Histology of the Cardiovascular System (Spring 2024) PDF

HISTOLOGY OF THE CARDIOVASCULAR SYSTEM SPRING 2024 María José Navarrete Talloni, DVM, MPVM, PhD...

HISTOLOGY OF THE CARDIOVASCULAR SYSTEM SPRING 2024 María José Navarrete Talloni, DVM, MPVM, PhD Department of Biomedical Sciences, RUSVM Disclaimer: Images and information in this presentation come from different sources including Drs. I. Irimescu, O. Illanes, M. Smith, M. Zibrin, P. Hanna and H. Bogdanovic‘s notes. Dellmann’s and Junqueira’s histology books, P. Hyttel et al. and K. Moore embryology books. This presentation is for teaching purposes only, please do not distribute.  Overview 1. Background and functions of the CVS 2. The Heart: Endocardium Myocardium Epicardium ossifies 3. Vessels Arteries (elastic a., muscular a., arterioles) Capillaries (continuous, fenestrated, discontinuous /sinusoids) Venules & veins CVS = Cardiovascular system Lymphatics  1. Background “The circulatory system consist of blood, a central pump (the heart), blood distribution (arterial) and collection (venous) networks, and a system for exchange of nutrient and waste products between blood and extravascular tissue (microcirculation)”. “A network of vessels (lymphatics) that parallel the veins also contribute to circulation by draining fluid from the extravascular spaces into the blood vascular system”. McGavin, 2007  Functions of the Cardiovascular System Maintenance of adequate blood flow (cardiac output) Delivery of oxygen, nutrients, hormones, electrolytes and water to peripheral tissues Removal of carbon dioxide (CO2) and other metabolic waste products Maintenance of normal thermoregulation Maintenance of normal glomerular filtration rate (GFR) and urine output have A and vein all hesler and  1 to Y open The right heart propels unoxygenated blood through the pulmonary circulation, and the left heart propels oxygenated blood through the systemic circulation. RA, Right atrium; RV, right ventricle; LA,left atrium,LV, left ventricle. FYI: Relationship between blood pressure, vascular permeability and structural characteristics of different types of blood vessels (bottom) Pressure Histology, a text atlas, 3rd edition, Ross, Romrell, and Permeability Kaye, 1995 Elastic substance + Thickness of the wall Smooth muscle Total cross-sectional area  Microcirculation Normal vascular pattern T 1 biggest Artery Metarteriole Capillaries Venule Vein Source: Junqueria’s Basic Histology, 2010  2. Heart First organ to form in the embryo Mammalian and bird hearts have 4 chambers: RA Left & right atria (LA, RA) Left & right ventricles (LV, RV) LA RV LV  Apt Heart tunics RLIIiigi IB.EE The heart is composed of 3 layers - Myocardium from inside to outside: 1. Endocardium (includes valves) 2. Myocardium (heart muscle)thicklayer Endocardium 3. Epicardium (visceral pericardium) thin layer Epicardium Heart tunics Myocardium Epicardial Epicardium adipose tissue Purkinje fibers Endocardium Right Right atrium ventricle Aorta Endocardium Left ventricle Myocardium Epicardium Interventricular septum 5x  Endocardium Forms the inner lining and the valves Equivalent to the tunica intima of BV Endothelial cells on the inner most Endocardium surface Direct contact with blood Important in hemostasis  Endocardium blood 3 layers: contactwth 1. Endothelium Endocardium 2. Basal lamina 3. Subendothelial connective tissue Purkinje Fiber Also contains part of the conductive system and Purkinje fibers Myocardium  Conduction system SEn E M P 0 Heart, sheep. Purkinje fibers. The endocardium (E) is a thin layer of connective tissue lined by endothelium. Between the endocardium and myocardium (M) is a layer of variable thickness called the subendocardial layer (SEn) containing small nerves and in the ventricles the Purkinje fibers (P) of the subendocardial conducting network. These fibers are cardiac muscle cells joined by intercalated disks but specialized for impulse conduction rather than contraction. Purkinje fibers are usually larger than contractile cardiac muscle fibers with large amounts of lightly stained glycogen filling most of the cytoplasm and displacing sparse myofibrils to the periphery.  