Dr. ANAND SRINIVASAN

 Able to :  Describe, identify and draw the histological features of :  Blood vessels

 Histologically 5 main types of blood vessels :  ARTERIES ▪ Large A. (Elastic A.) ▪ Medium sized A. (Muscular A.)  ARTERIOLES  CAPILLARIES ▪ Continuous capillary ▪ Fenestrated capillary ▪ Sinusoidal capillary  VENULES  VEINS ▪ Medium sized V. ▪ Large V.

 All blood vessel have same basic structure  TUNICA INTIMA ▪ Innermost lining endothelium (simple squamous) and subendothelial connective tissue  TUNICA MEDIA ▪ Smooth muscle and connective tissue  TUNICA ADVENTITIA ▪ Fibroelastic connective tissue

 Thick wall  E.g. Aorta and its branches  Tunica intima  Endothelium and subendothelial connective tissue  Subendothelial cells contain mactophages and smooth muscle like cells “Myointimal cells”  Separated from Tunica media by poorly defined internal elastic lamina.

 Tunica media  Mainly made of 40 – 70 layers of fenestrated elastic laminae arranged circularly.  Between elastic laminae there are smooth muscle cells and collagen fibers  Outermost / External elastic lamina is thick.  Tunica adventitia  Fibroelastic connective tissue carrying small blood vessels and sympathetic nerves

 Thickening of tunica intima – due to migration and proliferation of smooth muscle cells from tunica media  Accumulation of lipid in myointimal cells and macrophages  Formation of fibrofatty plaques in tunica intima (Atheroma)  Calcification of tunica media (Arteriosclerosis)

 Age related changes (Atheroma ± arteriosclerosis)  Not only in large A. but also coronary and cerebral A.  Can lead to ischemia / infarction

 Atrophy of tunica media – loss of elasticity of artery

 E.g. branches of external carotid A., radial A. etc.  Distributes blood to various parts of body  Tunica intima :  Endothelium and internal elastic lamina  No subendothelium  Internal elastic lamina clearly seen and thrown into folds due to contraction of smooth muscle in media

 Tunica media  Smooth muscle cells (~40 layers) arranged circularly  Also contains elastic and few collagen fibers  Tunica adventitia  Presence of external elastic lamina  Inner part of adventitia contains more elastic  Middle part contains more collagen  Outer part contains loose connective tissue

 Small artery having diamater less than 0.5 mm.  Meta arterioles – terminal branches of arterioles acting like sphincter  Tunica intima  Only endothelial lining  No subendothelium or internal elastic lamina  Tunica media  1 – 5 layer of circular layer of smooth muscle  Tunica adventitia  Thin and poorly developed & contain sympathetic

 CONTINUOUS  Commonest type present in tissue, mucle, brain  FENESTRATED  Found in kidney, intestines  SINUSOIDAL  Found in liver, spleen and bone marrow

 Larger diameter than arterioles (0.5 – 1 mm)  Tunica intima  Endothelium  Tunica media  1 – 2 layer of smooth muscle cells  Tunica adventitia  Thick made of collagen fibers

 Tunica intima  Endothelium & subendothelium  No internal elastic lamina  Tunica media  Few circular smooth muscle, mostly collagen  Tunica adventitia  Loose fibroelastic connective tissue  Compared to Medium sized A.  Collapsed lumen  Thin wall of tunica media with few smooth muscle & elastic fibers  No internal elastic lamina  Presence of valves

 E.g. Superior vena cava  Tunica intima  Well developed endothelium and subendothelium  Tunica media  Thin / absent  Tunica adventitia  Well developed and thickest coat  Longitudinally arranged smooth muscle

 Wheater’s Functional Histology  Cell Biology & Histology – Board Review Series  Microanatomy workbook – RAKMHSU