Histology of Arteries PDF

(002) ARTERIES DR. AGDAMAG | 01/04/21 OUTLINE If present, the external elastic lam...

(002) ARTERIES DR. AGDAMAG | 01/04/21 OUTLINE If present, the external elastic lamina (elastica externa) marks the boundary between the tunica media and tunica I. MICROANATOMY OF BLOOD VESSELS adventitia. II. STRUCTURES FOUND BETWEEN THE LAYERS The tunica adventitia contains loose to moderately dense OF THE BLOOD VESSEL fibroelastic connective tissue, +/- scattered smooth muscle A. Endothelium cells. B. Smooth muscle fibers Small and medium arteries and veins are present in the C. Connective tissues tunica adventitia of large arteries and veins. 1. Collagen fibers 2. Elastic fibers III. LAYERS OF THE BLOOD VESSEL A. Tunica intima B. Tunica media C. Tunica adventitia IV. VASA VASORUM V. CLASSIFICATION OF ARTERIES A. Elastic arteries/large arteries 1. Elastic arteries layers a. Tunica intima b. Tunica media c. Tunica adventitia 2. Vasa vasorum B. Muscular Fig. 1. Artery vs. Vein C. Arterioles VI. MED APP: ATHEROSCLEROSIS VII. ARTERIAL SENSORY STRUCTURES A. Carotid sinuses B. Complex chemoreceptors C. Glomus bodies VIII. MUSCULAR ARTERIES A. Tunica intima B. Tunica media C. Tunica adventitia Fig. 2. Flow of blood through the blood vessel IX. MED APP: RAYNAUD PHENOMENON X. ARTERIOLES A. Structure Oxygenated blood flows from the arteries to arteriole, exchange 1. Tunica intima occurs at capillaries and deoxygenated blood is carried on to the vein 2. Tunica media via venule. 3. Tunica adventitia II. STRUCTURES FOUND BETWEEN THE LAYERS OF B. Function THE BLOOD VESSEL XI. MEDICAL APPLICATION Transitions from “small arteries” to “arterioles” not clear-cut. The walls of all blood vessels larger than the microvasculature have many components in common and I. MICROANATOMY OF BLOOD VESSELS similar organization. Branching of the vessels helps produce reductions in their size which are accompanied by Most larger blood vessel walls contain three major layers with sub gradual changes in the composition of the vascular wall layering Tunica intima (luminal layer) - lumen is lined by endothelium of simple squamous A. ENDOTHELIUM epithelium. All larger vessels have walls with three concentric layers, or - a subendothelial layer of loose fibroelastic connective tunics (L. tunica, coat) tissue in most medium to large vessels and may contain Found in all blood vessels scattered smooth muscle in larger vessels. Semipermeable barrier between blood plasma and interstitial tissue fluid An internal elastic lamina (elastic lamina) marks the Squamous, polygonal, and elongated long axis in direction boundary between the tunica intima and tunica media. of blood flow Mediate bidirectional exchange of molecules by simple and The tunica media contains layers of either elastic active diffusion, receptor-mediated endocytosis, transcytosis laminae/lamellae (fenestrated sheets) or fibroelastic and other mechanism connective tissue alternating with layers of smooth muscle. Page 1 of 10 CMED 1E  (002) ARTERIES DR. AGDAMAG | 01/04/21 Functions ◦ non-thrombogenic surface III. LAYERS OF THE BLOOD VESSEL ◦ regulate local vascular tone and blood flow ◦ inflammation and local immune response A. TUNICA INTIMA Consists of: 1. Endothelium 2. Thin subendothelial layer - loose connective, - sometimes containing smooth muscle fibers Arteries and large veins o includes prominent limiting layer, internal elastic lamina: ⮚ composed of elastin, with holes allowing better diffusion of substances from blood deeper into the wall. B. TUNICA MEDIA Middle layer Concentric layers of helically arranged smooth muscle cells ◦ With variable amounts of elastic fibers and elastic Fig. 3. Small artery vs. small vein lamellae, reticular fibers, and proteoglycans, Walls of both arteries and veins have three tunics called the intima, produced by smooth muscle media, and the adventitia (or externa), which correspond roughly to Arteries - media may have thin external elastic lamina, the heart’s endocardium, myocardium, and epicardium. An artery has separating it from outermost tunic. a thicker media and relatively narrow lumen. A vein has a larger lumen and its adventitia is the thickest layer. The intima of veins is C. TUNICA ADVENTITIA often folded to form valves. Capillaries have only an endothelium, Tunica externa with no subendothelial layer or other tunics. connective tissue consisting principally of Type I collagen B. SMOOTH MUSCLE FIBERS and elastic fibers Continuous with and