CVS 4

• Endocardium


  ≈ Tunica intima
• Myocardium


  ≈ Tunica media, Thicker in left ventricle

  
• Epicardium


  ≈ Tunica adventitia, Thin layer, May contain adipose tissue along the course of coronary artery, Visceral layer of serous pericardium, Composed of simple squamous epithelium – mesothelial cells (Mes), Blood vessels and nerve to the heart lies in epicardium

  
• Myocardium


  • Contains cardiomyocytes, Divided into contractile & conducting cells, Similar features as in interventricular disc (except membranous portion), Intercalated disc - Specialized intercellular junction, Provide both mechanical & electrophysiological coupling, Allowing cardiac myocytes to act as a functional syncytium, Ventricle myocardium thicker than atria because higher pressure required to pump the blood
• Endocardium


  Lined by simple squamous epithelium (endothelial cells), Consists of: 1. Inner layer- endothelium & subendothelial connective tissue, 2. Middle layer- connective tissue & smooth muscle cells, 3. Deeper layer- subendocardial layer, location on conducting system

  
• Heart Valve


  • Dense irregular connective tissue, avascular, Composed of 3 distinct layers: 1. Fibrosa, 2. Spongiosa, 3. Venticularis/ atrialis, Valvular interstitial cells- sustain valve homeostasis, resemble fibroblast

  
• Conducting Cells


  SA and AV node- smaller modified cardiac muscle fibre than surrounding atrial muscle cells, fewer myofibril, lack intercalated disc, AV bundle, bundle branches & purkinje fibres- larger modified cardiac muscle cells
• Histology of Cardiac Muscle

  • Nucleus oval, central, Striated appearance, Branching & anastomosing fibre, Attached to and communicate with each other by intercalated disc, Innervated by autonomic nervous system – involuntary action

  
• General Histology Structure of Arteries & Veins

  • TUNICA INTIMA- INNERMOST: i. Endothelium- single layer of squamous epithelial cells, ii. Basal lamina of endothelial cells- collagen, proteoglycans, iii. Subendothelial layer- loose connective tissue, smooth muscles, internal elastic membrane, 2. TUNICA MEDIA: Between internal and external elastic membrane (elastin layer), Consists of circumferentially arranged vascular smooth muscles cells, elastin, reticular fibres, 3. TUNICA ADVENTITIA: Longitudinally arranged collagenous tissue & elastic fibres, Thinner in artery than veins, Contains vasa vasorum & nervi vasorum in large vessels

  
• Vascular Endothelium


  Lines by simple squamous epithelium (endothelium), Formed by endothelial cells- flattened, elongated, polygonal shape, Express surface adhesion molecules and receptors
• CLINICAL CORRELATION: ATHEROSCLEROSIS - Due to coronary heart disease, myocardial infarction, stroke, peripheral artery disease, Lesion primarily in tunica intima 🡪 endothelial dysfunction, Present of macrophages, foam cells, fatty streak, fibrofatty plaque, atheromatous plaque

  
• Elastic Artery


  • Aorta and branches & pulmonary artery, Characterized by the large number of elastic lamellae in tunica media, Tunica (I) thick, consist of: Endothelial lining (En) with its basal lamina, Cells parallel to blood flow direction, Vasoactive agent & physical barrier, Subendothelial layers, Collagen fibres (CF) & elastic fibres (EF), Internal elastic membrane, Tunica media (M) - Thickest layer with 40-50 concentric layers of elastic fibres, Elastin, vascular smooth muscle cells (SMC) (no fibroblast), collagen fibres, Tunica adventitia - Collagen fibres (CF), fibroblast & macrophages, vasa vasorum, nervi vasorum

  
• Muscular Artery


  • Predominant smooth muscle, elastic materials decreases, Prominent internal elastic membrane & external elastic membrane, Tunica intima - Thinner, endothelial lining (AEn) with its basal lamina, Sparse subendothelial CT, Prominent IEM, Tunica media (TM) - Thick. Predominantly muscular layer composed of up to 40 layers of circularly arranged smooth muscle (SM), No fibroblast, EEM is present, Tunica adventitia (TA) - Relatively thick and contains collagen bundles (C), elastic fibre (EF)
• Small Arteries & Arteriole (A)