Conduction system The Purkinje fibers are found in the I subendocardium. pepanfovconcuctinofimp.is Modified cardiomyocytes that function in conduction. Larger than cardiac muscle cells Fewer myofibrils Lots of glycogen and mitochondria No T-tubules. Connected by desmosomes and gap junctions Intercalated discs are variable in appearance and number depending on their location compared to myocardiocytes a They are specialized conducting fibers, which extend from the interventricular septum, to the papillary muscles, and up the lateral walls of the ventricles. Conduction system - FYI Two nodes generate the impulses for myocardial contraction. Both are small masses of specialized cardiac muscle fibers and associated connective tissue, supplied by nerve fibers from the autonomic nervous system that speed up and slow down the heart rate. First impulses are generated by the sinoatrial node (SA) (in the wall of the right atrium) - sending a wave of depolarization around the atria via gap junctions between the muscle fibers. Next the atrioventricular node (AV) (located in the interatrial septum) starts impulse generation around the ventricles. Impulses are sent from it to the AV bundle (bundle of His), which branches to form Purkinje fibers, which lie in the deepest layer of the endocardium and supply the papillary muscles The apex of the heart contracts 1st then the papillary muscles then the wave of depolarization spreads up the walls of the ventricles from the apex upwards, as shown in the diagram.  Valve cusp T Figure 11–6. (Mescher) Valve leaflet and fibrous skeleton. The fibrous skeleton of the heart consists of masses of dense connective tissue in the endocardium which anchors the valves and surrounds the two atrioventricular canals, maintaining their proper shape. Section through a leaflet of the left atrioventricular valve (arrows) shows that valves are largely dense connective tissue (C) covered with a thin layer of endothelium. The collagen—rich connective tissue of the valves is stained pale green here and is continuous with the fibrous ring of connective tissue at the base of the valves, which fills the endocardium (En) of this area between the atrium (A) and ventricle (V). The chordae tendinae (CT), small strands of connective tissue which bind distal parts of valve leaflets, can also be seen here. The interwoven nature of the cardiac muscle fibers, with many small fascicles, in the myocardium (M) is also shown. X20. Masson trichrome. Myocardium Myocardium Cardiomyocytes Cross striated Central single nucleus Intercalated disks: - gap junctions - anchoring junctions Lipofuscin Sarcoplasmic reticulum Many mitochondria up to 20% cell volume FEE requires a lot of O2 Low regeneration capacity (mitosis) www.histology.leeds.ac.uk Myocardium Left ventricular myocardial thickness is ~ 2 to 4 times thicker than the right in adults due to higher pressure on the left side Involuntary striated muscle Arranged in sarcomeres Branched fibers connect via intercalated discs Contain nr. mitochondria Intercalated discs  Cardiac muscle cells 0 Figure 10–16. (Meshcer) Cardiac muscle. Diagram of cardiac muscle cells indicates characteristic features of this muscle type. The fibers consist of separate cells with interdigitating processes where they are held together. These regions of contact are called the intercalated discs, which cross an entire fiber between two cells. The transverse regions of the steplike intercalated disc have abundant desmosomes and other adherent junctions which hold the cells firmly together. Longitudinal regions of these discs contain abundant gap junctions, which form “electrical synapses” allowing contraction signals to pass from cell to cell as a single wave. Cardiac muscle cells have central nuclei and myofibrils that are less dense and organized than those of skeletal muscle. Also the cells are often branched, allowing the muscle fibers to interweave in a more complicated arrangement within fascicles that produces an efficient contraction mechanism for emptying the heart. Epicardium and Pericardium – Pericardial sac Visceral Pericardium = Epicardium Parietal Pericardium Fudging heat The outside Lung  Epicardium GO.EE Atrium A/V Valve Outer surface of the heart visceral pericardium Surface is covered by mesothelium (simple squamous epithelium), a thin layer of dense connective tissue, and depending on the location, a variably thick Ventricle layer adipose tissue with blood vessels (coronary arteries and veins) Is contiguous with the endocardium at Epicardium the level