Pathogenesis of various forms of double outlet right ventricle in mouse fetal trisomy 13

Nuchal Translucency and Congenital Heart Defects

Nuchal translucency comprises a temporary accumulation of fluid in the subcutaneous tissue on the back of a fetus's neck, which accompanies the crown-rump length and is observed through an ultrasound performed between 11 and 13 weeks + 6 days gestation. Nuchal translucency is considered to be above normal when values are higher than the 95th/99th percentile or equal to or higher than 2.5/3.5 mm. The first connection between increased nuchal translucency and the presence of congenital heart defects is described in the study of Hyett et al., who observed that they are directly proportional. Since that time, several studies have been conducted to understand if nuchal translucency measurements can be used for congenital heart defect screening in euploid fetuses. However, there is great variability in the estimated nuchal translucency cutoff values for congenital heart defect detection. The purpose of this review was to understand how increased nuchal translucency values and congenital heart defects are related and to identify which of these defects are more frequently associated with an increase in these values.

The outflow tract of the heart is recruited from a novel heart-forming field

As classically described, the precardiac mesoderm of the paired heart-forming fields migrate and fuse anteriomedially in the ventral midline to form the first segment of the straight heart tube. This segment ultimately forms the right trabeculated ventricle. Additional segments are added to the caudal end of the first in a sequential fashion from the posteriolateral heart-forming field mesoderm. In this study we report that the final major heart segment, which forms the cardiac outflow tract, does not follow this pattern of embryonic development. The cardiac outlet, consisting of the conus and truncus, does not derive from the paired heart-forming fields, but originates separately from a previously unrecognized source of mesoderm located anterior to the initial primitive heart tube segment. Fate-mapping results show that cells labeled in the mesoderm surrounding the aortic sac and anterior to the primitive right ventricle are incorporated into both the conus and the truncus. Conversely, if cells are labeled in the existing right ventricle no incorporation into the cardiac outlet is observed. Tissue explants microdissected from this anterior mesoderm region are capable of forming beating cardiac muscle in vitro when cocultured with explants of the primitive right ventricle. These findings establish the presence of another heart-forming field. This anterior heart-forming field (AHF) consists of mesoderm surrounding the aortic sac immediately anterior to the existing heart tube. This new concept of the heart outlet's embryonic origin provides a new basis for explaining a variety of gene-expression patterns and cardiac defects described in both transgenic animals and human congenital heart disease.

A heart segmental defect in the anterior-posterior axis of a transgenic mutant mouse

A recessive lethal insertional mutation on chromosome 13 has been identified in a transgenic mouse line that displays a segmental form of cardiac defect along the anterior-posterior axis in all homozygous mice identified. The most anterior segment (future conus and right ventricle) of the single heart tube fails to develop normally and the endocardial cushions in both the conus and the atrioventricular regions are missing. Analysis of the beta-galactosidase reporter portion of the transgene during embryonic development shows a segmental expression of activity primarily in the defective outlet of the primitive heart. In addition to expression in the heart tube, hemizygous embryos show transgene expression in the chondrogenic regions of first and second branchial arches, the appendicular skeleton, and the dermal papillae of the vibrissae. The restricted pattern of beta-galactosidase expression in the heart can be disrupted with retinoic acid exposure and extended posteriorly along the anterior-posterior axis in hemizygous mice. Although cushion mesenchyme fail to form in the homozygous mutant, the myocardial and endothelial cells explanted from the mutant atrioventricular, but not the conus, are capable of forming mesenchyme in vitro. Mice trisomic for chromosome 13 have also been shown to display segmental anomalies associated with the anterior primitive outlet segments of the heart. Our data show that this insertional mutation identifies a new gene locus, hdf (heart defect), on mouse chromosome 13 that may be required for mechanisms that initially establish and/or maintain continued development of the anterior limb of the developing heart. The hdf mouse mutation also provides a new model system to evaluate the molecular requirements of normal endocardial cushion formation and the segmental interactions that form the adult heart.

Mouse fetal trisomy 13 and hypotrophy of the spinal cord: effect on calbindin-D28k and calretinin expressed by neurons of the spinal cord and dorsal root ganglia

Trisomy 13 was detected in 10% of mouse embryos obtained from pregnant females which were doubly heterozygous for Robertsonian chromosomes involving chromosome 13. The developing dorsal root ganglia and spinal cords were examined in trisomy 13 and littermate control mice between days 12 and 18 of gestation (E12-18). The overall size of the dorsal root ganglia and number of ganglion cells within a given ganglion were not altered, but the number of neurons immunoreactive for calbindin and calretinin was reduced. The trisomic spinal cord was reduced in size with neurons lying in a tightly compact distribution in the gray matter. In trisomic fetuses, the extent of the neuropil of the spinal cord was reduced, and may represent a diminished field of interneuronal connectivity, due to reduced arborization of dendritic processes of the neurons present, particularly of calbindin-immunostained neurons. Furthermore, the subpopulation of calretinin-immunoreactive neurons and axons was also reduced in developing trisomic gray and white matter, respectively. Thus, overexpression of genes on mouse chromosome 13 exerts a deleterious effect on the development of neuropil, affecting both dendritic and axonal arborization in the trisomy 13 mouse. The defect of calbindin or calretinin expression by subsets of dorsal root ganglion or spinal cord neurons may result from deficient cell-to-cell interactions with targets which are hypoplastic.