CHIMERIC ANALYSIS REVEALS ROLE OF PDGF RECEPTORS IN ALL MUSCLE LINEAGES

Blood vessels originate as simple endothelial cell tubes. It has been proposed that platelet-derived growth factor B polypeptide (Pdgfb) sec reted by these endothelial cells drives the formation of the surroundi ng muscular wall by recruiting nearby mesenchymal cells(1,2). However. targetted inactivation of the Pdgfb gene(3) or the Pdgf receptor beta (Pdgfrb) gene(4), by homologous recombination, does not prevent the d evelopment of apparently normal large arteries and connective tissue. We have used an in vivo competition assay in which we prepared chimaer ic blastocysts, composed of a mixture of wild-type (Pdgfrb(+/+)) and P dgfrb(+/-) or wild-type and Pdgfrb(-/-) cells, and quantified the rela tive success of cells of the two component genotypes in competing for representation in different cell lineages as the chimaeric embryos dev eloped. This study revealed that the participation of Pdgfrb(-/-) cell s in all muscle lineages (smooth, cardiac, skeletal and pericyte) was reduced by eightfold compared with Pdgfrb(+/+) cells, and that partici pation of Pdgfrb(+/-) cells was reduced by twofold (eightfold for peri cytes). Pdgfrb inactivation did not affect cell contribution to non-mu scle mesodermal lineages, including fibroblasts and endothelial cells. Chimaera competition is therefore a sensitive, quantitative method fo r determining developmental roles of specific genes, even when those r oles are not apparent from analysis of purebred mutants; most likely b ecause they are masked by homeostatic mechanisms.