Chorionic Mesoderm Cells (ChB)

Chorionic mesoderm (stromal) cells make up the inner layer of the chorion bilayer. At the time of gastrulation (E6.5) in the mouse embryo, the epiblast-derived extraembryonic mesoderm contributes cells to form the chorion bilayer at E7.5, which later fuses with the allantois at E8.5-9.  In the human embryo, these cells migrate to the primary villi and transform them into secondary villi, as the mesenchyme expands to the villous tips. These cells give rise to the villous stromal cells and Hofbauer macrophage-like cells of the placenta. In addition, these cells are thought to be the source of de-novo placental hemangioblast differentiation (forming the first fetal capillaries at 18-20dpc) and of hematopoietic stem cells which reside in the primitive capillaries. Later, allantoic vasculature reaches the chorionic plate and fuses to form the more mature fetal vasculature.