1. Histology for Pathology Circulatory System Theresa Kristopaitis, MD Associate Professor Director of Mechanisms of Human Disease Kelli A. Hutchens, MD, FCAP Assistant Professor Assistant Director of Mechanisms of Human Disease Loyola Stritch School of Medicine

2. OBJECTIVES 1. Given a histologic section of a large or medium sized artery, identify the a) tunica intima b) tunica media c) tunica adventitia 2. Identify the following structures in a histologic section of an artery a) Endothelium b) Internal elastic lamina c) External elastic lamina d) Vasovasorum 3. Distinguish the characteristics which separate large, medium and small arteries and arterioles. 4. In a tissue section, distinguish a medium sized artery from a medium sized vein 5. In a tissue section identify capillaries, describe their structure and function 6. Compare and contrast general structural features of the arterial vs venous system 7. Describe the structure and function of a lymphatic vessel

3. Vessel Types Arteries – Large artery – Medium artery – Small artery – Arteriole Veins – Large Vein – Medium Vein – Small Vein – Venule Capillaries – Continuous – Fenestrated – Discontinuous

4. Three layers (tunica) – Tunica intima(inner most layer) Endothelium Subendothelium Internal elastic lamina (IEL) in arteries – Tunica media Smooth muscle layer External elastic lamina (EEL) in arteries – Tunica adventitia (outer most layer) Connective tissue and fibroblasts Longitudinal smooth muscle in veins Vasovasorum in large vessels Overall Structure of Vessels

5. Structure of Vessels

6. a = tunica intima - endothelial lining plus thin layer of underlying connective tissue called the subendothelium. b = tunica media - alternating layers of elastic membranes (elastic lamina) and smooth muscle. c = tunica adventitia - fairly dense connective tissue carrying small blood vessels, the vasa vasorum Large Artery a b c Endothelium Subendothelium Elastic fibers Vasa vasorum

7. High Power of AortaTunica Media

8. Endothelium Endothelium: Composed of single layer of squamous cells, provides a permeable barrier, angiogenesis, release of single molecules.

9. Medium Artery Also called muscular artery because the wall is dominated by smooth muscle. Similar to large artery but internal and external elastic lamina are well defined and lack prominent vasovasorum.

10. Small Artery & Arterioles Small Arteries: Generally have same structure as medium artery but have a smaller diameter and no external elastic lamina. The tunica media also has fewer layers of smooth muscle cells. Arterioles: The smallest arteries, lead blood flow into capillary beds. Only two layers of smooth muscle cells. Internal elastic lamina, external elastic lamina, and subendothelial layers usually absent. Arteriole

11. Large ArteriesMedium Arteries Small arteriesArterioles Tunica MediaSmooth muscle cells + Large quantity of elastic fibers Dominated by multiple layers of smooth muscle cells (6-40) 2-6 layers of smooth muscle 1-2 layers of smooth muscle cells FunctionConduct blood from heart. Walls recoil. Accommodate pressure changes. Maintain continuous blood flow during diastole. Distributing arteries Help control and modulate blood pressure Control blood flow to capillaries. Important role in regulating blood pressure. ExamplesAorta and its large branches – subclavian, carotid, iliac Coronary, RenalWithin substance of tissues and organs

12. Capillary Smallest vessels Connect arterioles and venules One layer of endothelial cells with a basal lamina A capillary lying in the endomysium between skeletal muscle fibers. This one shows very dark endothelial nuclei and has 3 pink red blood cells* lined up in a row inside *

13. Venules and Small Veins Similar except Small veins may have slightly larger lumen and more visible smooth muscle layer Venules have small lumens, thin walls, and only single layer of endothelum. Have surrounding connective tissue

14. Vein Medium-sized vein with a much less compact muscle layer than in arteries. a - tunica media b – tunica adventitia, which is at least as wide as the media, and often even wider. LUMEN (filled with RBCs)

15. Medium Vein Tunica intima Smooth muscle bundles Quite similar to a large vein but smaller lumen, tunica adventitia contains fewer bundles of longitudinal smooth muscle and vasa vasorum is not prominent. Tunica media Tunica adventitia

16. Valves Folds in the intima seen in medium and larger veins Number of valves increase with size of vessel Prevent backflow of blood Also present in the lymphatic vessels

17. Large Veins Vasa vasorum Smooth muscle bundles c a b a.Tunica intima b.Tunica media with a circular smooth muscle layer c.Tunica adventitia: thickest layer with many longitudinal smooth muscle bundles and vasa vasorum.

18. Artery vs Vein Arterial systemVenous system LumenSmaller, rounder. Prominent internal elastic lamina Larger, flatter Tunica mediaThicker than tunica adventitia Tunica adventitiaThicker than tunica media Longitudinal smooth muscle bundles present ValvesNoYes Artery Vein

19. Lymphatic system Composed of lymphatic capillaries, vessels, and ducts Collect and drain interstitial fluid from tissue in the large veins Have large lumens and relatively thin walls Single layer of endothelium Connective tissue outer layer with few smooth muscle cells Also have valves

20. Lymphatics Thinned walled vessels with large lumens and valves. There are some fat cells and lymphocytes in the surrounding connective tissue. Valve