bound to the stromal connective tissue Fibers occur in all vessel walls larger than capillaries of the organ through which the blood vessel runs Can be found in between layers. Arranged helically in layer Arterioles and small arteries, smooth muscle cells connected IV. VASA VASORUM by gap junctions and permit vasoconstriction and vasodilation (regulate BP) “Vessels of the vessel” Arterioles, capillariies, and venules in the adventitia and outer part of the media C. CONNECTIVE TISSUE Provide metabolites to cells in those tunics in larger vessels Connective Tissue components are present in vascular walls because wall is too thick to be nourished solely by diffusion in variable amounts and proportions based on local from the blood in the lumen functional requirements. Luminal blood alone does provide the needs of cells in the 1. Collagen fibers - subendothelial layer intima. Between smooth muscle layers, and in the outer Because they carry deoxygenated blood, large veins covering. commonly have more vasa vasorum than arteries 2. Elastic fibers The adventitia of larger vessels also contains a network of Resiliency for vascular wall expansion under unmyelinated autonomic nerve fibers- pressure o Vasomoto nerves network of unmyelinated In large arteries form parallel lamellae, regularly autonomic nerve fibers, which release the distributed between muscle layers. vasoconstrictor norepinephrine. Elastin is a major component in large arteries Density is greater in arteries than in veins. where it forms parallel lamellae, regularly distributed between the muscle layers. Page 2 of 10 CMED 1E  (002) ARTERIES DR. AGDAMAG | 01/04/21 V. CLASSIFICATION OF ARTERIES o Well developed, with many smooth muscle cells in the subendothelial A. ELASTIC/LARGE ARTERIES connective tissue, and often shows Conducting or conduit folds in cross section as a result of Closest to heart the loss of blood pressure and Largest contraction of the vessel at death E.g aorta and pulmonary, common carotid, subclavian, and common iliac arteries. b. Tunica media – thick The most prominent feature of elastic arteries is the thick o Elastic lamellae alternate with layers tunica media in which elastic lamellae alternate with layers of smooth muscle fibers. of smooth muscle fibers. The adult aorta has about 50 elastic o adult aorta = 50 elastic lamellae lamellae (more if the individual is hypertensive). (more if hypertensive). Aorta, pulmonary artery, and their largest branches; → o Between elastic laminae - conducting arteries fibroblasts, elastic fibers, collagen Major role is to carry blood to smaller arteries fibers, spiral (to circular) smooth muscle Ventricular contraction (systole) _-> forceful movement of o The internal elastic lamina, which is blood through arteries → elastin stretched → distend wall more well-defined than the elastic within limit set by the wall’s collagen. laminae of the media Ventricles relax (diastole) → ventricular pressure drops → elastin rebounds passively, helping to maintain arterial c. Tunica adventitia - thin; consists mainly of pressure collagen fibers, blood vessels, nerves; some Aortic and pulmonary valves prevent backflow of blood into elastic fibers, fibroblasts, macrophages may the heart, so the rebound continues the blood flow away from the heart also be present. - much thinner than the media. Arterial blood pressure and blood velocity decrease and become less variable as the distance from the heart increases 2. VASA VASORUM - arterioles (A), capillaries, and 1. ELASTIC ARTERIES LAYERS venules (V) in adventitia of larger arteries - contains supply of microvasculature to bring O2 and nutrients to local cells that are too far from lumen to be nourished by blood there. - adventitia of large arteries also supplied more sparsely with small sympathetic nerves (N) for control of vasocontriction. Above the adventitia in this section can be seen muscle fibers (SM) and elastic lamellae (E) in the media. X100. H&e. Fig. 4. Elastic artery layers a. Tunica intima - thin relative to other layers in this type of vessel. o Endothelium o Subendothelial layer contains some smooth muscle, elastic fibers, collagen fibers o Internal elastic lamina - not as distinct as in other arteries because it is similar to elastic laminae of next layer o Elastic laminae → making blood flow more uniform Fig. 5. Vasa vasorum Page 3 of 10 CMED 1E  (002) ARTERIES DR. AGDAMAG | 01/04/21 B. MUSCULAR ARTERIES VII. ARTERIAL SENSORY STRUCTURES Distributing arteries A. CAROTID