a, A schematic diagram of a normal placental villus in vivo compared to a trophoblast organoid. The basement membrane (BM) beneath the VCT is contiguous with the stromal villous core in vivo and with the Matrigel in vitro. The SCT contacts maternal blood in the intervillous space in vivo, and forms in the centre of the organoids. b, Immunohistochemical staining for TP63 in first trimester placenta and trophoblast organoids (representative images from n = 14). TP63 is expressed in VCT. Scale bars, 50 μm and 20 μm (insets). c, Representative images of TP63, Ki67 and DAPI staining in trophoblast organoids by confocal microscopy (n = 3). Cells on the outside of the organoids are TP63+ and Ki67+. Scale bars, 20 μm. d, Confocal microscopy images of trophoblast organoid stained for EdU, EPCAM and DAPI, showing fewer proliferating cells (white arrowheads) as the organoids enlarge. Scale bars, 50 μm. Representative images from n = 3. e, Immunohistochemical staining for the SCT markers CD46 and CD71 in first trimester placenta and trophoblast organoids (representative images from n = 20). CD46 and CD71 stain the syncytial brush border. Scale bars, 50 μm and 20 μm (insets). f, Carnegie stage 5b embryo (approximately 9 days after fertilization) from the Carnegie Collection at the early lacunar stage (number 8171). Courtesy of A. Enders and the Centre for Trophoblast Research (https://www.trophoblast.cam.ac.uk/Resources/enders). Arrows point to examples of cavities that appear in the primitive syncytium owing to fluid accumulation before the coelomic cavity and the embryo have fully developed. g, Similar cavities in placental tissue samples from first trimester (6–9 weeks gestation) and in syncytium in the centre of trophoblast organoids. Boxed areas are shown at higher magnification (bottom). Scale bars, 200 μm (top) and 50 μm (bottom). Similar morphology seen in at least five early placental villi and in all organoids. ICM, inner cell mass; Pr.Syn., primitive syncytium; Str, stromal core.