Heart & Blood Vessels Histology By Dr. Nabil, Khouri

1. Heart & Blood Vessels HistologyBy Dr. Nabil, Khouri The Cardiovascular System IV

2. The Heart Wall

3. The myocardium. • Note the endocardial layer, which consists of endothelium supported by a rather thick layer of sub-endocardial connective tissue (green). The ventricular lumen is indicated. Lumen

4. Cardiac muscle orientation • The tissues that comprise the myocardium, as well as the adjacent tissues of the endocardium and pericardium, are continuous, which means that the cardiac muscle is one single tissue that wraps around itself to form the heart. • The myocardial tissue in normal heart spirals up from the base to the apex, causing a series of clear intersections of cardiac muscle tissue. • The double spiral formation of the myocardial tissue allows a 60% increase in ejection fraction with a fiber shortening of 15%.

5. Different Cardiac muscle layers & Orientations Tricuspid & bicuspid Valves

6. cell nucleus Onecell Intercalated Discs Cardiac Muscle Longitudinal Section • Cardiac muscle consists of Branched muscle cells with one centrally placed nucleus. • The Nuclei are oval, rather pale and located centrally in the muscle cell which is 10 - 15 µm wide. • Cardiac muscle exhibits cross-striations. • Cardiac muscle is for these reasons also called involuntary striated muscle. X40 Magnification

7. Cardiac Muscle N

8. Cardiac Muscle Cross section & Purkinje fibers X20 Magnification X40 Magnification

9. This medium power view of the inner wall of the left ventricle shows a cluster of Purkinje fibers, that are partof the conduction system of the heart. The fibers run within the subendocardial connective tissue.

10. At high power, the structure of individual Purkinje fibers can be seen. • Much larger than normal cardiac muscle fibers, these cells still contain centrally-placed nuclei and, although they do still contain myofibrillar elements, their cytoplasm appears somewhat vacuolated Purkinje fibers Sub-endocardial CT Cardiac Cells

11. Note the coronary vessels surrounded by epicardial fat. Can you determine which are arteries and which are veins

12. Blood Vessels • 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 • www.lab.anhb.uwa.edu.au/.../Vascular.htm

13. Generalized Structure of Blood Vessels • Arteries and veins are composed of three tunics – • tunica externa • tunica media • tunica interna • Capillaries are composed of endothelium with sparse basal lamina • Lumen – central blood-containing space surrounded by tunics

14. Tunics • Tunica interne (tunica intimae) • Endothelial layer that lines the lumen of all vessels • In vessels larger than 1 mm, a sub-endothelial connective tissue basement membrane is present • Tunica media • Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system • Controls vasoconstriction/ • vasodilatation of vessels • Tunica externa (tunica adventitia) • Collagen fibers that protect and reinforce vessels • Larger vessels contain vasa vasorum

15. Histological Structure of Blood Vessels

16. Large Elastic (Conducting) Arteries • Thick-walled arteries near the heart; the aorta and its major branches • Large lumen allow low-resistance conduction of blood • Contain internal elastic lamina and fibers in all three tunics • Withstand and smooth out large blood pressure fluctuations • Allow blood to flow fairly continuously through the body

17. Elastic (Conducting) Arteries Tunica intima: Large arteries often have a large subendothelial layer. The border of the intima is delineated by the internal elastic membrane. The internal elastic membrane may not be conspicuous because of the abundance of elastic material in the tunica media. Tunica media: Both connective tissue and smooth muscle are present in the intima. It is the thickest of the three layers. The smooth muscle cells are arranged in a spiral around the long axis of the vessel. They secrete elastin in the form of sheets, or lamellae, which are fenestrated to facilitate diffusion. These lamellae, and the large size of the media, are the most striking histological feature of elastic arteries. In addition to elastin, the smooth muscle cells of the media secrete reticular and fine collagen fibers and proteoglycans (all not identifiable). No fibroblasts are present. Tunica adventitia: This is a relatively thin connective tissue layer. Fibroblasts arethe predominant cell type, and many macrophages are also present. Collagen fibres predominate and elastic fibres (not lamellae) are also present. Blood vessels supplying the adventitia and outer media are also present, these are called vasa vasorum ("vessels of the vessels").

