University:
Kentucky State University
Course:
BIO 301 | Human Anatomy
Academic year:

2019
Views:

80

Pages:

37
Author:

Tyshawn Shaffer

Histology of the myocardium and blood vessels Histology of blood vessels • The walls of arteries and veins are composed of endothelial cells, smooth muscle cells and extracellular matrix (including collagen and elastin). • These are arranged into three concentric layers: intima, media and adventitia. Vessel tunics I. Tunica intima: • Endothelium • Subendothelial connective tissue • Internal elastic lamina (membrane) II. Tunica media: • Smooth muscle • Elastic fibres • External elastic lamina (membrane) III. Tunica adventitia: • Connective tissue • Vasa vasora Tunica Intima • The innermost layer of the vessel • Consists of three components: (A) Endothelium (B) Basal lamina of the endothelial cells (C) Subendothelial layer The subendothelial layer of the intima in arteries and arterioles contains a sheet like layer or lamella of fenestrated elastic material called The internal elastic membrane. Endothelium • In the adult human body, a circulatory system consists of about 60,000 miles of different-sized vessels that are lined by a simple squamous epithelium called endothelium • The endothelium is formed by a continuous layer of flattened,elongated, and polygonally shaped endothelial cells Functions of Endothelium 1 Maintenance of selective permeability barrier  simple diffusion, active transport…. 2Maintenance of non-thrombogenic barrier  secretion of anticoagulants, antithrombogenic agents…. 3- Modulation of blood flow and vascular resistance  secretion of vasoconstrictors and vasodilators 4- Regulation of cell growth  secretion of growth stimulating or inhibiting factors 5- Regulation of immune responses  expression of adhesion molecules…. 6 Maintenance of extracellular matrix  synthesis of basal lamina 7 Involvement in lipoprotein, cholesterol, metabolism  oxidation of LDL Sheet note #1 LDL is short for Low Density Lipoproteins *‫*اخذناهم بالـ جينيرال فسيولوجي‬ Subendothelial Connective Tissue Composed of: •Loose connective tissue. •Few scattered longitudinally arranged smooth muscle cells Internal Elastic Lamina • Well developed in muscular arteries. • Composed of fenestrated sheet of elastin. • Permits diffusion of substances into deeper layers. Tunica Media • The largest layer in arteries. • is the thickest layer and provides structural support, vasoreactivity and elasticity. • Contains smooth muscle fibres, elastic fibres, collage type III and proteoglycans which vary in amount depending on the type of vessel. • In capillaries and post capillary venules this layer is replace by pericytes. • The fibres and ground substance are secreted by smooth muscle cells. • There are NO fibroblasts in this layer. • Smooth muscle cells contract (vasoconstriction) or relax (vasodilatation), which is controlled by autonomic nerves (nervi vasorum) and local metabolic factors. • The media is separated from the adventitia by a dense elastic membrane called the external elastic lamina (EEL) Fibroblasts are not present in the tunica media Smooth muscle cells synthesize The collagen Elastin and other molecules of the extracellular matrix Sheet note #2 Why is the Tunica Media the largest layer in atteries? As written before: 1- Structural support. We need this structural support in the arteries because of the pressure that the walls of the arteries endure, unlike veins. (which we are later going to see)2- Vasocreactivity & elasticity. Because the arteries need to contract and relax which requires this layer, which contains lots of smooth muscles, to be very thick. - Usually fibroblasts produce fibers and ground substance, but here in this layer, smooth muscle secretes them instead. (there are no fibroblasts in the Tunica Media) External Elastic Lamina Present in large and muscular arteries. More delicate than the internal lamina. It is also fenestrated. Tunica Adventitia • Thickest layer in veins. • Composed mainly of fibroblasts, type I collagen and few elastic fibres. • It is continuous with the surrounding connective tissue. • The adventitia is composed of nutrient vessels (vasa vasorum) and autonomic nerves (nervi vasorum). The intima and inner part of the media are nourished by diffusion of oxygen and nutrients from blood in the lumen, and the adventitia and outer part of the media are nourished by vasa vasorum Sheet note #3 - External Lamina CAN NOT be found in veins and arterioles (small arteries ) - More delicate means thinner - Vasa Vasorum literally means “vessels of vessels”, nervi vasorum means “nerves of vessels” Arteries • The walls of arteries are thicker than that of veins to withstand pulsatile flow and higher blood pressures. • As arteries become smaller, wall thickness gradually decreases but the ratio of wall thickness to lumen diameter increases • Arteries are divided into three types according to size and function. • The constituents of the media of these vessels differ in their relative amounts accordingly. Classification of Arteries 1. Large, elastic (conducting) arteries: Aorta, Brachiocephalic, Common carotid, Subclavian, Common iliac and pulmonary trunk. 