Ultrastructural analysis of chromosome translocation-induced neural tube defects

Osteogenesis of the mandibular arch in Ts16 mice

Males and females homozygous for the Robertsonian translocation specific for chromosomes 16 and 17 Rb(16.17)Bnr and male and females homozygous for the Robertsonian translocation for chromosomes 6 and 16 Rb(6.16)24Lub were bred to produce double heterozygotes [Rb(16.17)Bnr/Rb(6.16)24Lub]. Experimental data were based on 156 features: 70 euploid (control), 86 trisomic. Affected fetuses were identified by decreased size, shortened faces (flattened snouts), oedema, petechiae, open eyelids, and dysplastic ears. Identification of trisomics was substantiated by karyotyping the metaphasic spreads. Five gestational days were studied (14-18). Fetal age was assumed to be accurate as fertilization occurred within half an hour of copulation. Euploid specimens followed normal developmental paths of chondrogenesis, osteogenesis and related tissues. In trisomics, developmental faults increased unequally between gestational days 15 and 17: some tissues were mildly and others acutely affected. Among the trisomic disorders were diminished growth, lagging mitotic activity, and retarded and poorly ordered tissue development, especially of bone. All of these contributed to hypoplasia, hypocellularity, reduced vascular supply and enlarged intercellular spaces. Intensities of the mandibular abnormalities varied among litters and littermates. The severity of the developmental disorders of an individual Ts16 specimen differed among the tissue components studied. Of the trisomic mandibular tissues, bone was most frequently involved and Meckel's cartilage least.

Basal lamina and extracellular matrix alterations in the caudal neural tube of the delayed Splotch embryo

Regional patterns of deposition of laminin (LN), fibronectin (FN), type IV collagen (IV), and heparan sulfate proteoglycan (HSPG) were examined during the formation of the caudal neural tube in embryos homozygous for the delayed Splotch gene and in their normal littermates. Delayed Splotch embryos had neural tube closure defects which extended from the posterior neuropore into the region formed by secondary neurulation. During posterior neuropore closure these components were normally restricted to forming basal laminae, with FN and HSPG additionally deposited in the mesenchyme. Unlike control embryos in which medial regions of the neuroepithelial basal lamina contained greatest amounts of all four, the dorsolateral zone contained less LN and IV and more FN and HSPG, in affected embryos these components were less densely deposited medially, reflecting perhaps the poor structural organization of the notochord. The neuroepithelial basal lamina was often disorganized and wavy compared to the linear pattern typical of controls. By the 12th day, the posterior neuropore of controls had closed and secondary neurulation was underway; however in delayed Splotch embryos, the neural folds remained widely splayed and epithelium newly formed via secondary neurulation extended that abnormally open configuration to the tip of the tailbud. In controls, with mesenchymal cell aggregation FN and HSPG were displaced from between cells to the forming basal lamina. As a central lumen formed within the aggregate LN and IV were added to the basal lamina, and the newly formed epithelium merged with the anterior neural tube. In delayed Splotch embryos, FN and HSPG were incompletely removed from aggregating cell surfaces, the normal morphogenetic cell shaping changes failed to occur and in many embryos a central lumen did not form; the overgrown, aggregated cells merging with the abnormally splayed anterior neural folds. In addition, the critical enrichment of FN and HSPG present between newly formed and consolidated neuroepithelium was displaced in delayed Splotch embryos.

Developmental consequences of autosomal aneuploidy in mammals

Autosomal aneuploidy in mammals adversely affects developmental processes. In human beings, for example, trisomy 21 is the most frequent aneuploidy detected among newborns and the most common known genetic cause of mental retardation. In this review, several hypotheses are discussed that have been proposed to explain the mechanisms by which aneuploidy (especially trisomy) disrupts development. These mechanisms included specific gene dosage effects, generalized disruption of genetic homeostasis, and the influence of the parental origin of the duplicated chromosome. The availability of specific chromosomal rearrangements in mice, coupled with selective breeding schemes, permits generation of aneuploidy of specific chromosomes or chromosomal segments on controlled genetic backgrounds, thus enabling the systematic study of the causes and consequences of defined aneuploidy. Phenotypic characteristics associated with a number of specific aneuploidies in the mouse are discussed. Emphasis is placed on the effects of trisomy 16. Genetic homology between mouse chromosome 16 and human chromosome 21 has led investigators to suggest that analogous mechanisms will be responsible for the developmental abnormalities produced in these respective aneuploidies. Analysis of trisomy 16 mice from the organismal to the subcellular level has revealed a number of phenotypic characteristics (particularly neurobiologic ones) shared with human trisomy 21. The dosage effects of shared genes (or their products) may contribute to the development of these features.

Autosomal aneuploidy in mice: generation and developmental consequences

Spontaneous aneuploidy in the mouse is uncommon, but specific mating schemes have been developed that produce aneuploid conceptuses at high frequencies. The most commonly reported aneuploid condition in the mouse is autosomal trisomy, in which there is an extra copy (in whole or in part) of a chromosome. In this review, we present several of the schemes used in producing trisomic, partially (tertiary) trisomic, and monosomic conceptuses and summarize the developmental consequences that are associated with each of the autosomal trisomies of the mouse.