exposed groups at these times was approximately 19% and 13.5%, respectively, with a similar number of monosomic and trisomic conceptuses observed at these times. In addition, about 2% of all conceptuses examined were triploid. Further females were autopsied on the 10th or 11th day of gestation, though the chromosome constitution of only the morphologically abnormal or developmentally retarded embryos was determined. Eight embryos out of a total of 16 studied in the ethanol-exposed group were either aneuploid or triploid, whereas in the control group only one out of 11 examined proved to be aneuploid. The triploids and ethanol-induced aneuploid conceptuses appeared to be capable of surviving to the morula stage but generally failed to survive to the 10th/11th day. No monosomics were in fact observed in the postimplantation series. The present findings are briefly discussed with reference to the possible pathogenesis of spontaneous abortions in man, which often possess similar types of chromosomal anomalies.

Morphologic development of the fetal trisomy 19 mouse.

Trisomy 19 mice show a characteristic pattern of delayed morphologic development when compared with normal littermates during the second half of gestation. Examinations of the external phenotype and sectioned fetuses, and skeletal maturation at later days, suggest that trisomy 19 has little recognizable effect on morphogenesis prior to 10 days. Trisomic conceptuses collected at subsequent 24-hour intervals show a 1-day lag in weight gain and begin to show a progressive and uniform delay in differentiation so that 14- and 15-day animals resemble 13- and 14-day normal fetuses, respectively. There is no further recognizable increase in this delay through term. There was no striking change in numbers of viable trisomic conceptuses collected throughout the period from 10 days to birth. These observations suggest that most trisomies surviving at 10 days will develop to term, with the major effect of the aneuploid genome being to delay the otherwise normal-appearing growth and development of the affected conceptuses. In all cases, the litters were from crosses between mice doubly heterozygous for the (5.19) and (9.19) Robertsonian translocation chromosomes.

Hypotelorism in trisomy 1-producing mice.

Trisomy 1 embryos in mice are smaller in all dimensions, showing a developmental retardation as compared with euploid mice. Very rarely the trisomic embryos develop a typical hypotelorism with holoprosencephalon and missing olfactory nerves. Corneal distances, angles between the optic nerves and further microscopic examination showed no intermediate forms between the trisomy 1 embryos and the rare trisomy 1 embryo with hypotelorism. There seems to be a threshold, beyond which the major developmental derangement occurred. This is an experimental model showing parallels to the occasional varying phenotypes in human trisomies.

Abnormal neural fold development in trisomy 12 and trisomy 14 mouse embryos

ABSTRACT The early development of the exencephalic malformation in trisomy 12 (Tsl2) and trisomy 14 (Tsl4) mouse embryos was examined by means of scanning electron microscopy and compared with cranial neural tube formation in euploid litter-mates. Embryos from normal laboratory mice were used as additional controls. The euploid control embryos of the trisomy-inducing breeding system showed a slight delay and some variation in the timing of cranial neurulation. The pre-exencephalic trisomic embryos showed hypoplasia, and lower somite number when compared with euploid littermates; there was also a retardation of development of the whole neural tube, when related to the somite stage. External differences from the control embryos were observed at the late pre-somite stage, when the anterior part of the neural plate showed a crumpled appearance. At 6 somites the lateral edges of the forebrain were everted instead of elevated in Tsl2 and Tsl4 embryos. At later stages, however, the forebrain showed a tendency towards the normal morphogenetic pattern, so that the optic vesicles were eventually formed and the most anterior part fused. The caudal forebrain and the midbrain were more permanently affected by the disturbance of trisomic conditions and grew laterally, failing to appose or fuse in the midline in both Tsl2 and Tsl4 embryos. Hindbrain morphogenesis was different in Tsl2 and Tsl4 excencephaly : in Tsl2 embryos it did not close rostral to the otic pits, whereas in Tsl4 embryos it showed a normal closure up to the hindbrain/midbrain junction. These observations support the hypothesis that in mammalian embryos the mechanism of neural tube formation of the future brain region is more complex than that of the spinal neural tube and therefore may be more likely to react to a general delay of neurulation with a gross malformation. Tsl2 and Tsl4 exencephaly are due to a primary non-closure of the neural tube.

Maize pollen test systems to detect nondisjunction.