of the endocardial cushion Epicardium and Pericardium Epicardium Myocardium  Heart Structure - Summary ENDOCARDIUM (can be compared to the tunica intima of blood vessels): - endothelial lining of the heart chambers surface and covers the surface of the valves. - the subendocardium contains a thin layer of connective tissue and the Purkinje fibers. MYOCARDIUM (can be compared to the tunica media of blood vessels): Cardiac muscle mass – involuntary striated muscle. EPICARDIUM (can be compared to the tunica adventitia of blood vessels): - single layer of flattened epithelial cells the mesothelium (simple squamous epithelium) - supported by connective tissue, including fat. - similar mesothelial layer lines the opposing parietal surface of the pericardial sac. - mesothelial cells secrete a small amount of serous fluid that lubricates the movement of the epicardium on the opposite parietal pericardium. - the epicardium represents the visceral layer of the pericardial sac.  Cardiac skeleton 4 dense bands of fibrous connective tissue encircle the base of the pulmonary trunk, aorta and the AV valves – provides structural support to the heart. A triangular mass of fibrous connective tissue – the fibrous trigon, connects the aortic a. ring and the L and R atrioventricular rings. This area undergoes Fibrous trigon osseous differentiation and forms the “Os Cordis” – primarily seen in cattle.  3. Vessels Arteries Elastic arteries Muscular arteries Arterioles Capillaries Continuous Fenestrated Discontinuous /sinusoids Veins Venules Lymphatic vessels  thehigh bigger xg of pressure m B agaft lowpass Source: Junqueria’s Basic Histology, 2010  Tunics of vessels INNERMOST: TUNICA INTIMA Eubank Endothelium, Internal elastic membrane Subendothelial connective tissue MIDDLE: TUNICA MEDIA Ayoub Smooth muscle and elastic lamellae/fibers OUTERMOST: TUNICA ADVENTITIA / EXTERNA Eggadir Collagen, may contain blood vessels, nerves, capillaries.  Vascular Endothelium Role in hemostasis Modulates perfusion Role in inflammation Fig. 2-6 (McGavin) Structure and function of the endothelium. Endothelium is both a physical barrier between intravascular and extravascular spaces, and it is an important mediator of fluid distribution, hemostasis, inflammation, and healing. Hemostasis - It is a physiological response to vascular damage. - Provides a group of mechanisms to seal an injured vessel to prevent blood loss. - It is the result of a complex and well-regulated process which maintains blood as a flowing fluid within the cardiovascular system. ARTERIES  Elastic Artery (e.g. Aorta) All 3 tunics exist: Tunica intima (1) is endothelium + loose connective tissue. Tunica media (2) consists largely of repeating ELASTIC LAMELLAE. D Tunica adventitia (3) contains vasa vasorum to assist in supplying Lelastic nutritional needs of thick tunica media. waves  Muscular Arteries (e.g. Femoral Artery) Tunica media is primarily SMOOTH MUSCLE. Thickest tunic is tunica media. Generally, they have a round appearance. 40X Van Giesson stain for elastic fibers  Tunica media of smooth muscle Tunica adventitia Tunica intima Lumen Tunica intima  r Tunica Media TUNICA MEDIA - Vascular smooth muscle - Smooth muscle cells are circumferentially arranged within the tunica media - Regulates DIAMETER and TONE (vasodilation/vasoconstriction) Tunica Media o T.intima Source: Dellmann’s Texbook of Veterinary Histology, 2006.  ARTERIOLES 1- 3 layers of smooth muscle cells Greatest effect on blood pressure Nuclei bulge into lumen Round appearance of vessel No internal elastic membrane in the smallest arterioles with one smooth muscle cell Metarteriole is a terminal vessel, has precapillary sphincters that can regulate flow to the capillary bed  RBC s Arteriole Smooth muscle nuclei 1000x  Arteriole vs. venule Smooth muscle nuclei V A relay Fibroblasts Small vessels embedded in loose connective tissue. Nuclei of fibroblasts can be seen along with collagen bundles.  Arteriole vs. venule haveshithmusle if Source: Dellmann’s Texbook of Veterinary Histology, 2006.  Arteriole in longitudinal section Smooth muscle cells Smooth muscle nuclei cell Epithelial  CAPILLARIES 1. Thin walled tubules of mesenchymal origin; in a cross section they are made of only one endothelial cell rolled into the tube. 2. Represent the site of exchange between blood and surrounding tissue. 