Continuous fenestrated vessels of the intestinal villi and endocrine glands have a similar continuous structure but contain diaphragmed fenestra throughout their membrane. Continuous nonfenestrated capillaries of the skin and lung are joined together by cellular junctions, have a complete basement membrane (BM), and lack fenestra (pores) in their plasma membrane. There are three main structural classes of capillaries. In particular, the microvasculature, made up of the capillaries and postcapillary venules, has different properties to meet the unique requirements of the tissue they vascularize. Each segment has different properties depending on where they are in the vascular tree as well as which organ they vascularize. The vascular tree is comprised of arteries and arterioles, which deliver blood to the tissues, the capillary bed, which is essential for gas and nutrient exchange within tissues, and venules and veins, which drain blood from tissues. Understanding how these different cell populations interact to regulate the barrier properties is essential for understanding how the brain functions during health and disease.īlood vessels convey blood from the heart to each tissue and organ throughout the body, which is essential to deliver oxygen and nutrients to the tissues, remove carbon dioxide and metabolic waste from tissues, convey hormonal signaling among tissues, as well as mediate the interaction of the peripheral immune system with each tissue. The physiological barrier is coordinated by a series of physical, transport, and metabolic properties possessed by the endothelial cells (ECs) that form the walls of the blood vessels, and these properties are regulated by interactions with different vascular, immune, and neural cells. This precise control of CNS homeostasis allows for proper neuronal function and also protects the neural tissue from toxins and pathogens, and alterations of these barrier properties are an important component of pathology and progression of different neurological diseases. The blood vessels that vascularize the central nervous system (CNS) possess unique properties, termed the blood–brain barrier, which allow these vessels to tightly regulate the movement of ions, molecules, and cells between the blood and the brain. Blood vessels are critical to deliver oxygen and nutrients to all of the tissues and organs throughout the body.

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