SINUSES Regulate blood flow to organs and parts of the body by contraction and relaxation of smooth muscle in their walls. Slight dilations of bilateral internal carotid arteries where they E.g femoral and brachial arteries branch from the (elastic) common carotid arteries Baroreceptors - monitoring arterial blood pressure C. ARTERIOLES Media is thinner - allow greater distension when BP Adventitia contains sensory nerve endings from cranial Smallest nerve IX, the glossopharyngeal nerve 100 mm or less in diameter Brain’s vasomotor centers process afferent impulses and small-resistance vessels that mainly regulate systemic blood adjust vaso-constriction, maintaining normal blood pressure. pressure Similar baroreceptors present in aortic arch. walls contain one or two layers of circularly arranged smooth At these sinuses the tunica media is thinner, allowing greater muscle distension when blood pressure rises, and the adventitia contains many sensory nerve endings from cranial nerve IX, the glossopharyngeal nerve. Brain’s vasomotor centers process afferent impulses and adjust vaso-constriction, maintaining normal blood pressure. Similar baroreceptors are present in the aortic arch transmit signals pertaining to blood pressure via cranial nerve X, the vagus nerve. These receptors sense changes in blood pressure and send the signal to the brain. B. COMPLEX CHEMORECEPTORS Monitor blood levels of CO2 and O2, H+ found in carotid bodies (in walls of carotid sinus) and aortic bodies (wall of aortic arch) Parts of ANS called paraganglia with rich capillary networks VI. MED APP: ATHEROSCLEROSIS Large, neural crest-derived glomus (type I) cells surround Disease of elastic arteries and large muscular arteries and capillaries. large muscular arteries o changes such as hypoxia, hypercapnia, Play a role in nearly half of all deaths in developed parts of or acidosis are sensed by this and sent to the world. the brain. Initiated by damaged or dysfunctional endothelial cells o Filled with dense-core vesicles containing oxidizing low- density lipoproteins (LDLs) in the tunica dopamine, acetyl- choline, and other Nt , intima, which Induces es adhesion and intima entry of supported by smaller satellite (type II) monocytes/macrophages to remove the modified LDL cells macrophages (called foam cells) Respond to stimuli in the arterial blood, primarily hypoxia Lipid-filled macrophages (called foam cells) accumulate and, (low O2), hypercapnia (excess CO2), or acidosis, by along with the free LDL, produce a pathologic sign of early activating release of neurotransmitters. atherosclerosis called fatty streaks Sensory Fibers branching from the Glossopharyngeal nerve During disease progression these develop into fibro-fatty form synapses with glomus cells and signal brain centers to plaques, or atheromas, consisting of a gruel-like mix of initiate cardiovascular and respiratory adjustments that smooth muscle cells, collagen fibers, and lymphocytes with correct the condition. necrotic regions of lipid, debris, and foam cells. Predisposing factors include dyslipidemia (> 3:1 ratios of LDL to HDL [high-densitylipoprotein]), hyperglycemia of diabetes, hypertension, and the presence of toxins introduced by smoking. In elastic arteries atheromas produce localized destruction within the wall, weakening it and causing arterial bulges or aneurysms that can rupture. In muscular arteries such as the coronary arteries, atheromas can occlude blood flow to downstream vessels, leading to ischemic heart disease. Page 4 of 10 CMED 1E  (002) ARTERIES DR. AGDAMAG | 01/04/21 Internal elastic lamina - prominent very distinct, usually folded - Is relatively thinner in muscular arteries than in elastic arteries and consists of an endothelial lining with its basal lamina, a sparse subendothelial layer of connective tissue, and a prominent internal elastic membrane. - In some muscular arteries, the subendothelial layer is so scanty that the basal lamina of the endothelium appears to make contact with the internal elastic membrane. In histologic sections, the internal elastic membrane usually appears as a well-defined, undulating or wavy structure because of contraction of the smooth muscle 2. Tunica media - thick, contain up to 40 layers of smooth muscle cells interspersed with a variable number of elastic lamellae (depending on the size of the vessel) Thickness decreases as diameter of vessel decreases External elastic lamina - present only in the larger muscular arteries (may be distinct in smaller muscular arteries) less thick compared to larger arteries Fig. 6. Glomus bodies /aorta which have 50 layers. C. GLOMUS BODIES - The tunica media of muscular arteries Two small (0.5–5 mm-diameter) ganglion- like structures consists of smooth muscle cells amid found near the common carotid arteries collagen fibers and relatively little elastic Contain many large capillaries (C) intermingled with clusters material. The smooth muscle cells are of large glomus cells (G) filled with vesicles of various arranged in a spiral fashion in the arterial neurotransmitters. wall. Their contraction helps maintain Supportive satellite cells (S) with elongated nuclei ensheath blood pressure. As in elastic arteries, each glomus cell. there are no fibroblasts in this layer. The Glomus cells form synaptic connections with sensory fibers. smooth muscle cells possess an external Significant changes in the blood CO2, O2, or H+ (basal) lamina except at the sites of gap concentrations are detected by the chemoreceptive glomus junctions and produce extracellular cells, which then release a neurotransmitter that activates collagen, elastin, and ground substance. the sensory nerve to relay this information to the brain. X400. PT. 3. Tunica adventitia - thick; lymphatic capillaries, chemoreceptors vasa vasorum, and nerves are also found in the adventitia, and these structures may penetrate to VIII. MUSCULAR ARTERIES the outer part of the media. - The tunica adventitia of muscular arteries Medium to small arteries (also called muscular arteries) = consists of fibroblasts, collagen fibers, - Muscular arteries also called distributing arteries, elastic fibers, and, in some vessels, distribute blood to the organs. scattered adipose cells. Compared with - Generally, in the region of transition between elastic elastic arteries, the tunica adventitia of arteries and large muscular arteries, the amount of elastic muscular arteries is relatively thick— material decreases, and smooth muscle cells become the about the same thickness as the tunica predominant constituent of tunica media. media. Collagen fibers are the principal 1. Tunica intima - thin endothelium, thin extracellular component. subendotheliial layer consisting of scattered fine - However, a concentration of elastic collagen and elastic fibers and a few fibroblasts material immediately adjacent to the Page 5 of 10 CMED 1E  (002) ARTERIES DR. AGDAMAG | 01/04/21 - tunica media is often present and as such constitutes the external elastic membrane. Nerves and small vessels IX. MED APP: RAYNAUD’S PHENOMENON travel in the adventitia and give off Raynaud phenomenon - brief episodes of vasospasm in branches that penetrate into the tunica walls of small arteries and triggered by changes in media in the large muscular arteries as temperature (cold or hot) and stress - leads to discoloration the vasa vasorum. because there's no flow to the fingers or toe. Function - distribute blood to the organs and help regulate X. ARTERIOLES blood pressure by contracting or relaxing the smooth muscle Muscular arteries branch repeatedly into smaller and smaller in the media. arteries, until reaching a size with three or four layers of medial smooth muscle. The smallest arteries branch as arterioles, which have only one or two smooth muscle layers; these indicate the beginning of an organ’s microvasculature where exchanges between blood and tissue fluid occur. Arterioles are generally less than 0.1 mm in diameter, with lumens approximately as wide as the wall is thick. smallest type of artery A. Structure a. Tunica intima - very thin consisting only of endothelium Elastic lamina is absent b. Tunica media - circularly arranged 1 or 2 layers of Figure 7. LM of the wall of a muscular artery. In this partly constricted artery, smooth muscle, some elastic fibers. the lumen (*) caliber is small relative to the muscular wall thickness. A prominent ○ thinner compared to the other artery internal elastic lamina (IEL) looks corrugated. Several layers of circular smooth muscle occupy the media (TM); loose connective tissue, the adventitia (TA). c. Tunica adventitia - thin, consisting of 320×. H&E. longitudinally arranged collagen and elastic fibers - In both small arteries and arterioles the adventitia With distance from the heart, arteries have relatively less is very thin and inconspicuous. elastin and more smooth muscle Most arteries, large enough to have name, are of the B. Function - to redistribute blood