Abstract
Embryos carrying null mutations of both retinoid X receptors alpha and beta (RXRalpha-/-/RXRbeta-/- mutants) were generated. These mutant embryos die between 9.5 and 10.5 days of gestation and display a wide range of abnormalities. The cause of the lethality appears to be the lack of formation of the labyrinthine zone of the chorioallantoic placenta. In a thorough analysis of mutant conceptuses, we establish that RXRs, through heterodimerization with retinoic acid receptors, are essential for postimplantation embryonic development before placentogenesis. RXRs are also essential for the formation of the chorioallantoic placenta, most probably through RXR/peroxisomal proliferator-activated receptor-gamma heterodimers. Interestingly, as a RXR ligand appears dispensable, placentogenesis must be controlled by a yet unknown hormonal ligand(s) activating the heterodimerization partner(s) of RXRs.