Evaluation of maternal and fetal factors in reduced postnatal survival of NC-eob mutant mice.

It has been reported that mouse pups bearing a newly identified mutation, eyelids open at birth (eob), as a homozygous condition often die within 7 days after birth although they have no external malformations other than open eyelids. We sought to determine what factors influence the viability of eob pups by performing a cross-fostering experiment using NC and NC-eob mice which are co-isogenic with each other. Viability indices of NC pups during days 0 to 7 of lactation were approximately 95% or more when they were fostered by either NC or eob mothers. However, the viability indices of eob pups were reduced to 87.3% and 36.7% when they were fostered by NC and eob mothers, respectively. Body weight gains of both NC and eob pups were slightly inhibited during the entire lactation period when they were fostered by eob mothers. In a second experiment, eob mothers were examined for milk-yielding capability and the fetuses examined for the presence of congenital visceral and skeletal malformations. Neither decreased amounts of suckled milk nor malformations were observed. Based on these results, we concluded that a remarkably high mortality of eob pups would be caused only when weak lethal factors in eob pups were combined with the slightly depressed pup-rearing capability in eob mothers.