Examination of the Axial Skeleton of Fetal Rodents

The axial skeleton represents one product of the metameric segregation of the mesoderm in the developing embryo. The mechanisms underlying this pattern formation remain poorly understood. Genetic alterations, either resulting from spontaneous mutation or as a result of xenobiotic exposure, may disrupt this patterning and lead to a variety of skeletal alterations. Chemical agents including valproic acid (1 ), retinoic acid (2 ), salicylate (3 ), and acetazolamide (4 ) have been shown to cause supernumerary ribs in rodents. Fewer agents cause a reduction in the number of ribs or vertebrae. Agents or conditions in the latter category include boric acid (5 ,6 ), arsenate (7 ), methanol (8 ), 2-chlorodeoxyadenosine (9 ), and hyperthermia (10 ). Effects on axial development have also been associated with changes in homeotic gene expression (11 –13 ) as well as deletion of the bmi-1 proto-oncogene (14 ). Posteriorization of Hoxa10 expression has been associated with lumbar ribs in mice following prenatal exposure to sali-cylate (15 ) as well as retinoic acid (2 ). Careful characterization of the morphology of the axial skeleton is critical in identifying homeotic shifts and other changes produced by xenobiotics or altered gene expression. Here we present our methods for preparing and examining rodent skeletons, including anatomical landmarks and a brief discussion of methods for analyzing and presenting these data.