flow to capillaries and to alter muscular types blood pressure by altering peripheral resistance to blood flow Arterioles can change diameter very drastically therefore affecting blood pressure and flow patterns. - major determinants of systemic BP Arterioles are referred to as peripheral resistance vessels - Arterioles almost always branch to form anastomosing networks of capillaries that surround the parenchymal cells of the organ. At the ends of arterioles the smooth muscle fibers act as sphincters and produce periodic blood flow into capillaries. - Muscle tone normally keeps arterioles partially closed, resisting blood flow, which makes these vessels the major determinants of systemic blood pressure. - In certain tissues and organs arterioles deviate from this simple path to accommodate various Figure 8. A transverse section through a muscular (medium-caliber) specialized functions. artery shows a slightly folded intima with only sparse connective tissue - For example, thermoregulation by the skin involves between the endothelial cells (E) and internal elastic lamina(IEL). arterioles that can bypass capillary networks and Multiple layers of smooth muscle (SM) in the media are thicker than connect directly to venules. The media and the elastic lamellae and fibers with which they intersperse. Vasa adventitia are thicker in these arteriovenous shunts vasorum (V) are seen in the adventitia. (X100; H&E). Page 6 of 10 CMED 1E  (002) ARTERIES DR. AGDAMAG | 01/04/21 (or arteriovenous anastomoses) and richly innervated by sympathetic and parasympathetic nerve fibers. - The autonomic fibers control the degree of vasoconstriction at the shunts, regulating blood flow through the capillary beds. High capillary - blood flow in the skin allows more heat dissipation from the body, while reduced capillary blood flow conserves heat—important functions when the environmental temperature is hot or cold, respectively. - Another important alternative microvascular pathway is a venous portal system in which blood flows through two successive capillary beds separated by a portal vein. This arrangement allows for hormones or nutrients picked up by the Figure 10 & 11. (a) Arterioles are microvessels with an intima (I) blood in the first capillary network to be delivered consisting only of endothelium (E), in which the cells may have most efficiently to cells around the second capillary bed before the blood is returned to the heart for rounded nuclei. They have media (M) tunics with only one or two layers general distribution. of smooth muscle, and usually thin, inconspicuous adventitia (Ad). (X350; Masson trichrome) (b) Three arterioles (A) of various sizes and - The best examples are the hepatic portal system a capillary (C) are shown here. (X400; H&E) of the liver and the hypothalamic-hypophyseal portal system in the anterior pituitary gland, both of which have major physiologic importan Figure 12. LM of an arteriole in transverse section. Tightly arranged smooth muscle cells (SM) are oriented more or less circularly relative to the lumen (*). Their contraction causes the internal elastic lamina (IEL) to appear corrugated and endothelial cell (En) nuclei to bulge into the lumen. The adventitia (TA) contains connective tissue cells (mostly fibroblasts) and collagen fibers. 720×. H&E. Arterioles supplying a capillary bed typically form smaller branches called metarterioles in which the smooth muscle cells are dispersed as bands that act as precapillary sphincters Figure 9. Arterioles (A), capillaries (C), and venules (V) comprise the Thorough channels microvasculature where, in almost every organ, molecular exchange - distal portion of the metarteriole, lacks smooth takes place between blood and the interstitial fluid of the surrounding muscle cells and merges with the postcapillary tissues. Lacking media and adventitia tunics and with diameters of venule. only 4-10 µm, capillaries (C) in paraffin sections can be recognized by True capillaries nuclei adjacent to small lumens or by highly eosinophilic red blood - Smallest vessels branching from metarteriole cells in the lumen. As described in Figure 5–20, not all interstitial fluid and thoroughfare channel formed at capillary beds is drained into venules; the excess is called ❖ thoroughfare is at the opposite side at the lymph and collects in thin-walled, irregularly shaped lymphatic vessels venuole part. (L), such as those seen in connective tissue and smooth muscle here. - Lack smooth muscle