18. { Intima Media ELASTIC ARTERY { Adventitia { Endothelium Collagen Smooth muscle cells Vasa vasorum Elastic laminae

19. Elastic artery Aorta el = elastic lamellaeend = endothelial cell nuclein = smooth muscle cell nucleiTA = tunica adventitiaTI = tunica intimaTM = tunica media

20. vasa vasorum Elastic fibers Lumen Endothelium

21. Muscular Arteries and Arterioles • Muscular arteries – distal to elastic arteries; deliver blood to body organs • Have thick tunica media with more smooth muscle and less elastic tissue • Active in vasoconstriction • Arterioles – smallest arteries; lead to capillary beds • Control flow into capillary beds via vasodilation and constriction

22. Medium Arteries: • Tunica intima: • The tunica intima is thinner than in large arteries, there are fewer smooth muscle cells and less elastic tissue. • The outermost part of the intima is defined by a very prominent internal elastic membrane (not obscured by elastic lamellae as in large arteries). • The basement membrane of the endothelium may rest directly on the internal elastic membrane, or be separated by a sub-endothelial layer of CT. • Tunica media: • Smooth muscle cells predominate in the tunica media, and little elastic material is present. As in large arteries, no fibroblasts are present. • lastic fibres (few), collagen, and ground substance are produced by the smooth muscle cells. • In tissue preparation, the internal elastic membrane of the intima appears wavy due to the contraction of the smooth muscle of the media. • Tunica adventitia: • The main constituent of the adventitia is collagen fibres, secreted by fibroblasts. Elastic fibres are also present, a concentration of such fibres at the inner boundary of the adventitia is called the external elastic membrane. • The external elastic membrane is not as prominent as the internal, and as arteries get smaller (see small arteries, below) disappears much earlier. • The tunica adventita is relatively larger than in elastic arteries,

23. VESSEL-WALL LAYERING } Endothelium Lamina propria Tunica INTIMA Internal elastic lamina Tunica MEDIA Tunica ADVENTITIA This layering scheme works well for muscular arteries, but becomes strained when applied to other vessels, e.g., in regions of the vena cava there may be no T media; in arterioles, the intima has a convincing identity only in TEM External elastic lamina

24. artery tunica externa thickest layer of vein V vein A Muscular Artery and Vein

25. Artery and vein

26. Smallest ARTERIOLE 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 Contraction regulates flow by need Mechanical support Endothelial cell Smallest arteriole, is a capillary with smooth muscle cells wrapped around it, with modifications to the endothelial cells - less transport, more interaction with SMCs.

27. Arteriole arteriole (center) and an accompanying venule (right).

28. arteriole and an accompanying venule.

29. Capillaries • Capillaries are the smallest blood vessels • Walls consisting of a thin tunica interna, one cell thick • Allow only a single RBC to pass at a time • Pericytes on the outer surface stabilize their walls • There are three structural types of capillaries: continuous, fenestrated, and sinusoids

30. Capillary Structure Figure 21.4

31. Capillary

33. 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

34. Continuous Capillaries

35. 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

36. Fenestrated Capillaries

37. 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

38. Sinusoids

39. Capillary Beds • An interconnected network of vessels consisting of • Collateral arteries feeding an arteriole • Metarterioles • Arteriovenous anastomoses • Capillaries • Venules

40. 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

41. Capillary Beds

42. The Organization of a Capillary Bed

43. Capillary Capillary

44. 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

45. tunica externa tunica media tunica interna Vein

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

47. Large vien

48. Muscular vein

49. Medium size veins • The tunica Externa • consists of the endothelium and a thin subendothelial layer with smooth muscle cells among the connective tissue elements. A thin internal elastic membrane may or may not be present. (If present, it is not nearly as prominent as in arteries). • The tunica media • is much thinner relative to that of an artery, and consists mostly of circularly arranged smooth muscle but also contains collagen fibres. • The tunicas intima and media therefore tend to be less distinct from one another than is the case in arteries. • The tunica adventitia is usually thicker than the media and is made up mostly of collagen fibres. • It may contain longitudinally oriented smooth muscle bundles. (Remember gradations between the vessels of different sizes are continuous.)

50. Medium size vein • Medium-sized vein with a much less compact muscle layer than you saw in the preceding arteries. The tunica media is indicated by bar "a". Bar "b" = adventitia, which is at least as wide as the media, and often even wider. There is no evident inner elastic membrane. (Blood in the lumen stains red here.) To the right, compare sizes and walls of one small artery (d) and two very small veins (c) and (e