histology of blood vessels

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1 : Histology of the Blood Vessels Presented by Navid Zolfaghari Moheb
2 : Histology of the Blood Vessels The blood vessels are made of three layers, called from the luminal side outward, the tunica intima, the tunica media and the tunica adventitia. These three layers are analogous to the endo-, myo- and epicardium, respectively. The thickness of these three layers varies greatly depending upon the size and type of vessel (large, medium & small arteries and veins; capillaries). The tunica intima consists of an endothelium (present in all vessels) and any sub endothelial connective tissue that may be present (highly variable depending on vessel). The endothelium of vessels entering or leaving the heart is continuous with that of the heart. The tunica media is the layer of concentrically-arranged smooth muscle, the autonomic control of which can alter the diameter of the vessel and affect the blood pressure. Smooth muscle cells (in contrast to cardiac and skeletal) have secretory capabilities, and (depending on the vessel), the tunica media contains varying amounts of collagen fibres, elastic fibres, elastic lamellae, and proteoglycans secreted by the smooth muscle cells. The tunica media of arteries is larger than that of veins of similar size. The tunica adventitia is made chiefly of longitudinally arranged collagen fibres. It tends to be much larger in veins than arteries.
3 : Large Arteries The aorta and its branches (brachiocephalic, subclavian, pulmonary, beginning of common carotid and iliac) are distinguished by their great elasticity. This helps them smooth out the large fluctuations in blood pressure created by the heartbeat. During systole, their elastic laminae are stretched and reduce blood pressure. During diastole, the elastic rebound helps maintain arterial pressure. Tunica intima: Large arteries often have a large sub endothelial layer, which grows with age or disease conditions (arteriosclerosis). Both connective tissue and smooth muscle are present in the intima. The border of the intima is delineated by the internal elastic membrane. The internal elastic membrane may not be conspicuous because of the abundance of elastic material in the tunica media.
4 : Large Arteries Tunica media: This is the thickest of the three layers. The smooth muscle cells are arranged in a spiral around the long axis of the vessel. They secrete elastin in the form of sheets, or lamellae, which are fenestrated to facilitate diffusion. The number of lamellae increase with age (few at birth, 40-70 in adult) and with hypertension. These lamellae, and the large size of the media, are the most striking histological feature of elastic arteries. In addition to elastin, the smooth muscle cells of the media secrete reticular and fine collagen fibers and proteoglycans (all not identifiable). No fibroblasts are present. Tunica adventitia: This is a relatively thin connective tissue layer. Fibroblasts are the predominant cell type, and many macrophages are also present. Collagen fibres predominate and elastic fibres (not lamellae) are also present. The collagen in the adventitia prevents elastic arteries from stretching beyond their physiological limits during systole. Blood vessels supplying the adventitia and outer media are also present, these are called vasa vasorum ("vessels of the vessels"). (The inner part of the media is supplied from the lumen via pinocytic transport).
5 : Large Arteries
6 : Medium Arteries The majority of named arteries are medium (muscular or distributive) arteries. There is no sharp dividing line between elastic (large) and muscular (medium) arteries; in areas of transition, arteries may appear as intermediates between the two types. Medium arteries have less elastic tissue than large arteries, the predominant constituent of the tunica media is smooth muscle. Tunica intima: The tunica intima is thinner than in large arteries, there are fewer smooth muscle cells and less elastic tissue. The outermost part of the intima is defined by a very prominent internal elastic membrane (not obscured by elastic lamellae as in large arteries). The basement membrane of the endothelium may rest directly on the internal elastic membrane, or be separated by a sub endothelial layer of CT. The tunica intima increases in thickness with age, and may also become expanded by lipid deposits.
7 : Medium Arteries Tunica media: Smooth muscle cells predominate in the tunica media, and little elastic material is present. As in large arteries, no fibroblasts are present. Elastic fibres (few), collagen, and ground substance are produced by the smooth muscle cells. These are arranged in a spiral fashion and their contraction helps maintain blood pressure. In tissue preparation, the internal elastic membrane of the intima appears wavy due to the contraction of the smooth muscle of the media. Tunica adventitia: The main constituent of the adventitia is collagen fibres, secreted by fibroblasts. Elastic fibres are also present, a concentration of such fibres at the inner boundary of the adventitia is called the external elastic membrane. The external elastic membrane is not as prominent as the internal, and as arteries get smaller (see small arteries, below) disappears much earlier. The tunica adventitia is relatively larger than in elastic arteries, it can be up to the same size as the media. It will often blend in with the CT of surrounding structures. Adipose cells may be present.
8 : Medium Arteries
9 : Small Arteries Different textbooks will give slightly different numbers for what constitutes the diameter of small arteries and arterioles. Again there is a gradation to larger vessels. The general construction of small arteries is very similar to that of muscular arteries. The media is still muscular and has up to 8-10 layers of smooth muscle cells. This number is reduced as the arteries get smaller, the smallest arterioles have 1-2 layers of smooth muscle cells. The adventitia becomes thinner and the external elastic membrane disappears. The intima becomes smaller and the internal elastic membrane also eventually disappears. However, it persist much longer than the external, and it is not uncommon to see very small arteries which still have an internal elastic membrane. Small arteries also maintain their shape, and tend to be round or oval.
