Histology Mentor

1. Arteries - learn to distinguish elastic artery from muscular artery. Muscular arteries are distributing arteries, and their structure varies with their location.

a. Size - includes those arterial vessels down to about 0.05 mm in diameter (arterioles).

b. Described as having 3 layers:

1. Tunica intima - very thin in arterioles, moderately thick in elastic arteries. Consists of endothelium plus a fibroelastic subendothelium, that in some sites or conditions (hypertension) may contain myointimal cells, but it is negligible in arterioles. Myointimal cells produce elastic fibers where arteries divide. The intima of coronary arteries becomes thicker with age partly caused by myointimal fibers that are longitudinally oriented.

All endothelial cells are polarized. They present on their luminal surface membrane bound angiotensin converting enzyme, enzymes that degrade vasoactive hormones and a receptor that binds lipoprotein lipase. On the basal side they secrete components of the thrombogenic basal lamina containing collagen III as well as IV and V, laminin, fibronectin, endothelin(a vascular constrictor), and the vascular relaxant nitric oxide. Von Willebrand factor, which facilitates the coagulation of platelets, is produced by most endothelial cells but in arteries it is stored in vesicles called Weibel-Palade bodies.

The outermost layer of the intima is the internal elastic lamina. It is present in all arteries but is most conspicuous in muscular arteries and may consist only of fibers in arterioles.

Muscular Artery 1. Irregular C.T. 2. External elastic lamina 3. Tunica adventitia 4. Tunica media (smooth muscle) 5. Internal elastic lamina 6. Endothelium 7. Tunica intima

2. Tunica media - thickest layer. Circular smooth muscle in muscular arteries. Elastic laminae predominate in elastic arteries where they alternate with smooth muscle in a network of fine elastic fibers and collagen. The smooth muscle cells make the collagen and elastic fibers.

3. Tunica adventitia - fibroelastic layer is abundant sometimes equaling the media, in muscular arteries. Thin in elastic arteries and small arterioles. The innermost layer of the adventitia is the external elastic lamina. It is frequently present in muscular arteries, but is not as distinct as the internal elastic lamina.

2. Capillaries

3. Veins - see also inferior vena cava; artery, vein and nerve; and other vessels

4. Lymph vessels

If fully dilated, the vascular bed (arteries, capillaries and veins) can contain several times the volume of blood in the body. Therefore in order to maintain the flow of blood, contraction of all these vessels must be controlled in a coordinated fashion. This is achieved through a combination of neural and hormonal influences. Numerous reflexes combine to appropriately nourish the tissues (i.e., maintain adequate pressure to perfuse the brain), control the body temperature and ensure adequate ventilation. Sensory inputs for these reflexes come from pressure receptors around the large vessels, chemoreceptors in the carotid and aortic bodies and temperature receptors in the skin and hypothalamus. The effector limb of these reflexes is by way of sympathetic postganglionic fibers that terminate near vascular smooth muscle cells. The contractile influence of the neurotransmitter, norepinephrine, acting on nearby smooth muscle cells spreads by diffusion of norepinephrine and through gap junctions to other smooth muscle cells. Moreover, the influence of circulating hormones (epinephrine, norepinephrine, prostaglandins and plasma kinins) may be transmitted directly from endothelial cells to contractile elements, because specialized contact between these cells have been demonstrated. Of course, these hormones can act directly on smooth muscle cells when they diffuse through the intima to the media. A factor produced by endothelial cells called endothelial relaxing factor has been shown to be nitric oxide. Finally circulation at the local level is markedly influenced by metabolites, especially carbon dioxide and other acids.

Arteriovenous anastomoses provide a rapid means by which the nervous system can control the flow of rather large amounts of blood into specific regions. The glomus, a special form of A-V anastomosis, is found in the dermis of finer tips and consists of an arteriole joining to a venule without an elastic membrane but with many epithelioid contractile cells. Such arrangements permit the control of heat loss from the extremities.

Vas vasorum (vessel of vessel) is one of many vessels provided to nourish the wall of the larger blood vessels. In elastic arteries, they penetrate about halfway through the media. The inner part is nourished by diffusion from within and without. Vasa vasorum penetrate most of the thickness of the wall of larger veins.

1. Endocardium - thickness inversely related to the thickness of its overlying myocardium. It is thick in atria, thin in ventricles.

2. Myocardium - cardiac muscle fibers surrounded by endomysial tissue carrying abundant supply of blood capillaries. In the ventricles the cardiac muscle is arranged so that contraction results in a twisting motion. The myocardium is much thinner in the atrium. The atrium may function as a blood volume receptor, since atrial myocardial cells contain secretion granules that have natriuretic, diuretic and vasodilation activity, the atrial natriuretic factor. These are not found in the ventricles. there are other natriuretic factors from other sources. They all function to reduce blood volume.

3. Epicardium - It is also called the visceral pericardium and consists of a smooth mesothelial surface and a submesothelial layer that contains areolar tissue with large vessels, nerves, and fat.

1. Skeleton - dense fibrous connective tissue surrounds the valves, thus providing support, and separates the atria from the ventricles. This prevents the contraction impulse from spreading from atrium to the ventricle causing premature contraction.

2. Valves - reduplication of endocardium supported internally by extensions of the skeletal tissue.

1. Sino-atrial node - origin of impulse - embryonic appearing fibers with abundant connective tissue. The small spindle-shaped cells contain relatively few myofibrils randomly oriented. Autonomic nerve fibers and ganglion cells that control heart rate are closely associated with the cells in the S-A and A-V nodes. The impulse travels by several internodal pathways in the atrial muscle to the atrio-ventricular node.

2. Atrio-ventricular node - in septal wall of right atrium. Similar to S-A node in cytology.

3. Atrio-ventricular bundle of His - at its origin from the A-V node the fibers in this bundle resemble those of the node, but as they enter the subendocardial portion of the ventricles the fibers are larger and contain stored glycogen. These are the typical Purkinje fibers that form the right and left bundle branches on each side of the interventricular septum. Autonomic nerves are closely associated with the A-V bundle but direct synapses are not observed. They continue into the cardiac muscle, where they probably modulate the strength and velocity of contraction.