cells (although pericytes (200X; H&E). may be present). Page 7 of 10 CMED 1E  (002) ARTERIES DR. AGDAMAG | 01/04/21 2. What tissue is directly associated with and extends into the Precapillary sphincters heart valves? - sphincters regulate blood flow into true a. Myocardium capillaries. b. Epicardium - Uppermost portion - relax sphincters. Capillary bed is c. Atrioventricular bundle of His perfused. d. Cardiac skeleton - Lower portion - contracted sphincter. No blood flow. blood e. Pericardium passes the capillary bed. 3. Individuals with Marfan syndrome have mutations in the fibrillin gene and commonly experience aortic aneurisms. What portion of the arterial wall is most likely to be affected by the malformed fibrillin? a. Endothelium d. Tunica adventitia b. Tunica intima e. Vasa vasorum c. Tunica media 4. A 66-year-old man diagnosed with type II diabetes 10 years earlier presents with an aching pain in the muscles of his lower extremities. He says the pain is relieved by rest and worsened by physical activity. His lower limbs appear cold, pale, discolored, and he has a sore on the skin of his left heel. He has a weak tibial pulse on both sides and poor skin filling from dermal capillaries. The problems with blood distribution in this patient’s leg are most likely associated with what vascular structures? a. Veins and venules d. Lymphatic vessels b. Arterioles e. Ventricles c. Branches of the aorta 5. What causes the release of atrial natriuretic hormone (ANH)? a. Decreased blood volume in the atrium b. Decreased heart rate c. Decreased blood pressure d. Stretching or distention of the atrium e. Stretching or distention of the ventricle Figure 13. a) shows a well-perfused capillary bed with all the 6. The main function of atrial natriuretic hormone (ANH) is to: sphincters relaxed and open; b) shows a capillary bed with the blood a. increase blood flow into the atrium. shunted away by contracted sphincters. At any given moment, most b. increase blood pressure. sphincters are at least partially closed and blood enters the capillary c. reduce atrial distention. d. reduce ventricular distention. bed in a pulsatile manner for maximally efficient exchange of e. decrease pacemaker activity. nutrients, wastes, O2 , and CO2 across the endothelium. Except in 7. It is a specialized region in the walls of certain elastic the pulmonary circulation, blood enters the microvasculature well oxygenated and leaves poorly oxygenated. arteries contain tissues acting as chemoreceptors that provide information to the brain regarding blood chemistry XI. MEDICAL APPLICATION a. Vasa Vasorum BP depends on CO and TPR to blood flow b. Glomus Bodies Peripheral resistance - mostly due to arteriolar resistance c. Raynaud phenomenon HTn or elevated BP may occur secondary to renal or d. Atherosclerosis endocrine problems, but more commonly essential 8. What do you call the phenomenon wherein vasospasm of hypertension, due to a wide variety of mechanisms that arteries resulting in discoloration of fingers and toes? increase arteriolar constriction. a. Raynaud phenomenon TEST YOURSELF b. Atherosclerosis c. Vasa Vasorum 1. Vasa vasorum serve a function analogous to that of which of d. Glomus Bodies the following? 9. Which one is TRUE about Atherosclerosis? a. Valves d. Endothelial diaphragms b. Basal lamina e. Arterioles a. Phenomenon wherein vasospasm of arteries resulting in c. Coronary arteries discoloration of fingers and toes Page 8 of 10 CMED 1E  (002) ARTERIES DR. AGDAMAG | 01/04/21 b. To redistribute blood flow to capillaries and to alter blood pressure by altering peripheral resistance to blood flow c. Contain tissues acting as chemoreceptors that provide information to the brain regarding blood chemistry d. Initiated by damaged or dysfunctional endothelial cells oxidizing low- density lipoproteins (LDLs) in the tunica intima, which Induces es adhesion and intima entry of monocytes/macrophages to remove the modified LDL 10. Vasa vasorum function in a way that is similar to a. AV valves. b. Coronary arteries. c. Semilunar valves. d. Metarterioles e. Elastic arteries. 1) C 2) D 3) C 4) B 5) D 6) C 7) B 8) A 9) D 10) B REFERENCES Mesher, A (2016). Junqueira’s Basic Histology Text and Atlas. McGraw-Hill Education. (14th ed.). US: McGraw-Hill Education Eroschenko, V. P. (2017). Atlas of Histology with Functional Correlations (13th ed.). Philadelphia: Wolters Kluwer Page 9 of 10 CMED 1E  (002) ARTERIES DR. AGDAMAG | 01/04/21 APPENDIX Page 10 of 10 CMED 1E