  • The walls are thicker relative to the lumen, Diameter < 0.1mm, Tunica intima- Thinnest layer, thin IEM (may not present in arteriole), Tunica media- present of gap junction, 8-10 smooth muscle layers in small artery but 1-2 in arteriole, Tunica adventitia- thin, ill-defined sheath

  
• Capillary


  Smallest diameter blood vessels, interconnecting arterioles & venules, Single layer of endothelial cells with its basal lamina, Sufficient to allow individual RBC to pass, For gas and metabolite exchange between cells & bloodstream, Pericytes- modulates capillary blood flow & vascular support, large nuclei

  
• Types of Capillary


  • Continuous Capillary, 2. Fenestrated Capillary, 3. Sinusoidal Capillary
• Continuous Capillary


  • No interruptions in the wall, Have continuous basement membrane, Eg: Found in all kinds of nervous tissue, muscular tissue, connective tissue
• Fenestrated Capillary


  • Have fenestrae (filtration pore) in the wall (covered by nonmembranous diaphragm), Have continuous basement membrane, 60 – 80 nm diameter, Found in tissues where rapid interchange of substance between tissue and blood is needed; Eg: intestinal villi, renal glomeruli, endocrine glands

  
• Sinusoidal Capillary


  • Presence of multiple fenestrations in the wall, Tortuous path & greatly enlarged diameter (30 – 40 µm) that slows the circulation, Basement membrane is discontinuous, Found in tissues with phagocytic activity; eg: Liver, haematopoietic organs (bone marrow, spleen)
• Venous System


  Veins are thin walled, Carry blood from capillaries to heart, Tunic of vein less distinct, Large veins are formed by union of smaller vein like tributary of a river, Often provided with semilunar valves, Valves serve to prevent reflux of blood
• Types of Vein


  • Venules and Small Veins: Postcapillary venules (V), Muscular venules, Small veins, 2. Medium Vein, 3. Large Vein
• Postcapillary Venules and Collecting Venule

  • Endothelial lining & pericytes, Principal site for vasoactive agents: histamine & serotonin, No tunica media, *in lymph node has cuboidal/ columnar EC, referred as high endothelial venules (HEVs), Muscular venules - 1-2 layers of vascular smooth muscles in TM, No pericytes, thin TA, Small veins - Presence all tunics, 2-3 layers of smooth muscles, Thicker TA

  
• Medium Vein


  • Thin wall, collapsed larger lumen, TI: Often no IEM easily recognised endothelial cells (EN) than muscular artery, TM: Large quantity of collagen (few SM and less elastic fibers) that is the reason veins are easily compressed, TA: Thicker than TM but thinner than muscular artery, SM slight basophilia as compared to muscular artery, consists collagen and elastic fibres, Presence of valves to prevent back flow

  
• Large Vein


  • Eg: SVC, IVC, portal V, T. Intima - Well developed, endothelium and subendothelial CT+, Unclear boundary TI and TM, T. Media - Thin or not present, circular arranged SM, T. Adventitia - Well developed, thickest coat, many longitudinal bundles of smooth muscle fibres are embedded in CT, Myocardial sleeves- atrium myocardial extension to SVC and IVC- may lead to atrial fibrillation

  
• Atypical Blood Vessels - Coronary Arteries

  • Considered to be medium-sized muscular artery, Walls thicker due to large amount of circular smooth muscle in TM, Subendothelial layer of TI of younger peoples are not visible clearly, but progressively thicken with ageing, IEM may be lost in older individuals
• Atypical Blood Vessels - Great Saphenous Vein


  • Presence of smooth muscle (SM) in all tunics, Poorly developed IEM
• Atypical Blood Vessels - Central Adrenomedullary Vein


  • Unusual tunica media- longitudinal smooth muscle (SM) in irregular arrangement (muscle cushion), May absent SM (arrowhead), hence direct contact with tunica intima

  
• Lymphatic Vessels


  Lymphatic capillaries - Lack of continues basal lamina, Lymphatic vessels (L) - Present of valve (Val), Dense irregular connective tissue (DCT), Few endothelial cells (arrow)