Both primitive blood vessels and blood cells originate from the mesenchymal stroma of the mesoderm. In the early embryo, before the somites are formed, some cells are concentrated in the outer mesoderm of the yolk sac and body tip to form clusters of cells of different sizes called blood islands. The islands gradually extend and interconnect to form the primitive capillary plexus. Arteries and veins originate from the same plexus of capillaries at the same time. The embryonic development of blood vessels can be broadly divided into three stages: the plexiform stage, the reticular stage, and the tubular stem formation stage. During the reticular phase, arteriovenous fistulas can arise if enlarged vascular traffic clusters and tends to fuse together. Histologically, numerous parallel vessels may be seen to fuse incompletely and intersect each other in multiple places, often in extremely small pieces called microscopic arteriovenous fistulas. The following are some of the most important things that you can do to prevent fistulas: 1) Eat a light diet, less spicy and stimulating food, fresh eggs or lean meat, and avoid seafood. 2, usually more exercise, enhance physical fitness, improve resistance, in poor body resistance, inflammation is easy to invade. 3, it is best to control the complication of inflammation early in time, in the case of acute infection, you can control the inflammation by local hot compresses or the use of antibiotics, if you do not understand can see a doctor to follow medical advice for treatment. (If an abscess has formed, an incision and drainage should be performed, it is best to avoid to this step yet). 4, during pregnancy try to avoid exposure to all factors that can affect the development of the fetus. 5, congenital arteriovenous fistula is a congenital disease, so there are no effective preventive measures, and the diagnosis requires differential diagnosis with lesions of various sites, as well as prevention of complications. In particular, it is important to prevent the transfer of embolic material through the fistula to cause other embolic diseases. 6. The primitive blood vessels and blood cells originate from the mesenchymal stroma of the mesoderm, and in the early embryonic body segments before they are formed, some of the cells in the yolk sac and the external mesoderm of the body tip are concentrated to form clusters of cells of different sizes, called blood islands. Injury to the blood islands results in a compressed fistula test.