Circulatory System Histology & Embryology 2023-2024 PDF

Fevziye Figen Kaymaz M.D., Ph.D. Prof. of Histology & Embryology [email protected] 1 DHF230-3 Histology and Embryology 2023-2024 Fall Term Prof. Dr. Fevziye Figen Kaymaz Wk/day/hour Subject 1. wk: 7 october/ 9.00-12.00 registration week 2. wk: 14 October/9.00-12.00 registration week 3....

Fevziye Figen Kaymaz M.D., Ph.D. Prof. of Histology & Embryology [email protected] 1 DHF230-3 Histology and Embryology 2023-2024 Fall Term Prof. Dr. Fevziye Figen Kaymaz Wk/day/hour Subject 1. wk: 7 october/ 9.00-12.00 registration week 2. wk: 14 October/9.00-12.00 registration week 3. wk: 21 October /9.00-12.00 Histology of circulatory system 4. wk: 28 October /9.00-12.00 Histology of lymphoid system (cells) 5. wk: 4 November /9.00-12.00 Histology of lymphoid system (organs) 6. wk : 11 November/9.00-12.00 Histology of upper respiratory system 7. wk: 18 November/9.00-12.00 Histology of lower respiratory system 8. wk: 25 November/9.00-12.00 Histology of nervous system 9. wk: 2 December/9.00-12.00 Mid term exam 10. wk: 9 December/9.00-12.00 Development and anomalies of face 11. wk: 16 December /9.00-12.00 Development and anomalies of pharyngeal aparatus 12. wk: 23 December /9.00-12.00 Histology of integumantary system 13. wk: 30 December /9.00-12.00 Histology of eye 14. wk: 6 January /9.00-12.00 Histology of ear 15. wk: 15-26 January/ Mid term final 2 Circulatory System Fevziye Figen Kaymaz M.D., Ph.D. Prof. of Histology & Embryology [email protected] 3 OBJECTIVES • To learn the histological structures of the heart and vessel types to define them under the light microscope. 4 Learning Goals Heart Wall • • endocardium, • myocardium, • Epicardium • Heart Valves • Impulse Generation and Transmission System • Sinoatrial node (SA node, Keith and Flack) • Atrioventricular node (AV node) • Atrioventricular bundle (bundle of His) • Purkinje fibers § Arteries transport blood away from the heart § Capillaries: a network of thin-walled vessels providing molecular exchange bw blood and tissues § Veins drain capillary bed and return blood to heart § Lymphatic vessels (closed-ended, opening into great veins) § Arterio-venous anastomoses Circulatory system (CS) n Systemic circulation (from heart to body, from body to heart) n Pulmonary circulation (from heart to lungs, from lungs to heart) n Lympatic circulation 6 Circulatory system I. Heart II. Blood vessels § Arteries § Capillaries § veins III. Lymphatic vessels 7 Functions of circulatory system n CS carries blood in both directions bw heart and tissues (two-way circulation) n Assisting in the exchange of substances between cells and blood n Atrial Natriuretic factor and brain natriuretic factor n Lymphatic vascular system collects lymph, the excess extracellular tissue fluid and delivers it back to CVS (one-way transport) 8 HEART n n n n n n n Cardiac muscle Four chamber Contracts rhythmically Right ventricles propel blood to pulmonary circulation Left ventricles propels blood to systemic circulation Right atria receive blood from body Left atria receive blood from pulmonary veins 9 Layers of the heart wall n Endocardium n Myocardium n Epicardium (visceral pericardium) 10 Heart wall 11 n Pericardium q Fibrous q Serous n Parietal n Visceral (epicardium) q Pericardial cavity 12 Epicardium (visceral pericardium): n n n n outermost layer; homologues to TA of vessels, composed of simple squamous epithelium (mesothelium). Subepicardial layer of loose CT contains coronary vessels, nerves and ganglia. 