From bubble to tube
The vascular system can cause serious problems: Important arteries become clogged, threatening heart attacks and strokes. The worst could perhaps be averted if new veins could grow. But before doctors do it, they should first understand how nature accomplishes this feat.
Life pulsates in them. They carry nutrients and the vital oxygen to the furthest corners of the body, they drive signaling substances to their destinations and the immune cells use them to get to where they are needed. Under no circumstances should they clog or tear - the consequences would be fatal.
Blood vessels are far more than simple tubes, they are multifunctional highways through the body with a three-layer structure: They are lined with epithelial cells on the inside, with elastic fibers on the outside, followed by a layer of smooth muscle cells and closing on the very outside connect them with connective tissue cells that anchor the blood vessels in the tissue.
But how do these complex structures grow, which eventually ramify through the entire body? So far, the tube system has largely been able to keep the secret of its creation to itself – hidden in the body and hidden from the prying eyes of researchers. It's too difficult to look under the skin. Science therefore used a trick: it took cells from the vascular system and allowed them to grow in the Petri dish. Based on such experiments, it is assumed that blood vessels form through the fusion of vacuoles – fluid-filled sacs within the cells. However, no one can say with certainty whether this happens in the living organism in the same way as in the laboratory.
Brant Weinstein from the National Institutes of He alth and his team have now been able to solve the mystery of how blood vessels form in living animals. The scientists combined cell culture experiments with experiments on zebrafish. This small tropical fish is very popular in science: it is easy to genetically manipulate and its transparent embryo allows a revealing view of its innermost being.
First, the research team set about making the blood vessels in the fish embryo clearly visible. Using a genetic trick, it covered the epithelial cells with a fluorescent protein so that they shone bright green under the microscope. However, the animal didn't mind this treatment, it developed quite normally like untreated conspecifics.
The scientists then looked into the prepared fish using what is known as two-photon microscopy. In doing so, they discovered vacuoles in the growing blood vessels of the fish embryos that resembled those previously observed in cell cultures of human epithelial cells. But are these cavities really important for the formation of blood vessels?
When the team took pictures of the developing blood vessels at short intervals, they could observe how every minute new vacuoles formed, merged with each other, grew larger and finally formed a large cavity that almost filled the entire cell.
Now the researchers wanted to know whether these hollow cells also join together to form a long, multi-cellular tube, as would be necessary for the formation of a vascular system. To do this, they embedded endothelial cells in a three-dimensional gel and placed them under the microscope. In fact, the cells made contact and their vacuoles merged, but the cytoplasm did not mix. So, at least in the lab, this is how tubes were made.
But does this also happen in the living organism? To answer this question, the team injected a red fluorescent dye into the vascular system of zebrafish embryos. As expected, the dye spread from vacuole to vacuole in the direction of blood flow.
Blood vessels do indeed arise from vacuoles, which connect to those of neighboring cells, eventually becoming a long tube in which the precious lifeblood pulsates.