10 : Small Arteries
11 : large veins The large veins are the venae cavae and portal vein and their tributaries. The tunica intima consists of the endothelial lining with its basement membrane, a small amount of sub endothelial connective tissue and some smooth muscle cells. It blends in with the tunica media which is relatively thin, and in addition to smooth muscle cells may contain collagen fibres and some fibroblasts (in contrast with the media of arteries). The most distinguishing feature of large veins is the large tunica adventitia. The adventitia is the thickest layer in large veins and is made of collagen fibres, some elastic fibres and fibroblasts. Prominent bundles of longitudinally-arranged smooth muscle are a distinguishing feature.
12 : large veins
13 : Medium Veins The tunica consists of the endothelium and a thin sub endothelial layer with smooth muscle cells among the connective tissue elements. A thin internal elastic membrane may or may not be present. (If present, it is not nearly as prominent as in arteries). The tunica media is much thinner relative to that of an artery, and consists mostly of circularly arranged smooth muscle but also contains collagen fibres. The tunica's intima and media therefore tend to be less distinct from one another than is the case in arteries. The tunica adventitia is usually thicker than the media and is made up mostly of collagen fibres. It may contain longitudinally oriented smooth muscle bundles. (Remember gradations between the vessels of different sizes are continuous.)
14 : Medium Veins
15 : Small veins Different types of venules are described but are not distinguishable with the light microscope. Post capillary venules (which receive blood from capillaries) have only an endothelial lining (intima) and lack a smooth muscle media. They are surrounded by pericytes, which are undifferentiated mesenchymal cells. The basement membrane of the endothelial cells and pericytes may fuse. It is at the level of post-capillary venules that white blood cells leave the blood to enter the tissue. The endothelium of post-capillary venules is the main site of action for vasoactive agents such as histamine and serotonin which cause extravagation of fluid and WBCs during inflammation or allergic reactions. Collecting venules have a thin adventitia in addition to the pericytes surrounding the intima. The adventitia consists of longitudinally arranged collagen fibres with a few elastin fibres. Muscular venules have 1-3 layers of smooth muscle surrounding the intima, and an adventitia as described above. When looking at a venules which appears to have a media, it is generally not possible to tell if the nuclei of the media belong to smooth muscle cells or pericytes.
16 : capillaries Capillaries are the smallest diameter vessels and the site of exchange of metabolites between blood and tissues. Capillaries are just wide enough to allow the passage of red blood cells, only one cell at a time. (It is during this squeezing through the capillaries that mis-shapen RBCs, as in sickle cell anemia and thalassemia, rupture.) Capillaries consist of a single layer of endothelial cells and their basement membrane. The endothelial cells are joined together by tight junctions. At intervals, these tight junctions are interrupted, leaving small spaces allowing the passage of fluid between blood and ECF. These interruptions do not occur in the brain, and the lack thereof is responsible for the blood-brain barrier present in most of the brain. Endothelial cells also have pinocytotic vesicles which are involved in transporting macromolecules. Capillaries are classified according to the structure of their endothelial cells. Continuous capillaries (most capillaries) have a continuous endothelial cells with no fenestrations (openings) in their walls. They are found in nervous tissue, muscle tissue, lung, connective tissue and exocrine glands.
17 : capillaries a. Fenestrated capillaries have endothelial cells in which are found small openings, called fenestrated, of about 80- 100 nm in diameter. The fenestrations are covered by a small non-membranous diaphragm (which may be the remnant of the glycocalyx enclosed by pinocytotic vesicles from which the fenestrae may be formed). The basement membrane of endothelial cells is continuous over the fenestrated. Fenestrated allow greater permeability and the rapid passage of macromolecules smaller than plasma proteins. Fenestrated capillaries are found in the intestine and endocrine glands. b. A special type of fenestrated capillary with no diaphragm is found in the renal glomerulus. This capillary has a thick basement membrane. Sinusoids, also called discontinuous capillaries, have a large lumen and follow a tortuous path. They have many fenestrations with no diaphragm , and a discontinuous or absent basal lamina. The lumen is lined with phagocytic cells. They are found in the liver, hematopoietic organs (bone marrow, spleen) and some endocrine organs. At intervals, the endothelial cells of capillaries may be surrounded by pericytes, undifferentiated mesenchyme cells (as in post-capillary venules). Pericytes can differentiate into fibroblasts and smooth muscle cell, and function in wound repair and the formation of new vessels. Pericytes are associated much more frequently with continuous than with fenestrated capillaries. It is not possible to distinguish different types of capillaries (except sinusoids) with the light microscope. They are recognized by their small (1 RBC) diameter and thin wall.
18 : capillaries


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