13 Endocardium q Innermost layer q Endothelium: simple squamous epithelium q Subendothelium: supporting CT, smooth muscle fibers, myoelastic CT ØSubendocardium is absent; in q Subendocardium: loose CT papillary muscles, mitral and with small blood vessels, tricüspid valves free ends and nerves, Purkinje cells from the chorda tendinia conducting system 14 Endocardium 15 Myocardium n n n n n n n thickest layer Cardiomyocytes arranged in complex spirals around the orifices of the chambers Certain cardiac muscle cells extend to the fibrous skeleton atrial cardiac muscle Ventricular cardiac muscle Some are specialized for endocrine secretion Some are specialized for impulse generation or conduction 16 Cardiac muscle cells (cardiomyocytes) Single (some even double) nucleus in the middle n Branching (+) n Granules containing atrial natriuretic factor in the right atrium (+) n DOES NOT No mitosis REGENERATE. (After MI, microRNAs, hsa-miR-590 and hsamiR-199a can regenerate in cardiac muscle cells) n 17 Cardiac muscle cells (cardiomyocytes) • • n Diads Cross striations, A, I and H bands, sarcomere between two Z line n Intercalated disk n Large mitochondrions energy source n Glycogen n n n observe so much • • skeleton triat, cardiac "diad" Contractile cardiocyte helps pump blood Myoendocrine cardiocyte, Atrial natriuretic factor Nodal cardiocyte, those that coordinate the rhythmic contractions of the heart (in the SA and AV node, in the interatrial and interventricular septa) sinoatrial node 18 Intercalated discs Interfaces between adjacent cells Transverse region: 1- desmosomes 2- fascia adherens Longitudinal region 1- gap junctions cross the cell "transverse" Serve as electrical synapses promote rapid impulse conduction through cardiac muscle cells simultaneously 19 Atrial Natriuretic Factor Brain Natriuretic Factor n In granules (0,3-0,4µm) in cardiac muscle cells in the right atrium n It inhibits the secretion of renin from kidney and aldosterone from surrenal gland. n It antagonizes the effects of vasopressin and Angiotensin II.. n Inhibits Na-H2O reabsorption. n When the heart muscle is stretched; n q ANF is secreted. q Diuresis increases q Na excretion in the urine increases q The blood volume is reduced. BNF increases in the circulation in congestive heart failure. 20 Heart wall 21 Interventricular – Interatrial septum Interatrial Septum Interventricular Septum n no endocarium n Its surface is lined with n thinner than IV septum endocardium. n In the core: endocardium, and Except for the membranous part, it has cardiac muscle in its structure. the heart muscle. n There is fibrous tissue in localized areas. 22 Atrioventriculer Septum 23 Cardiac Fibrous Skeleton composed of dense CT, includes 3 components q Membranous interventricular septum: making upper portion of the IV septum q Anulus fibrosus (annuli fibrosi): formed around the base of aorta, pulmonary artery, and the AV orifices q Trigonum fibrosum (trigona fibrosa): formed close to aortic valve 24 Cardiac Fibrous Skeleton n At the base of the valves, the area where the muscle attaches, the dense connective tissue spread in all directions: It forms part of the interventricular and interatrial septum, surrounds the heart valves and extends from the cusps of the valves to the chordae tendinees. Function n support the heart valves n Creating spaces for the heart muscles to attach n Helping to coordinate heartbeat by making electrical isolation between atria and ventricles. 25 Heart Valves n Left q q n Aortic n LV-Aorta Mitral (Bicuspid) n LA-LV left verticle- Aorta Right q q Pulmonary n RV-Pulmonary V Tricuspid n RA-RV right atrium- right verticle 26 Heart Valves all same histological faces Layers n Endothelium n Dense CT (collagen and elastic fibres) n Endothelium both sides Features: n Base attached to anulus fibrosus (annuli fibrozi) n No blood vessel n No myocardium diffusion 27 Heart valves n They are attached to the fibrous skeleton. n Connective tissue consists of 3 layers. n Fibrosa n Spongiosa n ventricularis 28 Heart Valves 1. Fibrosa: n Irregular dense connective tissue extending from the fibrous skeleton n in the middle of the valves 2. Spongiosa: n the atrial surface of each valve or loose connective tissue adjacent to the vessel surface n Collagen and elastic fibrils, Proteoglycans: During the closing of the valves reduces vibration