Fig. 6. Model for mouse labyrinth formation from a pre-patterned chorion. Note that Ctsq expression within S-TGCs is not fully evident until E12.5. No exclusive marker has been identified for S-TGC precursors, although cells lining the early maternal sinusoids are Hand1⁺/Pl1^-. However, Hand1 expression is not restricted to cells lining the maternal sinusoids in the way that Ctsq is restricted to S-TGCs in mid-gestation; Hand1 is expressed in all TGC subtypes and within the ectoplacental cone and chorion (not shown). It is the combined Hand1⁺/Pl1^- nature of the cells lining the maternal sinusoids, together with their location, that implies they are S-TGC progenitors. For simplicity, cell bodies of Hand1⁺/Pl1^- cells are shown only along the top of the chorion. Light gray, allantois; gray, chorion; dark gray, ectoplacental cone (EPC); red, extraembryonic mesoderm-derivatives; yellow, Gcm1⁺ chorionic trophoblast cells; orange, Gcm1/Synb/Cebpa⁺ chorionic trophoblast cells; green, Syna⁺ trophoblast cells; blue, developing sinusoidal giant cells (S-TGCs) which will subsequently express Ctsq; mbs, maternal blood sinus.