Endocardial cushion development and heart loop architecture in the trisomy 16 mouse

Area of Science:

• Developmental Biology
• Cardiovascular Biology
• Genetics

Background:

• Murine trisomy 16 (Ts16) is a model for Down syndrome, exhibiting a high incidence of atrioventricular septal defects (AVSDs).
• The precise mechanisms underlying AVSD development in Ts16 models remain largely unknown.
• Previous hypotheses suggest abnormal endocardial cushion development contributes to AVSDs.

Purpose of the Study:

• To identify initial cardiac defects in Ts16 embryos.
• To characterize the pathogenesis of AVSDs in this model.
• To lay the groundwork for investigating molecular mechanisms of cardiac malformations.

Main Methods:

• Utilized an Rb(11.16)2H/Rb(16.17)7Bnr x C57BL/6J mouse cross to generate Ts16 embryos.
• Examined embryos at days 10-11 of gestation (15-43 somite pairs).
• Employed serial sectioning and stereomorphometry to analyze endocardial cushion volume and mesenchymal cell counts, and measured heart tube architecture.

Main Results:

• Ts16 embryos exhibited significantly reduced cell density in endocardial cushions, with increased cushion volumes compared to normal littermates.
• Abnormally enlarged cushion volumes in Ts16 embryos were observed as early as the 18-somite stage, preceding mesenchymal cell invasion.
• Subtle abnormalities in Ts16 heart tube architecture were detected, including an altered angle between the atrioventricular canal and the first pharyngeal cleft.

Conclusions:

• The primary cardiac defect in Ts16 mice may originate in the myocardium, influencing heart tube morphology.
• Endocardial cushion mesenchymal population changes appear to be secondary events.
• Further research is needed to determine if AVSDs result from myocardial or endocardial cushion abnormalities, or both.