2. Medium sized, muscular arteries : femoral, brachial, radial…… 3. Small sized, arterioles Large, elastic (conducting) arteries Tunica intima • Simple squamous epithelium, made of one layer of endothelial cells. The endothelium rests on a basal lamina • The subendothelial layer of connective tissue consists of a delicate, interlacing network of collagen and elastic fibers. • The internal elastic lamina is indistinct because the innermost elastic lamina of the media blends with adjacent laminae, without clear distinction between them Sheet note #4 Muscular Arteries ‫وال‬Elastic (conducting) arteries‫ليش مهم؟ عشان هاي خاصية خاللها بقدر اميّز بين ال‬ ‫اللي ستُذكر بعد قليل‬ Tunica media • consists of many fenestrated lamella of elastin. • The number of elastic lamella increase with age (35-70). • The extracellular matrix is secreted by smooth muscles. • A fenestrated external elastic lamina is present; it allow diffusion of nutrient from the adventitia to the media. • the media contains large amount of elastic fibres that allow it to expand with systole and recoil during diastole, thereby propelling blood forward. Tunica adventitia • • • • It is relatively thin. Composed of loose connective tissue. Contains few scattered smooth muscles. Vasa vasorum are abundant and may extend to the media. Histological sections for (A) aorta Verhoeff-Van Gieson stain (VVG, elastin) Sheet note #5 Please take a look at the next slide before returning to this one for better understanding. Another thing to notice here (other than special staining) is The section on the lower right belongs to a child because of the low number of elastic lamella (which means thinner Tunica media), whereas the one on the left belongs to an adult because of the large number of elastic lamella (as mentioned in slide #15) “The number of elastic lamella increase with age (35-70).” Muscular arteries Distributing arteries • • • • • Tunica intima thinner than in elastic arteries. Sub endothelial CT contains few smooth muscles. Prominent internal elastic lamina, which might be duplicated. Processes from endothelium pass through fenestrae in IEL and form gap junction with smooth muscles in the media. Tunica media • is composed mostly of smooth muscles. • Smooth muscles are circularly arranged. • The number of layers of smooth muscles is 4-40. • Each smooth muscle has a basal lamina. • Other components: type III collagen and chondroitin sulfate. • External elastic lamina is seen in large muscular arteries. Tunica adventitia • contains elastic and collagen fibres. • Ground substance contains dermatan sulfate and heparan sulfate. • Occasional longitudinal smooth muscles are seen. • Contains vasa vasorum and unmyelinated nerve fibres. Sheet note #6 This is why I mentioned (in slide #14) that the Internal Elastic Lamina is very important You can see here that it’s 1- very clear 2- wavy Which is very different from what we have seen in large elastic (conducting) arteries Arterioles • • • • • • Sheet #7: So you don’t get lost: IEL: Internal Elastic Lamina | EEL: External Elastic Lamina Their diameter is < 0.1 mm. They control the peripheral resistance and regulate flow to capillaries. IEL may be present. Tunica media is 1-3 layers of smooth muscles. No EEL. Tunica adventitia is small. Layers of smooth muscle are the primary site of vascular resistance. In vessels of this size, smooth muscle contraction causes dramatic changes in lumen diameter, thereby controlling systemic blood pressure as well as regional blood flow. Arterioles and small arteries exhibit a distinctive arrangement of endothelial cells and smooth muscle fibres in their walls. The endothelial cells are oriented longitudinally, whereas the smooth muscle fibres in the adjacent tunica media are wrapped around these vessels in a circular fashion. Sheet note #8 *quoting the doctor:* Notice that the Endothelial cells (labeled En) are longitudinal Compared to the ones outside the lumen which are smooth muscle cells (labeled SM) that are more circular *to distinguish arterioles* Metarterioles They directly supply capillary beds. The smooth muscle layer in the media is incomplete. Each smooth muscle encircles the endothelium. They act as sphincters. Veins •Compared to arteries, veins have larger diameters and thinner walls. •They therefore have larger lumens and contribute capacitance to the circulation, holding approximately two thirds of all circulating blood. •the