External examination of limb positions in near-term mouse fetuses: an experimental study and review of the literature published in Teratology

The accidental finding of near-term mouse fetuses exhibiting limb positions that could have been interpreted as malformations but that were, in fact, spurious, led us to study the posture of the limbs in the offspring of 100 CD-1 mouse dams. A total of 1,015 fetuses (18 days-old, vaginal plug: day 0), was examined. Fifteen of these fetuses presented positions that deviated markedly from the norm and were therefore considered to be "unusual," although the usual position was recovered in 6 of the 15 by placing the affected limb in a standard position and eventually eliciting locomotion. A broad review of the literature was made in order to evaluate our results fully. This review included 43 reports published in Teratology that recorded abnormal limb positions in near-term mouse and rat fetuses, and another 9 reports where these were not referred to but that included photographs showing such abnormalities. The data obtained from our experimental study and review led us to conclude that for a correct record of abnormal positions in near-term mouse and rat fetuses, authors must follow appropriate, clearly described technical procedures, including maneuvers to rule out spurious displacements; give adequate information in the legends of the photographs; use unequivocal terminology; and carry out further research to determine accurately the range in variation of normal limb positions.