The Heart

• Next, examine this specimen of heart  and the image below on the left. Compare the appearance of the cardiac muscle fibers (top half of image) and Purkinje fibers (lower half of image). They both contain intercalated discs, but the Purkinje fibers are larger and have paler staining cytoplasm due to the presence of abundant glycogen and fewer myofibrils. Intercalated discs are specialized intercellular junctions that provide adhesion and communication, allowing the muscle cells to coordinate their contraction.

• Finally, let’s consider the valves. Heart valves must be shaped so that their leaflets fit precisely together in order to prevent backflow or regurgitation of blood. They must also open freely and completely so as not to restrict blood leaving the chamber. If abnormal or misshapen due to infection or congenital malformation, regurgitation of blood back through the valve will occur and is detectable as a murmur.
• Heart valves are anchored to the fibrous cardiac skeleton and chordae tendinae. They are lined by endothelium and have a core of dense connective tissue with elastic fibers. Examine the histology of a heart valve in this trichrome stained slide of heart.
   

The elastic and large arteries comes next.