n Provides flexibility of covers n Spongiosa=Arterialis, in the aortic and pulmonary valves n Spongiosa= Auricularis in tricuspid and mitral valves 3. Ventricularis: n Dense connective tissue adjacent to the ventricular surface of each valve n Multilayered elastic fibers n Ventricularis chorda tendinea in AV valves 29 Heart Valves blue part connective tissues tibrostaining metalls n The fibrous cardiac skeleton consists of dense irregular connective tissue, primarily in the endocardium (En), which anchors the valves and surrounds the two atrioventricular canals, maintaining their proper shape. The micrograph shows a section through a cusp of an atrioventricular valve (arrow) and attached chordae tendineae (CT). These struc- tures are largely dense connective tissue (C) covered with a thin layer of endothelium. The collagen-rich connective tis- sue of the valves is stained pale blue here and is continu- ous with the fibrous ring of connective tissue at the base of the valves, which fills the endocardium between the atrium (A) and ventricle (V). The thick ventricular myocardium (M) is also shown. X20. Masson trichrome. 30 Clinical Correlation: Heart Valves anomalies n developmental problems n due to infection n due to CV problems (hypertension) n Lids do not close, backflow, Murmur n Surgical repair or renewal n In regenerated valves, the surface is not covered with endothelial cells. n Use of anticoagulants to prevent clot formation 31 Impulse generating-conducting system: n between A and V n devide into Right Atrium q Sinoatrial node (pacemaker) (Keith and Flack) q Atrioventricular node (AV node) Ventricle q Atrioventricular bundle (Bundle of His) q Subendocardial conducting network (Purkinje fibers) 32 Impulse generating-conducting system: Two nodes (in the Atrium) 1- Sinoatrial node (SA node, Keith and Flack) q vessel q q heart’s impulse generation 6-7 mm3 located close to the area where the sup vena cava enters the right atrium Modified heart muscle cells (Nodal cardiomyocytes) n n n Smaller size Fewer myofibrils Fewer intercalated discs 33 Impulse generating-conducting system: 2- Atrioventricular node (AV node) n Smaller than the SA node in the lower part of the right atrium near AV valve (in the lower part of the interatrial septum near the opening of the coronary sinus) n Modified muscle cells (Nodal cardiomyocytes) q small modified cardiac muscle cells q Discus intercalaris (+/-) q Few myofibrils 34 Impulse generating-conducting system: n The impulse starts at the SA node. n The atrium muscle reaches the AV node n AV node form AV bundle (His) passes through cardiac skeleton, 2 branch in Interventriculer Septum (left and right bundle) n It continues with Purkinje fibers (subendocardial conducting network) towards the apex of the IVS enters the myocardium. 35 Impulse generating-conducting system: n Bundle of His (Atrioventricular bundle) q Pass down the IV septım q Formed by Purkinje cells. q Subendocardially located in atrium q Located within the myocardium in ventricle 36 Impulse-conducting system: Purkinje cells n n n n Short, thick Low amount of myofibril, İntercalated disc + Numeruous mitochondria and glycogen granules..pale cytoplasm Round nuclei larger than those of myocardial heart muscle cells big amount n 37 Impulse-conducting system: Purkinje cells hemotoxin tricrom connective tissues ironhemotoxelin Purkinje cells characterized by a reduced number of myofibrils that are present mainly in the periphery of the muscle cell. The light area around the nuclei of the conducting cells is caused by a local accumulation of glycogen. 