Histology midterm

• Endocardium

  The lining of the interior surface of the heart Chambers
• Endocardium


  • Thin inner layer of endothelial cells that rest on a layer of collagen fibers
  • Fibrous layer called the subendothelial layer
  • Thicker middle layer of connective tissue that contains elastic fibers and smooth muscle fibers
  • Subendocardial layer merges with the myocardium and contains collagen fibers, elastic fibers, and blood vessels
• Myocardium

  The thickest layer, consists mainly of cardiac muscle with its fibers arranged spirally around each heart chamber
• Myocardium

  • Contacts between branching myocytes forms an anastomosing network with blood vessels in the spaces
  • Myocardial muscle bundles are oriented to make most efficient use of the force of contraction
  • Cells are butted together at their ends
  • Single nucleus
• Epicardium

  Thin serous membrane overlying the outer surface of the organ
• Epicardium

  • Inner portion of the CT envelope that surrounds the heart and allows the heart to move freely in its cavity without adhesion to surrounding structures
  • Simple squamous mesothelium supported by a layer of loose connective tissue containing blood vessels and nerves
  • Corresponds to the visceral layer of the pericardium
  • Site of the coronary vessels and contains considerable adipose tissue
  • Mesothelial cells secrete a lubricant fluid that prevents friction as the beating heart contacts the parietal pericardium
  • Underlying structures are cushioned by deposits of adipose tissue
• Cardiac skeleton

  Dense fibrous connective tissue that forms part of the interventricular and interatrial septa, surrounds all valves of the heart, and extends into the valve cusps and the chordae tendinese
• Cardiac skeleton

  • Anchoring and supporting the heart valves
  • Helping coordinate heart beating
  • Providing firm points of insertion for cardiac muscle
• Cardiac conduction system

  Modified cardiac muscle cells within the subendocardial layer and adjacent myocardium that make up the impulse conducting system
• Cardiac conduction system

  1. Atrioventricular AV node is continuous with specialized bundles of cardiac muscle fibers, the AV bundles that run along the interventricular septum to the apex of the heart, where they branch further as conducting (Purkinje) fibers that extend into myocardium of the ventricles

  2. Purkinje fibers are pale-staining fibers with peripheral myofibrils and much glycogen, a centrally-located single nucleus, and intercalated disks, may see striations

  3. Conducting system coordinates the contractions of cardiac chambers

  4. Both atria contract together, as do the ventricles

  5. Provides the heart its automatic rhythmic beat
• Cardiac vascularization and innervation

  • Extensive degree of vascularization in myocardium
  • Heart pumps blood to itself before it sends any to other parts
  • The first branches off the aorta are the coronary arteries of the myocardial circulation
  • Lymph capillaries form a network in the cardiac connective tissue
  • Small lymphatics are continuous with larger lymph vessels
  • Both parasympathetic and sympathetic neural components innervate the heart
  • The nerves that innervate the heart form extensive plexuses that are particularly dense around the sinoatrial and atrioventricular nodes where they affect heart rate and rhythm
  • Stimulation of the parasympathetic division slows the heartbeat
  • Stimulation of the sympathetic nerve accelerates activity of the pacemaker
  • Between fibers of the myocardium are afferent free nerve endings that register pain
• Abnormalities in the structure of heart valves

  • Can be produced by developmental defects, scarring after certain infections, or cardiovascular problems such as hypertension
• Arteries

  Pump blood away from the heart
• Veins

  Carry blood towards the heart
• Capillaries

  Small vessels within the tissues from which oxygen transfer takes place
• Walls of all blood vessels except capillaries bv structure

  • Contain smooth muscle and connective tissue in addition to the endothelial lining
  • Smooth muscle fibers are helically arranged, connected through gap junctions and permit vasoconstriction and vasodilation
  • Connective tissue components present in vascular walls in variable amounts and proportions
  • Collagen fibers are found in the subendothelial layer
  • Elastic fibers provide the resiliency required for the vascular wall to expand under pressure
  • Elastin is a major component in large arteries
• Artery structure

  • Tunica adventitia, media, intima and then endothelium with internal elastic fibers
• Vein structure

  • Same as artery but adventitia is more large and media thinner
• Arterioles, capillaries, and venules

  • Usually have vasa vasorum in the adventitia and outer part of the media
• Endothelium

  • Specialized epithelium that acts as a semipermeable barrier between blood plasma and interstitial tissue fluid
  • Cells are squamous, polygonal, and elongated with the long axis in the direction of blood flow
  • Highly differentiated to mediate and actively monitor the bidirectional exchange of molecules
  • Presents a nonthrombogenic surface on which blood will not clot and actively secretes agents that control local clot formation
  • Regulates local vascular tone and blood flow by secreting various factors that stimulate smooth muscle contraction or relaxation
  • Has several roles in inflammation and local immune responses
  • Secretes various growth factors, including proteins promoting proliferation of specific white blood cell lineages and cells that make up the vascular wall
  • Secretes angiogenic factors that stimulate endothelial cells to recruit smooth muscle cells and fibroblasts to form the other tissues of the vascular wall
• Elastic arterioles

