An unusual monolayer of cells lines the interface between the basal plate and the intervillous space in human term placenta but not the chorionic villi. Our recent descriptions of it are based on advanced microscopy, phenotyping and cytogenetic approaches. The papers show that the layer is partly epithelial (ectoderm) and partly endothelial (mesoderm): it is partly derived from the fetus and partly from the mother. This first accurate description of a naturally occurring human allo-epi-endothelium (monolayer of cells derived from two embryological germ layers and two individuals) is of interest in anatomy, obstetrics and gynaecology, developmental biology, histology and immunology. The most extensive evidence for this mosaic applies to the intervillous space lining layer of the basal plate where the endothelial proportion is of the order of 50%; it extends throughout central, intermediate and peripheral parts of the basal plate and is a consistent feature of the intervillous space lining of the chorionic plate also. Its presence lining chorionic plate is noteworthy as it includes the furthest parts of the sinus from the supplying and draining vessels which are endothelial lined.