Cardiovascular System - Heart Histology

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The heart is composed of cardiac muscle, specialised conductive tissue, valves, blood vessels and connective tissue.

Cardiac muscle, the myocardium, consists of cross-striated muscle cells, cardiomyocytes, with one centrally placed nucleus.

  • Nuclei are oval, rather pale and located centrally in the muscle cell which is 10 - 15 µm wide.
  • Cardiac muscle cells excitation is mediated by rythmically active modified cardiac muscle cells.
  • Cardiac muscle is innervated by the autonomic nervous system (involuntary), which adjusts the force generated by the muscle cells and the frequency of the heart beat.
  • Cardiac muscle cells often branch at acute angles and are connected to each other by specialisations of the cell membrane in the region of the intercalated discs.
    • Intercalated discs invariably occur at the ends of cardiac muscle cells in a region corresponding to the Z-line of the myofibrils.
  • Cardiac muscle does not contain cells equivalent to the satellite cells of skeletal muscle.

For development information see Cardiac Embryology. I have redirected Cardiac Muscle Histology to this current page.

Histology Links: Skeletal Muscle | Cardiac Muscle | Smooth Muscle

Cardiovascular Links: cardiovascular | Heart Tutorial | Lecture - Early Vascular | Lecture - Heart | Movies | 2016 Cardiac Review | heart | coronary circulation | heart valve | heart rate | Circulation | blood | blood vessel | blood vessel histology | heart histology | Lymphatic | ductus venosus | spleen | Stage 22 | cardiovascular abnormalities | OMIM | 2012 ECHO Meeting | Category:Cardiovascular
Historic Embryology - Cardiovascular 
1902 Vena cava inferior | 1905 Brain Blood Vessels | 1909 Cervical Veins | 1909 Dorsal aorta and umbilical veins | 1912 Heart | 1912 Human Heart | 1914 Earliest Blood-Vessels | 1915 Congenital Cardiac Disease | 1915 Dura Venous Sinuses | 1916 Blood cell origin | 1916 Pars Membranacea Septi | 1919 Lower Limb Arteries | 1921 Human Brain Vascular | 1921 Spleen | 1922 Aortic-Arch System | 1922 Pig Forelimb Arteries | 1922 Chicken Pulmonary | 1923 Head Subcutaneous Plexus | 1923 Ductus Venosus | 1925 Venous Development | 1927 Stage 11 Heart | 1928 Heart Blood Flow | 1935 Aorta | 1935 Venous valves | 1938 Pars Membranacea Septi | 1938 Foramen Ovale | 1939 Atrio-Ventricular Valves | 1940 Vena cava inferior | 1940 Early Hematopoiesis | 1941 Blood Formation | 1942 Truncus and Conus Partitioning | Ziegler Heart Models | 1951 Heart Movie | 1954 Week 9 Heart | 1957 Cranial venous system | 1959 Brain Arterial Anastomoses | Historic Embryology Papers | 2012 ECHO Meeting | 2016 Cardiac Review | Historic Disclaimer

Histology Links: stains | fixatives | artifacts | menstrual histology | placenta histology | heart histology | liver histology | Pancreas | Gall Bladder | Colon | Renal | Respiratory Histology | Bone | Category:Histology | UNSW Histology
  Historic Histology Textbooks: 1941 Histology] | 1944 Oral Histology
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Cardiac muscle histology.jpg

Cardiac muscle histology

Image of primate heart stained with Alizarin blue.
  • Red Blood Cells (orange cells) Cardiac Muscle Cells (blue)
  • Cardiac muscle cells are cut longitudinally.
  • At high magnification see both striations and the large nuclei of the cardiac muscle cells.
  • Follow the course of individual cardiac muscle cells and note fine, dark blue lines which seem to cross (traverse) the fibres.
  • Intercalated Discs that connect the individual muscle cells and permit the conduction of electrical impulses between the cells.
    • seen in longitudinal sections.

