Dissecting the developmental origins of organ-specific vascular beds [electronic resource]
Keywords: Biochemistry

One of the challenges facing modern medicine is to understand mammalian organ formation. Each organ has a specialized function and form, yet they are built from similar parts. One important part of each organ is the circulatory system, composed of arteries, veins, and capillaries, which is responsible for delivery of oxygen and nutrients and for the removal of carbon dioxide and metabolic waste. For centuries scientists have been trying to understand how the circulatory system functions and more recently how it forms. In this thesis, I focus on the development of the mouse pulmonary vasculature, the circulatory system that is responsible for transporting blood from the heart to the lungs where oxygenation occurs, and back to the heart where the oxygenated blood is pumped throughout the body. The lung is a complex organ made up of multiple hollow airway tubes that must form in coordination so that proper gas exchange can occur. In the first chapter I review the function and development of the circulatory system. In chapter two I present the experiments completed to determine the cellular origins of the blood vessels of the mouse lung and heart. Currently there are two major models for how the pulmonary arteries of the mouse form, vasculogenesis of lung mesenchymal endothelial progenitors and sprouting angiogenesis from vessels outside the lung. Using fate mapping I directly test the dominant model, vasculogenesis, and show that there are no endothelial progenitor cells resident in the lung mesenchyme. To determine if a vessel outside the lung is forming the pulmonary arteries I performed a clonal analysis labeling single endothelial cells before pulmonary artery formation and followed the fates of their daughter cells. I found a surprising multipotent progenitor population of endothelial cells, the primitive plexus of the lung, that gives rise to pulmonary arterial, venous, and plexus endothelial cells. I showed that the transition from plexus to arterial fate occurs in a leakproof manner in the lung by performing intracardiac perfusion experiments of fluorescently labeled dextrans and lectins. I also demonstrated a similar mechanism of leakproof plexus remodeling occurs in the heart during the development of the coronary arteries and veins. In chapter three I discuss experiments done to understand the molecular mechanisms controlling pulmonary artery formation. I describe an in silico in situ hybridization screen I performed to identify candidate molecules. I describe the expression pattern of BMP pathway activity in the lung as being restricted to the pulmonary arteries, a subset of the plexus and budding airways, using a transgenic mouse line that is only expressed in cells actively undergoing BMP signaling. I analyzed mice homozygously deficient for the BMP antagonists Gremlin and BMPER, and showed that there is no abnormal pulmonary artery formation or patterning phenotype. I also describe experiments in which I have deleted the BMP Receptor 2 from endothelial cells and again there is no abnormal pulmonary artery formation or patterning phenotype. I describe the expression pattern of VEGF in the lung as being mesenchymal during early lung formation, and show that mesenchymal deletions of VEGF do not result in abnormal pulmonary artery formation or patterning phenotypes. I found that multiple single gene manipulations do not result in abnormal phenotypes suggesting that pulmonary artery formation and patterning is robust and likely involves the interaction of multiple signaling pathways. This work describes a common mechanism of organ specific vascular bed formation that has not been previously described at a cellular and molecular level. I hope these studies provide a foundation for understanding how the vascular system of many organs form during organogenesis.