38 Atrioventricular bundle: Bundle of His 39 40 Innervasion n Parasympathetic and sympathetic innervation n Ganglion cells and nerve fibers located close to the SA and AV nodes n q Affects heart rate and rhythm q Parasympathetic slows down q Sympathetics speed up Free nerve endings between muscle cells in the myocardium q Detects pain (in angina pectoris) 41 Clinical correlation n n n n n n n n n n n n heart rheumatism ischemic heart disease Angina pectoris myocardial infarction Congestive heart failure Cardiac hypertrophy coronary artery disease Fibrillation (atrial/ventricular) Tachycardia/Bradycardia Murmur Pericarditis cardiac tamponade 42 Blood vessels q Arteries transport blood away from the heart q Capillaries: a network of thin-walled vessels providing molecular exchange bw blood and tissues q Veins drain capillary bed and return blood to heart q Lymphatic vessels (closed-ended, opening into great veins) q Arterio-venous anastomoses Blood vessels • Elastic artery • Muscular artery • Arteriole • Metarteriole • Capillary • Postcapillary venule • Medium type vein • Large vein Figure 12-1 Blood pressure and vascular anatomy Blood vessels q Elastic artery q Muscular artery (medium sized artery) q Arteriol q capillary q Postcapillary venule q Medium sized veins q Large veins General organisation of vessels 1- tunica intima 2- tunica media inner middle 3- tunica adventitia outer Vessel tunics 1. Tunica İntima q Endothelium (flattened cells secreting type II, IV, V collagens, lamin, endothelin NO, von Wille brand factor, ACE) Basal lamina (collogen, proteoglycans and glycoprotein) q Subendothelium (loose connective tissue, few smooth muscle cells) Internal elastic membrane (sheat of elastic fibers in arteries and arterioles, well developed in medium sized artey) Vessel tunics 2. Tunica Media q Concentric layers of helically arranged smooth muscle cells, elastic and reticular fibers, collagen type III q Proteoglycans, glycoproteins q External elastic membrane q Vasoconstriction/vasodilatation Vessel tunics 3. Tunica Adventitia q Type I, III collagen fibers q Longitudinally oriented elastic fibers q Mechanical support Endothelium n Total endothelial cell number ~ 6x1023 n Endothelial cell surface area 700-1000m2, weight ~1.5kg. n Tight junctions bw them n Pinocytotic vesicles, n Membrane-bounded Weibel-Palade bodies pinostosis Endothelium imp n Selective barrier n Synthesis and secretion : q Collagen type II, IV, V and proteoglycan (BM) q Coagulation factor FVIII (von Willebrand factor) WP bodies q Anticoagulan molecules: prostacyclin, NO, trombomodulin) q Vasoactive mediators (NO, prostacycline, endothelins) q Proinflammatory molecules (Interleukin 1, 6, 8) q Growth factors basel membrane Endothelium Clinical correlations n Endothelial damage formation n Exposure of subendothelial collagen n Aggregation of platelets n Fibrin formation n Thrombus formation if there is a damage Vasa Vasorum vessel of the vessels q Small arteries that enter vessel wall and provide nourrishment of it q More prevelant in veins q In the adventitia q Arteriole, capillary, venule Arteries elastic aretery midium size Efferents vessels that transport blood away from the heart to the capillary beds. 2 primary arteries that arises from R and L ventricles are the pulmonary tract and aorta. Pulmonary trunk branches into L and R pulmonary arteries. Aorta makes an arch to descend the thorasic cavity 3 types: n n n Elastic (conducting) q Large, conductinge q Aorta and its branches Muscular (medium type) q Medium sized, distributing q With a thick t media and elastic membranes Arteriole q Diameter less than 0.1 mm. Arteriole tonica media Elastic Artery n Walls appear yellow in the fresh state n Intima is thick, contains IEL n n n smooth muscles betweenn elastic tissues Media contains fenestrated elastic lamellae (newborn:40, adult: 50-70). An EEL is also present Adventitia is thin fibroelastic CT with vasa vasorum systole; elastic lamellas reduce pressure variations n diastole; Arterial pressure is stabilized by elastic rebound EEL Light micrograph of an elastic artery the fenestrated membranes (FM), tunica media (TM), and tunica adventitia (TA). Elastic Artery well stained Elastic Artery Carotid Body n Small oval structure located at the bifurcation of