Abstract
Development of the human placenta is characterized by continous angiogenesis which proceeds until term pregnancy which makes the placenta a unique model system for the studies of normal human angiogenesis. This report compares the patterns of PDGF-B and PDGF β-receptor protein expression in microcapillaries to those of larger blood vessels. Endothelial cells of microcapillaries co-express PDGF-B and PDGF β-receptor protein as analyzed by immunohistochemical staining of frozen sections of term placenta. Endothelial cells of larger vessels expressed the PDGF-B protein whereas PDGF β-receptor protein was not detectable. Positive staining with receptor antibodies was detected in the surrounding pericytes and smooth muscle cells. These data support the hypothesis that capillary endothelial cells proliferate via a PDGF-mediated autocrine loop. Endothelial cells situated in larger vessels, on the other hand, may stimulate growth of the vessel intima by paracrine interactions.