CVS HISTOLOGY Dr Nabil Khouri http anatomy kmu

CVS HISTOLOGY Dr. Nabil Khouri http: //anatomy. kmu. edu. tw/Block. His/Block 3/slides/block 4_24. html

The Heart Wall

Cardiac Muscle • Contract as a single unit • Simultaneous contraction due to depolarizing at the same time • Intercalated disk to speed depolarization automaticity

M -myocardium; E - endocardium; En -endothelium; S -ubendothelial layer

Cardiac Muscle Longitudinal Section • Cardiac muscle consists of muscle cells mononucleated with centrally placed nucleus. Nuclei are oval, rather pale and which is 10 15 µm wide. • Cardiac muscle is innervated by the autonomic nervous system. • Cardiac muscle exhibits cross-striations. • Cardiac muscle is for these reasons also called involuntary striated muscle. cell nucleus One cell Intercalated Discs X 40 Magnification

The Cardiac Muscle Cells

Adherens Junction Desmosome Macula adherens – (desmosomes) transverse and lateral components. Bind Fascia adherens – major portion individual myocytes to one of transverse component. another. stop separation during Anchoring sites for actin, and contraction by binding connect to the closest sarcomere. intermediate filaments, joining the cells together. Macula adherens junctions are also called desmosomes. Gap junctions - lateral component. Allow action potentials to spread between cardiac cells by passage of ions between cells, producing depolarization of the heart muscle. Allows muscle to act as syncytium.

Cardiac Muscle Tissue – Cardiac cells are connected by intercalated discs – Intercalated discs house desmosomes and gap junction. • Desmosomes provide strength so that the cell do not get ripped apart during contraction • Gap junctions are made of the connexin proteins and form a pore through which the cells can communicate.

Cardiac Muscle “Cross section” X 40 Magnification

• 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.

Purkinje fibers 40 X Are modified cardiac muscle cells. Compared to ordinary cardiac muscle thicker cells: Contain large amounts of glycogen fewer myofibrils.

Blood Vessels histology • Blood is carried in a closed system of vessels that begins and ends at the heart • The three major types of vessels are arteries, capillaries, and veins • Arteries carry blood away from the heart, veins carry blood toward the heart • Capillaries contact tissue cells and directly serve cellular needs

General Structure of Blood Vessels

Structure of blood vessel (Tunics) • Tunica interna (tunica intima) – Endothelial layer that lines the lumen of all vessels – In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present • Tunica media – Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system – Controls vasoconstriction/vasodilation of vessels • Tunica externa (tunica adventitia) – Collagen fibers that protect and reinforce vessels – Larger vessels contain vasa vasorum

General Histology Structure of Blood Vessels

A Comparison of a Typical Artery and a Typical Vein

Histological Structure of Blood Vessels

Elastic (Conducting) Arteries • Thick-walled arteries near the heart; the aorta and its major branches – Large lumen (2. 5 -1 cm diameter) allow low-resistance conduction of blood and act as conduits – Contain elastin in all three tunics – Withstand smooth out large blood pressure fluctuations – Allow blood to flow fairly continuously through the body

Large (Elastic) artery. • Elastic Arteries are classified by: • The tunica intimae consists of a lining of endothelial cells that rest on a thin layer of connective tissue. • The tunica media arranged as lamellae, interspersed with the smooth muscle cells of the tunica media and collagen fibers are found between the layers of elastic fibers • There are no elastic lamellae in the adventitia, but elastic fibers are present, though relatively few in number and can not be observed by H&E stain. • Brown adipose tissue is one of the two types of adipose tissue. Its primary purpose is to generate body heat. In contrast to white adipocytes (fat cells) which contain a single, large fat vacuole, brown adipocytes contain several smaller vacuoles and centrally located nuclei.

Elastic (Conducting) Arteries

Muscular arteries • The tunica intimae consists of an endothelial lining and a small amount of connective tissue. • The muscular arteries are characterized by a layer of internal elastic lamina separating the tunica intima from the tunica media. • The artery has a thicker tunica media, a narrower lumen than the similarly sized vein, and thickened elastic laminae that are not present in the vein. • Muscular arteries have more smooth muscle and less elastin in the tunica media than elastic arteries. • The less prominent and more variable external elastic lamina lies between the tunica media and the adventitia. • The tunica adventitia is composed of collagen fibers (pink), elastic fibers (black) and vasa vasorum.

Muscular arteries Are called distributing arteries – Middle sized. 3 mm 1 cm – Changes diameter to differentially regulate flow to organs as needed – Internal as well as external elastic lamina – Most of what we see as “arteries” Tunica media larger in proportion to the lumen, thus “muscular” 30

Muscular artery This slide is stained with Verhoeff's stain to visualize the elastic fibers, and with eosin to show the cellular structures.