media is much thinner due to significantly less smooth muscle and elastic tissue. •Internal and External elastic layers are not seen in the veins •Blood is propelled forward by contraction of surrounding muscles and pressure created during inspiration and expiration. •Reverse flow is prevented by the presence of venous valves. Venules and small veins Their walls are similar to capillaries. Venules (post capillary) have thin endothelium surrounded by pericyte and reticular fibres. In larger venules, pericytes are replaced by scattered smooth muscles. In small veins, smooth muscles form a continuous layer. has a clear tunica intima layer tunica media with one or two layers of muscle fibres. The tunica adventitia fuses with surrounding tissue. Medium veins Less than 1 cm in diameter. Drain most of the body. Occasionally the endothelium is surrounded by elastic fibres but no IEL. Tunica media: Smooth muscles are loosely organized with collagen fibres and fibroblasts in between. Tunica adventitia: collagen and elastic bundles are longitudinally arranged with few smooth muscles. Unlike muscular arteries, there are no internal or external elastic layers surrounding the muscle layer Sheet note #9 Other than the size of the lumen and the thickness of the Tunica Media layer, another way to distinguish these two is: in the artery\arteriole the lumen has a (more) well defined circular structure, whereas in the vein\venule it isn’t as circular in appearance and it may be bent. Large veins Include: SVC, IVC and veins forming them and their major tributaries, pulmonary and portal veins Tunica intima: similar to medium veins but with thick subendothelial tissue. Tunica media: poorly develop in most of the large veins. Tunica adventitia: well developed. DISTINGUISHING FEATURES Arteries: An internal elastic membrane is always present (although it is less distinctive in large elastic arteries). The tunica media is the thickest layer The tunica adventitia : is relatively thin. Veins: An internal elastic membrane is absent. The tunica media is relatively thin. The tunica adventitia: is the thickest tunic no external elastic membrane. Sheet note #10 Regarding the previous slide: External elastic membrane was mentioned the (veins) part but what about arteries? - It is NOT present in arterioles - It is present in arteries Regarding the next slide: The blood flow being slow in capillaries is very beneficial, why? Because it makes a room for proper nutrient exchange. Capillaries Capillaries connect arterioles with venules. They consist only of a single layer of endothelial cells on a basement membrane. There is no media or adventitia. The diameter is just wide enough for passage of a red blood cell, therefore flow is very slow. These features facilitate exchange of oxygen, nutrients and other substances between blood and tissues. Continuous capillaries Continuous capillaries are continuous in the sense that the endothelial cells provide an uninterrupted lining, and they only allow smaller molecules, such as water and ions to pass through their intercellular clefts. However lipid soluble molecules can passively diffuse through the endothelial cell membranes They are found primarily in skeletal muscles, fingers, gonads, and skin. They are also found in the central nervous system. These capillaries are a constituent of the blood–brain barrier Fenestrated capillaries Fenestrated capillaries Have pores in the endothelial cells These types of blood vessels have continuous basement membrane primarily located in the endocrine glands, intestines, pancreas, and the glomeruli of the kidney Sinusoidal (discontinuous) capillaries that have larger openings in the endothelium discontinuous basement membrane. These types of blood vessels allow red and white blood cells and various serum proteins to pass. primarily located in the bone marrow, lymph nodes and adrenal glands, and are present in the liver and spleen, where greater movement of cells and materials is necessary. Cardiac Muscle • Cardiac muscle has cross-striations and composed of elongated, branched cells bound to one another at structures called intercalated discs. •Contraction is involuntary, vigorous, and rhythmic. • Each cardiac muscle cell possesses only one (or two) centrally located, pale-staining nuclei •Surrounding each cardiac muscle cell is a delicate sheath of endomysium intercalated discs have many desmosomes and fascia adherentes; these bind cardiac muscle cells firmly together to prevent their pulling apart under constant contractile activity. •Cardiac muscle cells contain numerous mitochondria, which occupy 40% or more of the cytoplasmic volume (reflecting their need for continuous aerobic metabolism).

Histology of the Myocardium and Blood Vessels

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