The experiments described above strongly suggest that at least some endothelial cells express receptors for PSG1. Therefore, we hypothesized that PSG1 could play a role in uterine angiogenesis and capillary morphogenesis. To investigate this further, we performed in vitro endothelial tube formation assays. HDMECs were cultured and seeded onto a collagen gel at passage 3 or 4 and then cultured until confluence. Afterward, they were stimulated with PSG1-Fc or the control protein (FLAG-Fc) alone or in combination with VEGFA. HDMECs exposed only to VEGFA were used as a positive control and formed tubes as expected (Fig. 5A) . Cells cultured in basal medium, the negative control, showed no tube formation (Fig. 5B ). PSG1 alone, applied at concentrations of 100 and 500 ng/ml, induced tube formation as shown in Figure  5C (100 ng/ml of PSG1). Simultaneous application of PSG1 at 25 and 100 ng/ml of VEGFA increased the network of VEGFA-induced endothelial tubes (Fig. 5D) as assessed semiquantitatively under microscopic evaluation. When the control protein FLAG-Fc was used instead of PSG1, we could not observe the formation of tubes or the increase in VEGFAmediated tube formation (data not shown). Neither PSG1-Fc nor FLAG-Fc applied at concentrations 100 and 500 ng/ml had an effect on wound distance in comparison to the negative control, in which the cells were exposed to basal medium only (data not shown). In contrast, VEGFA treatment of the cells, which was used as positive control, reduced the wounding distance within 12 h after wound induction (not shown).