Vascularization of the Placenta

Abstract

The development of a functional vascular network involves vasculogenesis and angiogenesis. Vasculogenesis is the de novo formation of blood vessels from the differentiation of angioblasts (Flamme et al., 1997;Risau, 1997). These vessels then expand by angiogenesis, the formation of new blood vessels from preexisting vessels, by either sprouting or nonsprouting mechanisms (Risau, 1997). There have been intensive efforts to determine the molecular mechanisms regulating vascular growth and development (Carmeliet and Collen, 1997), and much of the rationale for this has stemmed from the increasing clinical importance (Folkman, 1995) and therapeutic potential of modulating angiogenesis during various disease states (Battegay, 1995;Isner and Asahara, 1999). In normal adult tissue, vascular growth is a relatively rare event, indicated by capillary endothelial cell proliferation rates of only 0.01% to 0.14% in most tissues (Engerman et al., 1967). Significant angiogenesis in the adult is generally limited to pathologic conditions such as arthritis, retinopathy, chronic inflammation, wound healing, and solid tumor growth. However, the female reproductive tract of primates undergoes substantial vascular growth and remodeling associated with the menstrual cycle and pregnancy (Findlay, 1986;Reynolds et al., 1992;Gordon et al., 1995;Torry and Torry, 1997;Rees and Bicknell, 1998).