HISTOLOGY OF THECIRCULATORY SYSTEM,BLOOD CELL PRECURSORSAND ERYTHROPOIESIS
Objectives¨ Learn about different vessels in the body.¨ Learn about red blood cell precursors and erythropoiesis.¨ Identify the histologic differences between vessels.¨ Identify the histologic differences between the erythroid precursors.
Histology of the Circulatory System¨ Heart - pumps the blood¨ Arteries - efferent vessels¨ Capillaries - anastomosing thin tubules where interchange between blood and tissue takes place¨ Veins - convergence of the capillaries into a system of larger channels to the heart¨ Lymphatic vascular system
Basic structure of arteries and veins¨ Tunica intima ¤ Endothelial layer. ¤ Subendothelial layer (loose connective tissue). ¤ Internal elastic lamina (ie. Elastin, in arteries and large veins).¨ Tunica media ¤ Concentric layers of helically arranged smooth muscle cells. ¤ Varying amounts of elastic fibers, reticular fibers, and proteoglycans. ¤ External elastic lamina, thin (larger muscular arteries).¨ Tunica adventitia ¤ Longitudinally oriented collagen (type I) and elastic fibers. ¤ Vasa vasorum (and in outer part of media) ¤ Unmyelinated autonomic nerve fibers (larger vessels)
Vasa Vasorum¨ In larger vessels¨ Small vessels branch profusely in the adventitia and outer part of the media¨ Nourish the media¨ Large veins usually have more vasa vasorum
Innervation¨ The vasomotor nerves for vasoconstriction (NE). ¤ Greater innervation in arteries than veins.¨ Sensory nerve endings – ¤ baroreceptors in the carotid sinuses and arch of aorta and ¤ chemoreceptors in the carotid and aortic bodies
Layers¨ Endothelium: squamous, polygonal and elongated with long axis in direction of blood flow.¨ Smooth muscle: in walls of all vessels except capillaries. Arranged helically in layers. Connected via gap junctions in arterioles and small arteries.¨ Connective tissue: in variable proportions in walls. ¤ Collagen fibers are in the subendothelial layer, between smooth muscle layers and in outer covering. ¤ Elastic fibers: in large arteries between the muscle layers.
Differences between vessels¨ Arteries have thicker media and narrower lumen.¨ Veins have larger lumen and the adventitia is the thickest layer. ¤ Intima of veins is folded to form valves.¨ Capillaries have only endothelium; no other layers.
Elastic arteries “conducting”¨ Aorta¨ Pulmonary artery¨ Their branches¨ Features: ¤ Well developed intima with many smooth muscle fibers that show folds. ¤ The internal elastic lamina NOT easily seen. ¤ Thick media, where elastic lamellae alternate with smooth muscle fibers. ¤ Thin adventitia.
Muscular arteries “distributing”¨ Distribute blood to organs and help regulate blood pressure.¨ Features: ¤ Thin subendothelial layer. ¤ Prominent internal elastic lamina. ¤ Have external elastic lamina.
Capillaries¨ Single layer of endothelial cells¨ Diameter 7 to 9 µm¨ Gap junctions are present¨ Held together by zonula occludens¨ Pericytes - partly surround the endothelial cells- they have myosin, actin, & tropomyosin--contractile function¨ Most endothelial cells are continuous ¤ Fenestrated - kidney, intestine, endocrine ¤ Discontinuous - liver, spleen, bone marrow
Capillary(b) and venule(a)
Arterioles¨ < 0.5mm in diameter¨ Endothelial cells¨ Thin subendothelial layer¨ Most have no internal elastic layer¨ 1 or 2 layers of smooth muscle¨ No external elastic lamina¨ Adventitia very thin.
Small artery &Vein (arrow) artery
Carotid Bodies¨ Small structures near the bifurcation of the carotid artery¨ Chemoreceptors (low 02 tension, high CO2 and low blood pressure)¨ Composed of glomus cells surrounded by fenestrated capillaries and many afferent nerve fibers “paraganglia”¨ Glomus cells have numerous dense core vesicles that store dopamine, norepinephrine and serotonin
Carotid Sinuses¨ Slight dilation of internal carotid artery¨ Contain baroreceptors that detect changes in blood pressure
Arteriovenous anastomoses¨ Direct communication between arterial and venous circulation¨ Play a role in regulation of blood pressure, flow and temperature in various areas¨ Glomus body- complex anastomoses between arterioles and venules. Seen in fingerpads, fingernail beds, and ears
Venules¨ Large diameter but thin wall.¨ Capillaries to venules:¨ Postcapillary venules (similar but bigger diameter than capillaries)¨ Collecting venules (more contractile cells)¨ Muscular venules (have recognizable tunica media)
Veins¨ Capacitance vessels - 70% of blood volume¨ Small and medium sized veins: ¤ thin intima (thin subendothelial layer, valves lined by endothelium on both sides) ¤ thin media (small bundles of smooth muscle fibers, reticular and elastic fibers) ¤ thicker adventitia¨ Large veins: ¤ Well developed intima ¤ Relatively thin media ¤ Larger adventitia than media with longitudinal bundles of smooth muscle.
Artery vein
Muscular vein
Muscular vein with valve
Red Blood Cells and Erythropoiesis
Erythrocytes (RBCs)¨ Small, biconcave discs¨ Transport oixs yfuglel nofahnedmCoOg2lo, bin cytoplasm molecules¨ Have no nuclei or organelles¨ Pick up Oit 2aat tbloudnyg capillaries and release tissue capillaries¨ Survive for ~100-120 days in the circulation. Worn out RBCs are removed by macrophages of the spleen and liver.
Erythrocytes…or red blood cells (RBCs)areanucleated cells that travel throughout the CirculatorySystem and carry Oxygen to all parts of the bodyvia the abundant hemoglobin found within them.
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