Placental development is particularly altered in trisomy of chromosome 21 (T21)-affected pregnancies. We previously described in T21-affected placentae an abnormal paracrine crosstalk between the villus mesenchymal core and villus trophoblasts. T21-affected placentae are known to be characterized by their hypovascularity. However, the causes of this anomaly remain not fully elucidated. Therefore, the hypothesis of an abnormal paracrine crosstalk between fetal mesenchymal core and placental endothelial cells (PLECs) was evocated. Villus mesenchymal cells from control (CMCs) and T21 placentae (T21MCs) were isolated and grown in culture to allow their characterization and collection of conditioned media for functional analyses (CMC-CM and T21MC-CM, respectively). Interestingly, PLEC proliferation and branching ability were less stimulated by T21MC-CM than by CMC-CM. Protein array analysis identified secreted proangiogenic growth factors in CMC-CM, which were reduced in T21MC-CM. Combined mass spectrometry and biochemical analysis identified spondin-2 as a factor decreased in T21MC-CM compared with CMC-CM. We found that exogenous spondin-2 stimulated PLEC proliferation and established that T21MC-CM supplemented with spondin-2 recovered conditioned media ability to induce PLEC proliferation and angiogenesis. Hence, this study demonstrates a crosstalk between villus mesenchymal and fetal endothelial cells, in which spondin-2 secreted from mesenchymal cells plays a central role in placental vascular functions. Furthermore, our results also suggest that a reduction in spondin-2 secretion may contribute to the pathogenesis of T21 placental hypovascularity.