The glycocalyx is present as soon as blood flow is initiated and is required for normal vascular development.

The glycocalyx, and the thicker endothelial surface layer (ESL), are necessary both for endothelial barrier function and for sensing mechanical forces in the adult. The goal of this study is to use a combination of imaging techniques to establish when the glycocalyx and endothelial surface layer form during embryonic development and to determine the biological significance of the glycocalyx layer during vascular development in quail embryos. Using transmission electron microscopy, we show that the glycocalyx layer is present as soon as blood flow starts (14 somites). The early endothelial glycocalyx (14 somites) lacks the distinct hair-like morphology that is present later in development (17 and 25 somites). The average thickness does not change significantly (14 somites, 182 nm ± 33 nm; 17 somites, 218 ± 30 nm; 25 somites, 212 ± 32 nm). The trapping of circulating fluorescent albumin was used to evaluate the development of the ESL. Trapped fluorescent albumin was first observed at 25 somites. In order to assess a functional role for the glycocalyx during development, we selectively degraded luminal glycosaminoglycans. Degradation of hyaluronan compromised endothelial barrier function and prevented vascular remodeling. Degradation of heparan sulfate down regulated the expression of shear-sensitive genes but does not inhibit vascular remodeling. Our findings show that the glycocalyx layer is present as soon as blood flow starts (14 somites). Selective degradations of major glycocalyx components were shown to inhibit normal vascular development, examined through morphology, vascular barrier function, and gene expression.