  • Large ones, with very strong and relatively elastic walls, whose function is to conduct the bulk of the blood to regions of the body
  • Examples include the aorta and pulmonary arteries
  • Carry a stream of liquid under high pressure
  • The most prominent feature is the thick media in which elastic lamellae, each about 10 μm thick, alternate with layers of smooth muscle fibers
  • Lumen is lined with a simple squamous epithelium
  • The intima is well developed
  • Adventitia is collagenous in nature and is much thinner than the media
  • Elastic fibers allow stretching and "springiness" in response to the pressure, and the collagen fibers limit the degree of stretch permitted
  • Internal elastic lamina is not easily visible because it is similar to the elastic laminae of the next layer
  • Large elastic arteries have their own internal blood circulation system and nervous supply
  • Smooth muscle in the wall is innervated by the nervi vasorum so that the CNS can control blood pressure and initiate contractions
• Histological structure of muscular arteries

  • Proportion of elastic fibers decreases, and the proportion of smooth muscle increases
  • Some elastic fibers and collagen fibers still there
  • Intima has a very thin subendothelial layer and a prominent internal elastic lamina
  • Media may contain up to 40 layers of large smooth muscle cells interspersed with a variable number of elastic lamellae
  • External elastic lamina present only in the larger muscular arteries
  • Adventitia consists of connective tissue
  • Lymphatic capillaries, vasa vasorum, and nerves are found
• Muscular type artery

  • Femoral artery
• Histological structure of muscular arterioles

  • 3 or 4 medial layers of smooth muscle
  • Smallest arterioles have only 1 or 2 smooth muscle layers, indicating the beginning of an organ's microvasculature where exchanges between blood and tissue fluid occur
  • Lumen approximately as wide as the wall is thick
  • Diameter less than 0.1mm
  • Subendothelial layer is very thin, elastic laminae are absent, and the media consists of the circularly arranged smooth muscle cells
  • Adventitia is very thin and inconspicuous
• Histological structure of capillaries

  • Small vessels with thin walls to allow diffusion of nutrients, oxygen and carbon dioxide
  • Single layer of endothelial cells rolled up as a tube
  • Diameter varies from 4 to 10 μm
  • 90% of the body's vasculature
  • Normally associated with perivascular contractile cells called pericytes with long cytoplasmic processes, which produce their own basal lamina and can proliferate and differentiate to form smooth muscle after tissue injuries
• Closed capillaries

  Can move material in and out using a process of sequential endocytosis and exocytosis, simple diffusion
• Continuous capillaries

  Have well-developed occluding junctions between slightly overlapping endothelial cells, which provide for continuity along the endothelium
• Fenestrated capillaries

  Allow more extensive molecular exchange across the endothelium, have actual pores in their walls, found in endocrine organs and in the kidney
• Discontinuous capillaries (sinusoids)

  Permit maximal exchange of macromolecules as well as allow easier movement of cells between tissues and blood, have incomplete basement membrane
• Histological structure of postcapillary venules

  • Immediate postcapillary venules are similar structurally to capillaries with pericytes, but in diameter from 15 to 20 μm
  • Primary site at which white blood cells adhere to endothelium and leave the circulation at sites of infection or tissue damage
  • Converge into larger collecting venules that have more contractile cells
  • Feature is the large diameter of the lumen compared to the overall thinness of the wall
• Histological structure of veins

  • Vessels leading blood back towards the heart
  • Have much thinner walls than arteries
  • Some veins have venous valves to prevent backflow, operate at low pressure
  • Thin walls are important because much of the pressure that drives blood through veins is generated by contraction of the muscles
  • Large veins have a muscular media layer that is very thin compared to the surrounding adventitia of dense irregular connective tissue
• Histological structure of lymph capillaries

  • Tiny, thin-walled vessels located in the spaces between cells which serve to drain extra-cellular fluid
  • Collected fluid and associated cells is known as lymph
  • Slightly larger in diameter than blood capillaries, and have closed ends
  • Their unique structure permits interstitial fluid to flow into them but not out
  • Attached to the lymphatic capillaries are anchoring filaments, which contain elastic fibers and attach lymphatic endothelial cells to surrounding tissues
  • When excess interstitial fluid accumulates and causes tissue swelling, the anchoring filaments are pulled, making the openings between cells even larger
• Histological structure of lymph vessels

  • Lymphatic capillaries converge into larger lymphatic vessels
  • Lymphatic circulation is aided by external forces as a vein with the valves responsible for keeping lymph flow unidirectional
  • Normally do not contain red blood cells, permit distinction from venules
  • Large lymph vessels (ducts) comprise three tunics: tunica interna (endothelium and layer of longitudinal interlacing collagen and elastic fibers), tunica media (smooth muscle cells surrounded by elastic and collagen fibers), and tunica externa (collagen and elastic fibers, may contain muscle cells)
  • Adventitia is relatively underdeveloped, but contains vasa vasorum and a neural network
  • Lymphatic vascular system is a major distributor of lymphocytes, antibodies, and other immune components