common carotid artery n Functions as a chemoreceptor monitoring changes in O2 and CO2, levels, as well as hidrogen ion concentration n 3-5mm in diameter, composed of multiple clusters of pale staining cells embedded in a CT. n Consisting of types of parenchymal cells q q Type I (glomus cells) (dopamin, serotonin, adrenalin storing cells) Type II sheath cells A highly vascularized structure sensitive to hypoxia. Its main cells have dense-core granules containing catecholamines that are surrounded by glia-like sustentacular cells. PT stain. A: Low magnification. B: Medium magnification. Carotid Sinus n Dilatations located on the arch of aorta bw right subclavian and the right common carotid artery and bw the left common carotid artery and left subclavian artey n Baroreceptor located in the adventitia which is thicker and media thinner n Monitors blood pressure changes n Nerve endings at intima and adventitia Aortic Bodies n Located on the arch of aorta bw right subclavian and right common carotid artery and bw the left common carotid artery and left subclavian artery n Structure and function similar to those of carotid bodies Muscular (Medium size) (Distributing ) Artery n Most vessels arising from aorta but also most of the named arteries with a diameter only 0.1 mm. It controls blood flow to organs n Thin TI; subendothelium realtively thicker than that of arteriole n Significant IEM n MEE (+) in large sized muscular arteries n Thick TM composed of ~40 layer of smooth muscle cells, with interspersed elastic and, reticular fibers, proteoglycans n TA; loose CT with vaso vasorum. Muscular (Medium size) Artery tonica E external more than two layyers Muscular (medium size) Artery stained with differed dye IMP higher magnification Arterioles n Diameter <0,5 mm n Narrow lumen n TI: Endothelium q Subendothelium is very thin or absent q n IEM (-/+) can be seen or not TM; 1-2 layers of circular smooth muscle q n EEM (-) TA; very thin absent Arterioles A small artery and its accompanying muscular vein. Because of vasodilatation, the arteriole is unusually filled with blood. IEL (-). Oblique section of a small artery from the mesentery. Note the transverse section of the smooth muscle cells of the media and the endothelial layer covering the lumen of the vessel (arrowheads). Arterioles smooth muscle cell Scanning electron micrograph of an arteriole illustrating its compact layer of smooth muscle and its attendant nerve fibers. (From Fujiwara T, Uehara Y: The cytoarchitecture of the wall and innervation pattern of the microvessels in the rat mammary gland: A scanning electron microscopic observation. Am J Anat 170:39-54, 1984.) Metarterioles n Arterioles with very small diameter that supply blood to capillary beds n Differ structurally from arterioles in that the smooth muscle layer is not continuous; rather the individual muscle cells are spaced apart and each encircles the endothelium of a capillary arising from the metarteriaole n Functions as a sphincter upon concentration ; controlling blood flow in the capillary bed Microcirculation Capillaries n Arising from the terminal endings of arterioles; from a capillary bed (network) bw arterioles and the venules n Smallest and thinest blood vessels composing of a single layer of endothelial cells (1-3 cells) siting on a BL. n Pericytes are located around the outside of the capillaries n Provide metabolic excahenge bw blood and tissues n Diameter: 7-9 µm, n Max length 50 µm n Total length in human body ~96000 km n Cytoplasm contains few organels (small Golgi, mitochondria, free ribosoms, few RER cisternae) n Tight junctions bw endothelial cells Pericytes proliferate postinjury and differentiate into endothelial cells forming new vessels Capillaries Scanning electron micrograph of a capillary displaying pericytes on its surface. (From Fujiwara T, Uehara, Y: The cytoarchitecture of the wall and innervation pattern of the microvessels in the rat mammary gland: A scanning electron microscopic observation. Am J Anat 170:39-54, 1984.) Light micrograph of a capillary in the monkey cerebellum. A capillary (Ca) is present in the field of view, and red blood cells (RBC) are evident in its lumen (L). Note the nucleus (arrow) of an endothelial cell bulging into the lumen. Capillary Classification: intrecellular junction promenant 1. Continuous (somatic) capillaries 2. Fenestrated (visceral) capillaries 3. 