Investigation of the possible roles of hofbauer cells in placental vascularization

The stroma of the placental villi contain numerous macrophages, so-called Hofbauer cells (HCs) which are of mesenchymal origin and are thought to function in many processes. Although there are many studies concerning placental vasculogenesis and angiogenesis, there has been a lack of evidence on the possible roles of HCs in these processes. We hypothesized that HCs play major roles in placental vasculogenesis and angiogenesis. Therefore we aimed to determine the potential roles of Hofbauer cells in vascularization by descriptive and functional experiments.Placental tissues were obtained from normal first-trimester pregnancies and were prepared for light and transmission electron microscopic investigations as well as for in vitro culture studies. Immunohistochemistry for VEGF, Flt-1, Flk-1, Ang-1, Ang-2, Tie-1, Tie-2 and Netrin-1 was applied to placental sections. The numbers and localizations of HCs and vascular structures were assessed by double immunohistochemistry with CD31 (endothelial cell marker) and CD68 (Hofbauer cell marker) under light and confocal microscopes. The ultrastructural features and types of HCs were identified by electron microscopy.HCs were isolated from fresh human first trimester placental tissues and characterized by CD68 and CD14 immunocytochemistry as well as TEM. The isolated placental macrophages were cultured and their conditioned media was collected for the determination of angiogenic factors by ELISA. Moreover, HUVECs were cultured with the collected conditioned media for the assay of angiogenesis.According to our results, majority of the HCs were found to be either in close contact with angiogenic cell cords and primitive vascular tubes or located in between them. Moreover, the number of HCs and vasculogenic structures were found to be significantly correlated. HCs expressed and secreted angiogenic factors such as VEGF, Ang-2 and Netrin-1. Strikingly, Hofbauer cell conditioned medium induced capillary formation of HUVECs cultured on a special matrix.Our study is of importance for being the first to determine that HCs not only have macrophagic properties, but also have conducting characteristics in the most important process for the placenta, which is vascularization. The findings of this study showed for the first time that there is a Hofbauer cell participation in vasculogenesis and angiogenesis and that these cells have angiogenic influences on endothelial cells. Further functional studies on the roles of HCs in placental vascularization will help elucidate the mechanisms that control normal placentation, which is important in uderstanding the pathophysiology of conditions associated with the impairment of vascular development during placentation such as intrauterine growth retardation and pre-eclampsia.