"Chemical surgery" as an approach to study morphogenetic events in embryonic mouse limb

Cytosine arabinoside (Ara-C) or retinoic acid (RA) was injected into pregnant mice in doses which induce a high incidence of limb defects. Within 4 hr of the treatment, extensive cell death was observed in the embryonic limb buds. However, the location of necrotic cells and the eventual limb defects were different for the two chemicals. Ara-C killed cells in those regions of the limb which were undergoing active proliferation. RA, on the other hand, had no effect on actively dividing cells but was lethal to cells of chondrogenic lineage at stages when their proliferation rate had fallen 7- to 10-fold below the original rate. In all cases, an excellent correlation between the location of dead cells (as seen 4 hr after drug treatment) and the eventual bony defects (as seen in the term fetuses) was observed. The unique properties of Ara-C and RA have been exploited in determining the relative levels of cytodifferentiation in the embryonic mouse limb buds. It is concluded that in the limbs of early 11th day mouse embryos (comparable to chick stage 19–20), differentiation of future skeletal elements has not yet begun. However, by the 12th day (comparable to chick stage 23), cell populations destined to form most of the future cartilages (except for digits) have already been established.