Placental circulation: Clinical significance.

Investigation of the mechanisms regulating development of new vessels is crucial to our understanding of both tumor biology and early pregnancy development. The placental vascular system develops through two distinct processes: vasculogenesis and angiogenesis. During vasculogenesis, endothelial progenitor cells angioblasts form a primitive vascular network. Angiogenesis represents the development of new vessels from preexisting vessels. Placental vascular growth begins as early as 21 days post conceptionem and continues throughout gestation. The development of villous capillaries from hemangioblastic cells can be observed until 10-12 weeks of gestation. From 12 weeks onwards, the capillaries coil, bulge, form sinusoids and protrude towards the trophoblastic layer. Although reliable signs of formation of new vessels are absent in the second half of pregnancy, some capillary sprouts can be seen. Human placenta is a rich source of angiogenic substances and these may play an important role in the regulation of placental vessel formation as well as in maternal vascular adaptation to pregnancy. Disturbances in the placental vascular development as demonstrated by reduced fetal capillary branching, maldevelopment of the villous tree and/or alterations in the expression of angiogenic factors in the fetomaternal unit, are associated with following pathologic conditions in pregnancy: early pregnancy losses preeclampsia intrauterine growth restriction (IUGR) gestational diabetes and maternal diabetes mellitus Our understanding of the different molecular and functional aspects of the placental vessel formation during gestation might allow the establishment of new therapeutic strategies for the treatment of pregnancy related pathology.