HISTOLOGY REVIEW Vasculature Dr. Tim Ballard Department of Biology and Marine Biology UNC Wilmington

Overview of vessel histology Blood vessels, with the exception of the capillaries and the smallest venules, have the same basic structure. There are three coats or tunics that form the wall of the vessel. Tunica intima endothelium basement membrane internal elastic lamina Tunica media circular smooth muscle external elastic lamina Tunica adventitia dense connective tissue Capillaries are essentially little more than an endothelium with a very thin basement membrane. As such, this allows them to be the level of the vasculature at which exchange between the blood and the tissue takes place.

Large or elastic artery A = tunica intima (interna) B = tunica media C = tunica adventitia (externa) Aorta–section – H&E – 4x objective A B C The aorta is a large elastic artery. As such it has 40 – 70 layers of elastin fibers in its wall, with some collagen fibers, and fewer smooth muscle fibers interspersed between.

Large or elastic artery With H&E, elastin fibers, collagen fibers, and smooth muscle cells are hard to discern. Under high power, focus up and down with the fine focus knob, and look for fibers that are birefringent (they look like they glow). These are elastin fibers. Aorta–section – H&E – 40x objective

Large or elastic artery Verhoeff’s stain is used to visualize elastin fibers since it stains them purple-black. How many layers of elastin fibers can you count in the tunica media? Large vessels are so thick that they must have their own blood supply. This is the vasa vasorum. A = tunica intima (interna) B = tunica media C = tunica adventitia (externa) arrows = vasa vasorum Aorta–section – Verhoeff– 4x objective A B C

Large or elastic artery The purple-black fibers are elastin fibers. The pink fibers are smooth muscle cells or collagen fibers. Aorta–section – Verhoeff – 40x objective

Medium arteries and veins A = tunica intima B = tunica media C = tunica adventitia Companion vessels – H&E– 10x objective vein artery Muscular arteries are smaller than elastic arteries and have more smooth muscle than elastin fibers in the tunica media. In addition, there are two specialized structures, the internal elastic lamina and the external elastic lamina, which are easily seen with the Verhoeff stain. As a general rule, medium arteries have at least one companion vein alongside. Be sure to compare the differences in relative thicknesses of the wall components. A B C B C

Medium artery artery – Verhoeff -- 10x objective internal elastic lamina (black lines) A = tunica intima B = tunica media C = tunica adventitia A B C external elastic lamina (black lines)

Arterioles and venules arteriole venule Arterioles and venules are microscopic, compared to small arteries, which can be seen with the naked eye. Typically, there are 1 – 5 layers of smooth muscle in the tunica media of the arterioles. Again, compare the relative sizes of the walls. companion arteriole and venule – H&E – 20x objective

Arterioles and venules companion arteriole and venule – H&E– 40x objective A = arteriole B = venule arrows = very small arterioles (single smooth muscle cell layer) Note the companion venule with the arteriole. Also, the thinner-walled venule tends to be collapsed while the arteriole tends to be more rounded and open. A B This arteriole has 3-4 layers of smooth muscle in its tunica media.

Capillaries Arrows = capillaries capillaries – cross section – H&E – 40x objective Capillaries have very small diameters and walls that consist of only a single layer of endothelium.

Capillaries Arrowhead = capillary Capillaries have very small diameters and walls that consist of only a single layer of endothelium. adipose tissue – section – H&E – 40x objective

Large veins (vena cava) vena cava– section – H&E – 10x objective A B C A = tunica intima B = tunica media C = tunica adventitia Like with the large elastic arteries, it is hard to tell where the tunica intima ends and the tunica media begins. The tunica media is very thin. The tunica adventitia is thicker, with circumferentially arranged smooth muscle cells, some collagen and sparse elastic fibers. end