Heat shock-induced homeotic transformations of the axial skeleton and associated shifts of Hox gene expression domains in mouse embryos.

Pregnant ICR mice were immersed in water at 42 degrees C for up to 15 min on Day 8.5 of gestation (plug day = Day 0), and their term fetuses were double stained with alcian blue and alizarin red S for skeletal examination. Heat exposure for 15 min induced homeotic vertebral transformations in more than one-third of the living fetuses, in which the morphologic identity of vertebrae (T6-S1) was shifted anteriorly by one or two segmental levels. The frequency of fetuses with vertebral transformations and the degree of the shift of vertebral identity were dependent on the length of heat exposure. The expression domains of Hoxa-7, Hoxc-8, and Hoxc-9 genes as examined by whole mount in situ hybridization were found to be shifted anteriorly in heated embryos. The heat-induced shifts of Hox gene expression domains were consistent with the observed vertebral transformations and suggested correlation or colinearity with the clustered organization of the Hox genes. The result of the present study indicates that a brief heat shock at a critical stage of differentiation can interfere with the normal establishment of Hox codes and subsequently, perturb the specification of vertebral identity.