Histology of the myocardium and blood vessels Prof

Histology of the myocardium and blood vessels Prof. Abdulameer Al-Nuaimi E-mail: a. al-nuaimi@sheffield. ac. uk E-mail: abdulameerh@yahoo. com

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 arranged into three concentric layers: intima, media and adventitia. The intima is the inner layer lining the vessel lumen. The adventitia is the outer layer of the blood vessel. The media is sandwiched between the intima and adventitia. 1. The intima is the thinnest layer. It is composed of a single layer of endothelial cells and a small amount of subendothelial connective tissue. The intima is separated from the media by a dense elastic membrane called the internal elastic lamina ( IEL ) 1. .

2. The media is the thickest layer and provides structural support, vasoreactivity and elasticity. It is composed of smooth muscle cells, elastic fibres and connective tissue, which vary in amount depending on the type of vessel. Smooth muscle cells contract (vasoconstriction) or relax (vasodilatation), which is controlled by autonomic nerves (nervi vasorum) and local metabolic factors. Elastic fibres allow the vessel to expand with systole and contract with diastole, thereby propelling blood forward. The media is separated from the adventitia by a dense elastic membrane called the external elastic lamina (EEL) 3. The adventitia is composed of connective tissue, 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

4 3 2 1 Wall of blood vessel 5

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.

1. Large elastic arteries (aorta, large aortic branches [eg. brachiocephalic, subclavian, common carotids, iliac arteries] and pulmonary arteries) – the media contains large amount of elastic fibres that allow it to expand with systole and recoil during diastole, thereby propelling blood forward.

Large elastic arteries

2 - Medium-sized muscular arteries (other aortic branches, eg. coronary and renal arteries) – The tunica intima has an endothelium of flattened endothelial cells. The media contains large amount of smooth muscle cells with some elastin and collagen and is sandwiched between the IEL and EEL. Smooth muscle cells vasoconstricte or vasodilate, thereby controlling lumen diameter and regional blood flow. The Tunica Adventitia is very broad, and mostly contains collagen and elastin.

Medium-sized muscular arteries

3 - Small arteries and arterioles (in the substance of organs and tissues) is a smalldiameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries The tunica intima is very thin, and mostly consists of a single layer of squamous epithelium. The tunica media consists almost entirely of one or two layers of smooth muscle cells, and there is no External Elastic Lamina. The Tunica adventitia is about the same size as the tunica media layer, merges in with surrounding tissue. 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.

Arteriole

Veins Post capillary blood flows into venules and then into progressively larger 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 intima and adventitia are similar in structure and function to arteries but 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 Veins therefore do not have the same capacity for elastic recoil and vasoconstriction as arteries. 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. The flaccid walls of veins predispose them to compression and penetration by tumor and inflammatory processes

Venule has a clear tunica intima layer, without any elastic fibres, and a tunica media with one or two layers of muscle fibres. The tunica adventitia fuses with surrounding tissue. A venule, the lumen (containing red blood cells) and endothelial cells.

Larger Vein You can identify the three layers of the vein: Tunica Intima: A thin endothelial lining, (in some veins, you may be able to see the valves). Tunica Media: This layer contains 2 -3 layers of muscle cells. Tunica Adventitia: This is the broadest layer. It contains longitudinal collagen fibres, and vasa vasorum.

Muscular veins You can identify the three layers, 1 - tunica intima (thin flattened endothelial cells), 2 - the thick muscular tunica media and 3 - the adventitia layer, which has vasa vasorum. Vasa vasorum are much more numerous in the muscular veins than in arteries of a similar size. Unlike muscular arteries, there are no internal or external elastic layers surrounding the muscle layer

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; it is predominantly muscular in arterioles and most arteries, and it is predominantly elastic in the largest arteries (the so-called elastic arteries such as the aorta and the common carotid). 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 and there is no external elastic membrane. An artery and vein frequently run together, and this facilitates their comparison. A useful generalization is that the artery has a relatively thick wall and a small lumen, whereas a vein has a relatively thin wall and a broad lumen. 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. This gives rise to a regular pattern of nuclear orientation that is lacking in the companion venous vessels.

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. These micro vessels, measuring around 5 to 10 micrometres (µm) in diameter, .

There are three main types of blood capillaries: Photography of the major types of capillaries, showing Continuous, fenestrations as well as intercellular gaps in sinusoid 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 lipidsoluble 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 Have pores in the endothelial cells (60 -80 nm in diameter) These types of blood vessels have continuous basement membrane and are primarily located in the endocrine glands, intestines, pancreas, and the glomeruli of the kidney

Sinusoidal (discontinuous) capillaries Sinusoidal capillaries are a special type of open-pore capillary, that have larger openings (30 -40 µm in diameter) in the endothelium and there is discontinuous basement membrane. These types of blood vessels allow red and white blood cells (7. 5 µm - 25 µm diameter) and various serum proteins to pass. Gaps present in cell junctions to permit transfer between endothelial cells, and hence across the basement membrane. Sinusoid blood vessels are 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 The intercalated discs represent the interface between adjacent muscle cells and contain many junctional complexes. intercalated discs are Transverse regions of the step-like 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 • During embryonic development, the mesoderm cells of the primitive heart tube align into chain-like arrays. • Has cross-striations and is composed of elongated, branched cells bound to one another at structures called intercalated discs. Contraction is involuntary, vigorous, and rhythmic.

Cardiac Muscle • They exhibit a cross-striated banding pattern comparable to that of skeletal muscle. • Each cardiac muscle cell possesses only one (or two) centrally located, pale-staining nuclei (Unlike multinucleated skeletal muscle ). • Surrounding the muscle cells is a delicate sheath of endomysium with a rich capillary network (cell membrane which is a thin layer of areolar connective tissue that ensheaths each individual cardiac muscle cell, it contains capillaries and nerves).

Cardiac Muscle • The structure and function of the contractile proteins in cardiac cells are the same as in skeletal muscle. • Cardiac muscle cells contain numerous mitochondria, which occupy 40% or more of the cytoplasmic volume (reflecting their need for continuous aerobic metabolism).

Cardiac Muscle Fatty acids, transported to cardiac muscle cells by lipoproteins, are the major fuel of the heart and are stored as triglycerides in the numerous lipid droplets seen in many cardiac muscle cells. Glycogen granules may also be present. Lipofuscin pigment granules are often found near the nuclei of cardiac muscle cells Lipofuscin are fine yellowbrown pigment granules composed of lipid-containing residues of lysosomal digestion

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