Thyroxin-induced differential mortality of mouse embryos with cleft lip.

Administration of thyroxin in midgestation has been reported to prevent spontaneous cleft lip in term fetuses of genetically predisposed mice. Attempts to repeat this finding have given ambiguous results. The experiment was therefore repeated using CL/Fr and A/HeJ mice, and various thyroxin dosages. The embryos were examined on days 13 and 14, earlier in gestation than in previous studies. The usual complement of cleft-lip embryos, 21% in CL/Fr and 10% in A/HeJ, was present, but they were often dead. At all dosages of thyroxin that caused embryonic mortality, differential mortality of cleft-lip embryos at term after thyroxin treatment is thus due to their increased mortality, not to prevention of the lip defect.