3. The diameter is 7–9 µm, the length 0.25 – 1mm, in some organs (adrenal cortex, kidney medulla) can be up to 5-10 cm long. CAPILLARIES A capillary is composed of simple squamous cells which if roll to produce a tube.  0 PERICYTES (Roujet cells) Mesenchymal-like cells that wrap around the endothelium of capillaries and venules. They “communicate” with endothelial cells by physical contact and paracrine signaling. They help to maintain homeostatic and hemostatic functions in the PERICYTES brain and also sustain the blood– (TEM) brain barrier. Proliferate after injury. Pericyte = p Stem cell source. Tight junction, capillary endothelial cell = t Important in angiogenesis new Skeletal muscle = m Capillary = c vessels formation. Collagen fibers in transverse section = cl  Classification of capillaries NN QDPorff 1. Continuous: Most common. Found in muscle, brain, bone, lung, etc. E.g. Blood-brain and blood-testis barriers. 2. Fenestrated (gaps 10-100 nm): In tissues with substantial fluid exchange, e.g.: intestinal villi, choroid plexus, ciliary process, glomerular capillaries. 3. Discontinuous (Sinusoidal): Hepatic and splenic sinusoids > large molecules can exit (RBC in the spleen). liver bonemarrow  Classification of capillaries A. Continuous: Most common. Found in muscle, brain, bone, lung, etc. E.g. Blood-brain and blood-testis barriers. B. Fenestrated (gaps 10-100 nm): In tissues with substantial fluid exchange, e.g.: intestinal villi, choroid plexus, ciliary process, glomerular capillaries. C. Discontinuous (Sinusoidal): Hepatic and splenic sinusoids > large molecules can exit (RBC in the spleen). Continuous capillaries Very swell 1. Nucleus of endothelial cell 2. Pinocytotic vesicles Capillary 3. Tight junctions (arrowheads) 4. Basement membrane/lamina 2 3 4 3 Consists of a tube of endothelium 1 4 TEM Fenestrated capillaries in SEM P a b Glomerular capillary (a) View from the inside of (b) View from the outside of fenestrated capillary arrow: capillary covered by pedicels of podocyte tight junctions E - Endothelium of fenestrated capillary B - Basement membrane P – Podocyte  RENAL CORPUSCLE (glomerulus) The renal glomerulus (a tightly coiled network of fenestrated capillaries) is responsible for the filtration of plasma. Fenestrated capillary In this stained section red blood cells appear orange.  PODOCYTES Lining the Bowman's capsules (nephrons). Help prevent proteins and other large molecules from being filtered. Their foot processes (pedicels) extend and wrap around the capillaries of the glomerulus to form the filtration slits. The pedicels increase the Pudo Feet area of the cells enabling efficient ultrafiltration. Source: Junqueria’s Basic Histology, 2010  Urinary Space PODOCYTE process EC Fenestrae Glomerular Capillary Lumen Robbins’Pathology Discontinued capillaries (Sinusoids) liar benarrow spleen 1. Lumen is enlarged and irregular; diameter 20 - 40 m 2. Lining endothelium is discontinuous and fenestrated. 3. Basal lamina may be absent (discontinuous) Sinusoid - S Bone marrow Hepatic sinusoids VENULES Called POSTCAPILLARYVENULES Very ‘leaky’ vessels NO SMOOTH MUSCLE LEUKOCYTE diapedesis possible here 5mm Hg pressure in vessel. VEINS Thebigonehas  VEIN VS. ARTERY Large, wide lumen, thin walls in comparison to same-size arteries Valves present Thin tunica media The tunica adventitia is the thickest tunic Large veins may have vasa vasorum May be collapsed in histological sections the me faster collapse  y LYMPHATIC VESSELS Complementary to the cardiovascular system Transport lymph throughout the lymphatic system Valves are present Freely anastomose with one another Lined by endothelial cells Thin layer of smooth muscles Adventitia that bind the lymph vessels to the surrounding tissue Circulate the chyle or linguistic fluid Lymphatic capillary Anchoring fibrils (arrows) pull endothelial cells apart, fluid enters vessel Junqueira’s Histology  LYMPHATIC VESSELS Very thin wall, very low pressure, may contain valves. No RBCs in lymph, so appear clear.  at THANK YOU VETERINARY.ROSSU.EDU Sluded ©2021 Ross University School of Veterinary Medicine. All rights reserved.  tunionmedia thick Vein Triandin air Artery IT Homethink sundy Myocardiogtes Interclat disk Branches Striations Ende endi Ede tulin her Punke f Lamin prop counter tissue Values Tumor Arterioles Admit Media m.IE gdomthwm