Arterioles – Smallest: . 3 mm-10 um – Only larger ones have all 3 layers – Regulated 2 ways: • Locally in the tissues • Sympathetic control – Systemic blood pressure can be regulated through them – Deliver blood into capillaries Tunica media has only a few layers of smooth muscle cells 36

• Arterioles – smallest arteries; lead to capillary beds – Control flow into capillary beds via vasodilation and constriction

“muscular” middle sized artery

Smallest ARTERIOLE Endothelial cell For fast flow & non -stick, until clotting is needed Controls passage through the wall Helps control blood flow Smooth muscle cell SMC/ VSMC Reticular fibers Mechanical support Contraction regulates flow by need Vasoconstriction Smallest arteriole, in essence, is a capillary with smooth muscle cells wrapped around it, with modifications to the endothelial cells - less transport, more interaction with SMCs.

Capillaries Heart to arteries to capillaries to veins to heart • Capillaries are smallest – 8 -10 um – Just big enough for single file erythrocytes – Composed of: single layer of endothelial cells surrounded by basement membrane • Universal function – Oxygen and nutrient delivery to tissues – CO 2 and nitrogenous waste (protein break-down product) removal • Some also have tissue specific functions

The Organization of a Capillary Bed

Capillary Beds • A microcirculation of interwoven networks of capillaries, consisting of: – Vascular shunts – metarteriole–thoroughfare channel connecting an arteriole directly with a postcapillary venule – True capillaries – 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed

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Capillary Structure Figure 21. 4

Continuous Capillaries • Continuous capillaries are abundant in the skin and muscles, and have: – Endothelial cells that provide an uninterrupted lining – Adjacent cells that are held together with tight junctions – Intercellular clefts of unjoined membranes that allow the passage of fluids • Continuous capillaries of the brain: – Have tight junctions completely around the endothelium – Constitute the blood-brain barrier

Fenestrated Capillaries • Found wherever active capillary absorption or filtrate formation occurs (e. g. , small intestines, endocrine glands, and kidneys) • Characterized by: – An endothelium riddled with pores (fenestrations) – Greater permeability to solutes and fluids than other capillaries

Sinusoids • Highly modified, leaky, fenestrated capillaries with large lumens • Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs • Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues • Blood flows sluggishly, allowing for modification in various ways

Veins • Collect blood from all tissues and organs and return it to the heart • Are classified according to size – Venules – Medium-sized veins – Large veins

The transition from capillaries to venules occurs gradually • The immediate postcapillary venules are similar structurally to capillaries, with pericytes, but range in diameter from 15 to 20 m. • A. Postcapillary venules participate in the exchanges between the blood and the tissues and, are the primary site at which white blood cells leave the circulation at sites of infection or tissue damage. • B. Venules converge into larger collecting venules which have more contractile cells. With greater size the venules become surrounded by recognizable tunica media with two or three smooth muscle layers and are called C. Muscular venules. • A characteristic feature of all venules is the large diameter of the lumen compared to the overall thinness of the wall

Venules • Venules collect blood from capillary networks and gradually merge to form veins.

PCV: Postcapillary venules. CV: Capillary venules. MV Musculsr venules

Veins • Blood entering veins is under very low pressure and moves toward the heart by contraction of the tunica media and external compressions from surrounding muscles and other organs. Valves project from the tunica intima to prevent back-flow of blood. • Most veins are small or medium veins with diameters less than one centimeter. • Located in parallel with corresponding muscular arteries. • The intima usually has a thin subendothelial layer • The media consists of small bundles of smooth muscle cells intermixed with reticular fibers and a delicate network of elastic fibers. • The collagenous adventitial layer is well-developed.

Small Veins. • (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. X 200. H&E.

Medium sized vines • Vein with a much less compact muscle layer than you saw in the preceding arteries. unica media and adventitia, which is at least as wide as the media, and often even wider. • There is no evident inner elastic membrane • (b): Micrograph of a convergence between two small veins showing valves (arrow). Valves are thin folds of tunica intima projecting well into the lumen which act to prevent backflow of blood. X 200. H&E.

(c): Micrograph of a medium vein (MV) showing a thicker wall, but still less prominent than that of the accompanying muscular artery (MA). Both the media and adventitia are better developed, but the wall is often folded around the relatively large lumen. X 100. H&E.

• (d): Micrograp h of a medium vein containing blood and showing valve folds (arrows). X 200. Masson trichrome.

LARGE VEIN Details Adventitia { { Intima Occasional circular SMC Numerous elastic fibers Bundles of longitudinal smooth muscle

• The big venous trunks, Paired with elastic arteries close to the heart, are large veins Large veins have a well-developed tunica intima, but the tunica media is relatively thin, with few layers of smooth muscle and abundant connective tissue. The adventitial layer is thick in large veins and frequently contains longitudinal bundles of smooth muscle. Both the media and adventitia contain elastic fibers, but elastic laminae like those of arteries are not present. • Most veins have valves, but these are most prominent in large veins.

Special features of veins • veins contain valves – Prevent backflow of blood • Valves – Prevent backflow – Most abundant in legs (where blood has to travel against gravity) • Muscular contraction – Aids the return of blood to heart in conjunction with valves Mechanical issues… (really good to know)

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