1. Diaphram+ 2. Diaphram- large holes in pink Sinusoidal (discontinuous) capillaries -no basel lamina -the holes are very huge Continuous (somatic) capillaries n No pores or fenestrae in their walls n Continuous cytoplasm n Continuous BL n Pinocytotic vesicles at both sides of the cell n Present in muscle, nervous and connective tissues; in the brain classified as modified continuous capillaries Continuous (somatic) capillaries q Muscle tissue (+) q Connective tissues (+) q Nervous tissue (+) q Exocrine glands (+) q Thymus (+) q Bone (+) q Skin (+) q Lung (+) Continuous (somatic) capillaries Electron micrograph of a section of a continuous capillary. Note the ruffled appearance of its interior surface, the large and small pinocytic vesicles, and numerous microfilaments in the cytoplasm. Arrows show the basal lamina. Continuous (somatic) capillaries Testicular capillary. CL, capillary lumen; MC, myoid cell; E, nucleus of endothelial cell. Arrows represent the basal lamina. (From Meyerhofer A, Hikim APS, Bartke A, Russell LD: Changes in the testicular microvasculature during photoperiod-related seasonal transition from reproductive quiescence to reproductive activity in the adult golden hamster. Anat Rec 224:495-507, 1989.) Fenestrated (Visceral) Capillaries maybe n Possess pores (60-80nm) covered by pore diaphrams in their walls n Basal lamina is continous n 2 types: q Diaphragm+ (most of the capillaries), bridged by ultrathin diaphragm q Diaphragm- (Glomerule capillary) in kedney Fenestrated (Visceral) Capillaries Fenestrated (Visceral) Capillaries with diaphragm Intestinal villi Choroid plexus Processus ciliaris (eye) Endocrine glands Renal tubules Fenestrated (Visceral) Capillaries with no diaphragm Fenestrated capillary; BM basement membrane BMp pericyte basement membrane C collagen D diaphragm E endothelial cell nucleus EC endothelial cell cytoplasm F fenestrations M marginal fold P pericyte Renal glomerular capillaries Fenestrated (Visceral) Capillaries with diaphragm Arrows: fenestrae closed by diaphragms. Golgi complex (G), nucleus (N), centrioles (C). Note the continuous basal lamina on the outer surface of the endothelial cell (double arrows). (Courtesy of J Rhodin.) Fenestrated (Visceral) Capillaries with no diaphragm f: frnestration Sinusoidal (Discontinuous) Capillaries n Possess discontinuous endothelial cells with large, irregular (30-40 µm)pores bw endothelial cells and BL n Many large fenestrae without diaphragms enhancing exchange bw blood and tissue n Macrophages near the endothelial cells n BL is discontinuous enter vessels and take what it needs Sinusoidal (Discontinuous) Capillaries S: sinusoida A: adipos n Bone marrow n Liver n Spleen n Lymphoid organs n Certain endocrine glands Microcirculation (Small Blood Vessels) 1. Arteriole –> metarteriole –> Capillary –> venule and vein 2. Arteriovenous anastomoses 3. Arterial portal system (renal glomerule) 4. Venous portal system (liver) Microcirculation (Small Blood Vessels) v Portal system nBlood vessels entering and sorting the capillary net are the same q Venous portal system nIn liver nHypotalamus, nPortal q hypophyisis vein – Capillary – Hepatic vein Arterial portal system nRenal cortex nAfferent arteriole - Capillary -Efferent arteriole Endothelial Transport The various methods of transport across capillary endothelia. A: Pinocytotic vesicles, which form on the luminal surface, traverse the endothelial cell, and their contents are released on the opposite surface into