Figure 1219

Photomicrograph of the left atrial and left ventricular wall. a. This photomicrograph shows a sagittal section of the posterior wall of the left atrium and left ventricle. The line of section crosses the coronary (A-V) groove containing the coronary sinus and circumflex branch of the left coronary artery. Note that the section has cut through the fibrous A-V ring of the mitral valve, which provides the attachment site for the muscle of the left atrium and the left ventricle and the cusp of the mitral valve. The ventricular wall consists of three layers: (1) endocardium (arrowheads); (2) myocardium; and (3) epicardium. The visible blood vessels lie in the epicardium and are surrounded by adipose tissue. The layers of the mitral valve are shown at higher magnification in Figure 12.20. x35. b.This high magnification of the area indicated by the rectangle in a shows the characteristic features of the inner surface of the heart. Note that the endocardium consists of a squamous inner layer of endothelium (End), a middle layer of subendothelial dense connective tissue (DCT) containing smooth muscle cells (SMC), and a deeper subendocardial layer containing Purkinje fibers (PF). The myocardium contains cardiac muscle fibers (CMF) and is seen on the left. x120.

is much thinner than the interventricular septum. Except in certain localized areas that contain fibrous tissue, it has a center layer of cardiac muscle and a lining of endocardium facing each chamber.

Heart valves are vascular structures composed of connective tissue with overlying endocardium

The heart valves attach to the complex framework of dense irregular connective tissue that forms the fibrous rings and surrounds the orifices containing the valves. Each valve is composed of three layers (Fig. 12.20):

• Fibrosa forms the core of the valve and contains fibrous extensions from the dense irregular connective tissue of the skeletal rings of the heart.

• Spongiosa represents loose connective tissue located on the atrial and/or blood vessel side of each valve. It is composed of loosely arranged collagen and elastic fibers infiltrated with a large amount of proteoglycans. The spongiosa acts as a shock absorber as it dampens vibrations associated with the closing of the valve. It also confers flexibility and plasticity to the valve cusps. In the aortic and pulmonary valves, spongiosa located on the blood vessel side is called arterialis. It corresponds to the loose connective tissue located on the atrial side of the A-V (tricuspid and mitral) valves, called the auricularis.

• Ventricularis is immediately adjacent to the ventricular and/or atrial surface of each valve and is covered with endothelium. It contains dense connective tissue with many layers of elastic fibers. In the A-V valves, the ventricularis continues into the chordae tendineae, which are fibrous, thread-like cords also covered with endothelium. They extend from the free edge of the A-V valves to muscular projections from the wall of the ventricles, called papillary muscles.

Valve cusps are normally avascular. Small blood vessels and smooth muscle can be found only in the base of the cusp. The surfaces of the valve are exposed to blood, and the cusps are thin enough to allow nutrients and oxygen to diffuse from the blood.

Several diseases affect the valves of the heart, causing their degeneration (e.g., calcification, fibrosis) and resulting in heart malfunction due to insufficiency or stenosis of valvular orifices. These conditions, known collectively as valvular heart disease, include rheumatic heart disease, vegetative endocarditis, degenerative calcific aortic valve stenosis, and mitral annular calcification. Rheumatic fever, e.g., causes inflammation of the heart valves (valvulitis). Inflammation induces angiogenesis in the valve and vascularization in the normally avascular layers of the valve. These changes most commonly affect the mitral valve (65 to 70%) and aortic valve (20 to 25%). This inflammation can lead to progressive replacement of elastic tissue by irregular masses of collagen fibers, causing the valve to

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