Fig. 5.

In situ hybridization of 14.5 dpc mouse placenta for syncytin-A and -B expression. Scheme of a mouse mature placenta cross section: the labyrinthine zona (LZ) is the site of exchange between the maternal blood (Mb) and the fetal blood vessels (Fbv) and contains mononuclear trophoblast cells (MT) that terminally differentiate, by cell fusion, into syncytiotrophoblasts (ST) directly lining the fetal blood vessels; in Muridae, both cell types are arranged as a three-layered barrier (two syncytial and one mononuclear layer, not represented) between the maternal and fetal blood circulation; the spongiotrophoblast zona with the spongiotrophoblast cells (SPT) and the outermost layer with the trophoblast giant cells (GC) (producing hormone/growth factors) overlie the labyrinthine zona. (A) Negative control: tissue section probed with a digoxigenin-labeled sense syncytin-A riboprobe. (B and C) Tissue sections probed with digoxigenin-labeled antisense syncytin-A and -B riboprobes, respectively, disclosing hybridization signals only within the labyrinthine zona (the dotted lines delineate the separation between the labyrinthine and spongiotrophoblastic zona). (D and E) Magnification of the labyrinthine zona in B and C, with the arrowheads pointing to labeled structures with several nuclei. Nuclei were stained in blue by using DAPI. A–C, ×100; D and E, ×400.