Spatial and temporal distribution of Tie-1 and Tie-2 during very early development of the human placenta.

Vasculogenesis in the human placenta comprises differentiation and growth of newly forming blood vessels derived from hemangiogenic stem cells within the mesenchymal core of villi. In a second stage, angiogenesis leads to the expansion and remodeling of the already existing vessels. At present, relatively little is known about the regulatory mechanisms of vasculogenesis and angiogenesis during very early placentation. Using placental villous tissues from days 22 to 48 of pregnancy, we analyzed the spatial and temporal expression of Tie-1 and Tie-2 in parallel to vascular maturation in the human placenta. In immunohistochemistry both receptors, Tie-1 and Tie-2 show a cell and villous type specific expression during this early phase of placental development. Especially, cytotrophoblast and hemangiogenic cell cords in mesenchymal villi and Hofbauer cells in immature intermediate villi have the strongest immunoreactivities. Western blot analysis showed that no significant changes were detected for Tie-1 and Tie-2 as pregnancy advanced. Moreover, phospho-Tie-2 levels did not change significantly in parallel to pregnancy ages. We conclude that both receptors are involved in angiogenesis as well as vascular modulation of early vessels. Due to their spatial distribution we speculate on an additional role in regulation of villous and extravillous trophoblastic behavior.