Synthesis of glycoproteins by endothelial cells in embryonic blood vessels

Abstract

Blood vessels developing in the chorioallantoic membrane (CAM) of the chick embryo were examined for the ability of endothelial cells to synthesize and secrete extracellular matrix glycoproteins (including proteoglycans). CAMs, aged 4 to 16 days, were labeled in vivo for 24 hr by injection of [ 3H]glucosamine or H 2 35SO 4. Biochemical analysis revealed that these precursors were incorporated predominantly into glycoproteins. Light microscope autoradiography was used to quantitate the extent of endothelial cell labeling. Synthesis of [ 3H]glucosamine-labeled glycoproteins was high in 4- to 8-day CAMs, decreased from Days 10 to 12, and was low thereafter. These glycoproteins remained associated with the cell and were not secreted into the extracellular matrix. Synthesis of 35SO 4-labeled glycoproteins by endothelial cells was moderate in 4- to 10-day CAMs, increased from Days 10 to 12, and remained higher thereafter. Pulse-labeling of 12-day CAMs in vitro demonstrated that 35SO 4-labeled material was secreted into the extracellular matrix within 1 hr where it accumulated during the next 24 hr. Correlation of these results with those of a previous study indicate that endothelial cells in CAM vessels synthesize mainly nonsulfated glycoproteins for intracellular use during the early phases of blood vessel development when endothelial cells are proliferating, and synthesize mainly sulfated glycoproteins (probably proteoglycans) during their later states of development when endothelial cells undergo cytodifferentiation. Further characterization of the secreted glycoproteins is required before they can conclusively be identified as proteoglycans.