The inner compartment of the rodent placenta - the labyrinth - contains the villi where nutrients pass from the maternal blood into the foetal blood. These villi are covered with a layer of mononuclear sinusoidal trophoblast giant cells (S-TGCs) and two layers of multinucleated syncytiotrophoblast cells (SynT-I and SynT-II). Now, on p. 2083, Simmons and colleagues solve the long-standing mystery of the developmental origins of this trilaminar structure. By examining gene expression in the mid-gestation mouse labyrinth, the researchers identify specific markers for each layer. They show that these markers are expressed in distinct layers in the chorion(the embryo's outer membrane) before villous formation begins and that the induction of the S-TGC and SynT-I precursors does not require the presence of SynT-II precursors, the first of the precursors to appear. Thus, they conclude, the three differentiated trophoblast cell types in the rodent labyrinth arise from distinct, autonomous precursors in the chorion that are patterned before its morphogenesis begins.