Three pollen test systems to detect the induction of nondisjunction in maize are actively being explored. (1) Each member of a tetrad of haploid microspores produced by meiosis contains a single chromosome 6 which carries the only nucleolar organizing region in the maize genome. Thus, each member of a normal tetrad contains one nucleolus. If nondisjunction took place at the first or second meiotic division, 2:2:0:0 or 2:1:1:0 tetrads would be produced respectively. (2) If a male parent carrying a dominant endosperm marker is crossed by a female carrying a recessive allele of this gene, all normal kernels would be heterozygous and would express the dominant phenotype in their endosperm. If nondisjunction of the chromosome carrying this gene took place at the second microspore division and the nullisomic and disomic sperm fertilized the polar nuclei and egg of a given embryo sac respectively, an exceptional kernel is produced which would express the recessive endosperm phenotype and contain a trisomic embryo. (3) Complementing null mutations of genes expressed in individual pollen grains can be utilized to detect nondisjunction. Normal haploid pollen grains from plants heterozygous for two complementing null alleles of a locus would each express the recessive phenotype. If nondisjunction took place during either meiotic division, disomic pollen grains containing both alleles could be produced expressing the dominant phenotype due to complementation. We are exploring this test system utilizing appropriate alcohol dehydrogenase mutants.

Maize Pollen Test Systems to Detect Nondisjunction

Three pollen test systems to detect the induction of nondisjunction in maize are actively being explored. (1) Each member of a tetrad of haploid microspores produced by meiosis contains a single chromosome 6 which carries the only nucleolar organizing region in the maize genome. Thus, each member of a normal tetrad contains one nucleolus. If nondisjunction took place at the first or second meiotic division, 2:2:0:0 or 2:1:1:0 tetrads would be produced respectively. (2) If a male parent carrying a dominant endosperm marker is crossed by a female carrying a recessive allele of this gene, all normal kernels would be heterozygous and would express the dominant phenotype in their endosperm. If nondisjunction of the chromosome carrying this gene took place at the second microspore division and the nullisomic and disomic sperm fertilized the polar nuclei and egg of a given embryo sac respectively, an exceptional kernel is produced which would express the recessive endosperm phenotype and contain a trisomic embryo. (3) Complementing null mutations of genes expressed in individual pollen grains can be utilized to detect nondisjunction. Normal haploid pollen grains from plants heterozygous for two complementing null alleles of a locus would each express the recessive phenotype. If nondisjunction took place during either meiotic division, disomic pollen grains containing both alleles could be produced expressing the dominant phenotype due to complementation. We are exploring this test system utilizing appropriate alcohol dehydrogenase mutants.

Trisomies and triploidies in hamster embryos: induction by low-pressure hypoxia and pH imbalances.

The authors produced trisomic and triploid embryos by mating hamster females approximately 1 hr before estimated ovulation and subsequently submitting them to low-pressure atmospheres equivalent to those found at mountain altitudes for 6 hr. During hypoxic exposures in the low-pressure chamber, the continuous monitoring of pH levels--both subcutaneously and intramuscularly--registered pH decreases from inital values as high as 7.6 to below 6.9. These findings indicate that trisomies in some embryos and triploidies in others have their origin in different, but related mechanisms involving maternal pH imbalances between ovulation and fertilization.

Lack of effect on the chromosomal non-disjunction in aged female mice after low dose X-irradiation

Karyotypes were determined in 1064 embryos of aged C57/BL mothers. The virgin female mice were irradiated with 0, 4, 8 or 16 R of X-rays, respectively, and placed with young untreated males 5 days after irradiation. 10.5-days old embryos were recovered from the uterus. Aneuploid embryos classified as alive (heart beats observed at the dissection) were 1 monosomic in the control group (496 embryos) and 2 trisomics in the irradiated group (568 embryos). The number of aneuploid embryos classified as dead was 4 trisomic cases in the control group and 3 trisomics in the irradiated group. The data indicate that trisomic embryos are not uncommon in the mouse but are eliminated in post-implantation death. In contrast to the results of Yamamoto et al. the present data do not demonstrate an increased frequency of chromosome abnormalities in embryos of aged mice X-irradiated before mating as compared to non-irradiated ones.

Laboratory mice carrying three pairs of Robertsonian translocations: establishment of a strain and analysis of meiotic segregation.