the connective tissue spaces. B: Trans Golgi network-derived vesicles possessing clathrin coats and receptor molecules fuse with the luminal surface of the endothelial cells and pick up specific ligands from the capillary lumen. They then detach and traverse the endothelial cell, fuse with the membrane of the opposite surface, and release their contents into the connective tissue spaces. C: In regions where the endothelial cells are highly attenuated, the pinocytotic (or trans Golgi network-derived) vesicles may fuse with each other to form transient fenestrations through the entire thickness of the endothelial cell, permitting material to travel between the lumen and the connective tissue spaces. Arteries Very useful Veins n n n n n n Return blood from capillaries to the heart Parallel to and thinner and less elastic than the arteries Having the same three layers Large lumen Valves (+) Classified into 3 groups on the basis of their diamater and wallthickness: small, medium and large Vein Tunics Large Veins n n n n TI, very well developed, endothelium, BL, valves in some, subendothelial CT TM, CT and smooth muscle cells TA, thick dense CT with smooth muscle cells oriented in longitudinal bundles Valves are CT surrounded by endothelium at both sides Large Vein Aorta – Vena Cava The media is much thicker in large arteries than veins, with relatively more elastin. Elastic fibers are also present in the outer tunica adventitia (A), which is relatively thicker in large veins. Vasa vasorum (V) are seen in the adventitia of the aorta. The connective tissue of the adventitia always merges with the less dense connective tissue around it. Both X122. Elastic stain. Muscular (Medium Sized) Vein Less than 1 cm, draining most of the regions of the extremities Muscular Artery and Vein n TI and TM are well developed in artery Muscular Artery and Vein Artery and vein Walls of both arteries and veins have three tunics called the intima, media, and the adventitia (or externa), which correspond roughly to the heart’s endocardium, myocardium, and epicardium. An artery has 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 often folded to form valves. Capillaries have only an endothelium, with no subendothelial layer or other tunics. Vein Valves Venule n Similar to but larger than capillaries n Larger venules (>1mm) possess smooth muscle cells instead of pericytes n Postcapillary venules with 15-20 µm diameter, receive blood from capillaries Postcapillary venule Diameter 0.1-0.5mm TI, endothelium and thin subendothelium TM, pericyte only (+) (a) Micrograph of small vein (V) shows a relatively large lumen compared to the small muscular artery (A) with its thick media (M) and adventitia (Ad). The wall of a small vein is very thin, containing only two or three layers of smooth muscle. X200. H&E (b) Micrograph of a convergence between two small veins shows valves (arrow). Valves are thin folds of intima projecting well into the lumen, which act to prevent backflow of blood. X200. H&E. (c) Micrograph of a medium vein (MV) shows a thicker wall but still less prominent than that of the accompanying muscular ar- tery (MA). Both the media and adventitia are better developed, but the wall is often folded around the relatively large lumen. X100. H&E. (d) Micrograph of a medium vein contains blood and shows valve folds (arrows). X200. Masson trichrome. Veins: Clinic correlation-Atherosclerosis Clinical Correlations: Atherosclerosis n Intimal focal thickining n Smooth muscle cell proliferation n Subendothelial ECM accumulation n Macrophages taking up the lipoproteins (foam cells) 106 Clinical Correlations: Thrombus 107 References 1. 2. 3. 4. 5. 6. Ross: Histology Text and Atlas Gartner: Color atlas of Histology Junqueira: Basic Histology Sadler TW, Langman's Medical Embryology 13E Moore KL, Persaud TVN, Torchia MG Developing Human 9e Carlson BM Human Embryology & Developmental Biology, 5E 108 QUESTIONS ????? 109