هیستولوژی:

  • Primarily consists of cardiac muscle called myocardium, whose inner surface is lined with endocardium and outer surface with epicardium

Myocardium:

  • Cardiac muscle is striated as a result of the arrangement of the actin and myosin filaments in the sarcomeres
  • Cardiac muscles fibers are smaller (about 15 micrometers) than most skeletal muscle fibers (10 - 100 micrometers)
  • Myocardium consists of individual muscle cells with 1 - 2 centrally placed nuclei which branch, anastomose and are arranged in a linear array, each fiber is about 85 - 100 micrometers long
  • The junction between two cardiac muscle cells, called an intercalated disk, is another distinguishing feature
  • The intercalated disk is made up of three types of cell junctions: fascia adherents, desmosomes and gap junctions
  • Cardiac muscle is more vascularized and has more abundant mitochondria than skeletal muscle (40% of volume vs. 2%); it also contains glycogen granules between the myofibrils
  • Physiologically, cardiac muscle is intrinsically rhythmic (contracts without outside stimulation) although it is regulated through nervous and hormonal mechanisms

Endocardium:

  • Lies on luminal side of myocardium
  • Its inner surface is covered with endothelial cells, a type of squamous epithelium lining the inside of the heart and blood vessels
  • Beneath the endothelium is a layer of fairly loose, well vascularized connective tissue, which becomes a bit denser closer to the myocardium
  • The thickness of the endocardium varies inversely with the thickness of the myocardium; i.e. thicker in the atria than the ventricles (which have more substantial muscular walls)
  • The connective tissue layer closest to the myocardium is slightly looser and is called the subendocardial layer; it contains veins and nerves, as well as the Purkinje fibers when present

Epicardium:

  • Delicate, inner visceral layer of the pericardium
  • The outer part of the epicardium is lined with mesothelium: the epithelium lining the walls and contents of the closed cavities of the body, such as the thoracic, pericardial and abdominal cavities
  • Large blood vessels and nerves are found in the epicardium, and adipose tissue can be abundant

Purkinje fibers:

  • Modified cardiac muscle cells with a diameter about twice that of regular fibers (30 vs. 15 micrometers)
  • Contain fewer myofibrils than regular cardiac muscle fibers and have large concentrations of glycogen
  • Their nuclei tend to be surrounded by a large perinuclear space with the myofibrils well toward the periphery of the muscle fiber
  • Purkinje fibers are much faster conducting than regular cardiac muscle fibers, with which they make contact via gap junctions
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