The evolution of a 2n = 34 combined Robertsonian translocation strain Rb(5.19)1Wh/(6.15)1Ald/(8.17)HEM is described. Homozygotes had three large ring translocation bivalents at M I and regular meiotic disjunction at M II. Male and female F<sub>1·</sub> heterozygotes had similar frequencies of abnormal meiotic disjunction at M II, but aneuploid embryos were more frequent in (F<sub>1</sub><i>♂ </i>X NIH GP ♀) crosses (25.6%) than (F<sub>1</sub> ♀ X GP <i>♂</i>) crosses (13.5%) at 10 days’ gestation. It could not be determined whether this discrepancy was genetic in origin or due to limitations in the techniques of M II analysis. Rb(5.19), as well as Rb(6.15) or (8.17), contributed to the aneuploidy, although previous M II analysis of singly heterozygous Rb(5.19)/+ males demonstrated comparatively regular meiotic disjunction. Trisomic embryos were not detected beyond 10 days in five (F<sub>1</sub> ♀ X GP <i>♂</i>) crosses, suggesting that their survival in F<sub>1</sub> females is limited. The ratios of the different types of balanced embryos from the (F<sub>1</sub> X GP) crosses indicated random meiotic segregation of the three translocation chromosomes in triple heterozygotes. Analysis of complementary groups of balanced embryos from the same crosses also demonstrated lack of preferential segregation of the translocations; embryos with translocations and those with their corresponding homologous acrocentrics were equally frequent. Triploid embryos (3.3 %) were also detected in (F<sub>1</sub><i>♂ </i>X NIH GP ♀) crosses. The translocations and Y chromosome served as paternal markers, permitting its origin to be determined in two of three cases (dispermy). One growth-retarded triploid had a distinctive anomaly: failure of lens induction and lack of optic cup development. The combined strain provided a complex of markers for determining polyploidy mechanisms, as well as a common genetic background for comparing behavior of the three translocations at M I.

Chromosome abnormalities in chicken (Gallus domesticus) embryos: types, frequencies and phenotypic effects.

Cytological screening of 4182 chick embryos from 10 strains and 5 strain crosses was performed to determine the types and frequencies of chromosome abnormalities. Gross phenotypic effects, such as growth retardation and malformation, were noted. Clues to the etiology of such chromosome aberrations were also sought. The following euploid series was observed: Haploid mosaics (A-Z/2A-ZZ, A-Z/2A-ZZ/3A-ZZZ, A-Z/A-W/2A-ZW/2A-ZZ, A-Z/A-W/2A-ZW/ 3A-Z?), diploid (2A-ZZ and 2A-ZW), triploid (3A-ZWW, 3A-ZZW, 3A-ZZZ, 3A-ZZZW) and tetraploid (4A-ZZWW and 4A-ZZZZ). Aneuploidy was observed as follows: Trisomy for chromosome numbers 1, 2, 3, 4 and double trisomy 2/5. Trisomy-4 with deletion of 50% of the long arm of one member of the trisomic triplet was observed. A 3A-ZWW embryo was found with two cell populations: one, disomic for chromosome 2 and 6; the other, tetrasomic for 2 and 6. Of the 4182 embryos sampled 1.4% were haploids, 97.5% diploids, 0.8% triploids, 0.1% tetraploids and 0.2% trisomics. On the average 10.8% of the early dead embryos were euploid (excluding diploid) or aneuploid. However, the range for euploidy and aneuploidy among strains was 2.3–23.7% of early deads. Haploid embryos were consistently underdeveloped at 4 days of incubation (D.I.), and died by 5–7 D.I. About 90% of (36) triploid embryos died at or before 4 D.I. The remaining 10% (normal embryos) died prior to hatching. Trisomic embryos were dead or underdeveloped at 4 D.I. Tetraploidy appeared to be lethal at a very early stage. The various strains examined had different overall rates of chromosome aberrations (0.4–8.9%), and also showed different varieties of such aberrations. The modes and possible causes of meiotic, mitotic and fertilization errors are considered. Genetic control of chromosome abnormalities, particularly haploidy, is postulated.

Trisomy-3,4 and triploidy (3A-ZZZW) in chick embryos: autosomal and sex chromosomal nondisjunction in meiosis.

The first known cases of trisomy-3, trisomy-4, and triploidy (3A-ZZZW) in chickens are reported. The trisomic embryos were near death at 4 days of incubation; the triploid embryo appeared normal and probably could have developed further. Although the triploid (3A-ZZZW) was found after sampling more than 2000 embryos, the trisomic-3 and trisomic-4 embryos were discovered in the first sampling of a line of chickens selected for low egg production. These studies suggest that the sex chromosomes (Z and W), as well as the autosomes, can undergo nondisjunction in meiosis of chickens.