Developmental analysis of cardiovascular system of 45,X fetuses with cystic hygroma.

There have been few pathological investigations of 45,X embryos and fetuses from a developmental point of view. Since most 45,X embryos and fetuses are lost prenatally, it is important to investigate them morphologically in order to elucidate the pathogenesis of the abnormalities. In this study, 13 45,X fetuses with cervical cystic hygroma were examined between 12 and 23 weeks of pregnancy. Every case had a hypoplastic thymus. The aortic valve was bicuspid in 11 cases and unicuspid in 2 cases. The aortic arch showed tubular hypoplasia between the left carotid artery and the left subclavian artery in 12 cases and type B interruption in one case. Smooth muscle cells and elastic fibers were reduced in number in the hypoplastic aortic arch. These results suggest hypoplastic development of the fourth branchial arch. Combined abnormalities between the aortic arch and aortic valve are not infrequently observed in DiGeorge anomaly. A similar developmental mechanism apparently underlies the pathogenesis of 45,X embryos. Possible genes causing the abnormalities are discussed.