Heart histology 002.jpg Heart histology 004.jpg

Heart histology 003.jpg Heart histology 001.jpg

Unlabeled Images

Cardiac Layers


  • Inner layer of the heart (lines the atria and ventricles and covers the heart valves) and contains blood vessels.
  • Has 3 sublayers:
    • Endothelium - innermost portion a simple squamous epithelium.
    • Smooth Muscle and Connective Tissue - middle layer of the endocardium is mix of connective tissue and smooth muscle.
    • Subendocardial Layer - outer layer of the endocardium is loose connective tissue joining the endocardium and myocardium.
  • equivalent to tunica intima.


  • Middle layer of the heart, thickest layer contains cardiomyocytes, blood vessels.
    • contains cardiac muscle fibres and loose endomysial connective tissue containing many capillaries.
  • Muscular layer.
  • equivalent to tunica media.


  • Outer layer of the heart, contains blood vessels and lymphatics.
    • fibroelastic connective tissue, blood vessels, lymphatics and adipose tissue.
  • Visceral layer of pericardium rather thin.
  • equivalent to tunica adventitia.


The sarcomere is the visible functional contractile unit within cardiac (and skeletal) muscle. The animation below shows the relative movement of the thick and thin filaments in the sarcomere (if animation not playing reload page).

  • A band - (anisotropic bands, light band) are composed of the thick myosin filaments.
  • I band - (isotropic bands, light band) are composed of thin actin filaments.
  • H band - (German, heller =brighter). are composed of the thick filaments that is not overlapped (superimposed) by the thin filaments.
  • Z line - (German, Zwischenscheibe = Intermediate plate) appears as a series of dark lines between the I bands, indicate the end of one sarcomere and the beginning of the next.
Sarcomere animation.gif
Links: Sarcomere animation | Sarcomere histology

Intercalated Discs

Cardiac muscle showing intercalated disc (electron micrograph)
Cardiac muscle EM01.jpg Cardiac muscle EM02.jpg
  • seen in longitudinal sections.
  • connect the individual muscle cells.
  • located in region corresponding to the Z-line of the myofibrils.
  • permit the conduction of electrical impulses between the cells.

Histology "step-like" appearance due to:

  • transverse part - crossing fibres at right angle to myofibrils.
  • lateral part - runs in parallel to myofibrils.
Junctional Components
Adherens Junction cartoon Desmosome cartoon Gap junction carrtoon
Adherens Junction Desmosome Gap junction
Fascia adherens – major portion of transverse component. Anchoring sites for actin, and connect to the closest sarcomere. Macula adherens – (desmosomes) transverse and lateral components. Bind individual myocytes to one another. stop separation during contraction by binding intermediate filaments, joining the cells together. Macula adherens junctions are also called desmosomes. Gap junctions - lateral component. Allow action potentials to spread between cardiac cells by passage of ions between cells, producing depolarization of the heart muscle. Allows muscle to act as syncytium.
Links: EM image - intercalated disc

Purkinje Fibres

Purkinje Fibres

Do not confuse these cardiac Purkinje fibres (cells) with neural Purkinje cells (neurons) located in the cerebellum.

  • modified cardiac muscle cells. Compared to ordinary cardiac muscle cells:
    • contain large amounts of glycogen.
    • fewer myofibrils.
    • thicker cells.
  • can be binucleated (cell contains 2 nuclei).
  • extend from the atrioventricular node, pierces the fibrous body, divides into left and right bundles, and travels, beneath the endocardium, towards the apex of the heart.
  • bundle branches contact cardiac muscle cells through specialisations similar to intercalated discs.
  • conduct stimuli faster than ordinary cardiac muscle cells (2-3 m/s vs. 0.6 m/s).
  • discovered in 1839 by Jan Evangelista Purkyně).

Cardiac Conduction System.jpg

Adult Heart Conduction System

Cardiac Valves

Note for the semilunar valves not anchored to the walls of the atria by chordae tendineae.

Only the mitral and tricuspid valves, located between the atria and the ventricles, are connected to the cardiac wall by chord tendineae and papillary muscles.

Adult Heart Valves.jpg Gray0498.jpg

Links: Heart Histology | Cardiac AZB Labeled | Cardiac AZB | Cardiac label LS | Cardiac LS | Cardiac label TS | Cardiac TS | Purkinje fibres | Purkinje fibres detail | Histology


  • cardiomyocyte -
  • chordae tendineae - tricuspid and mitral valves connective tissue bands attached on the other end to the papillary muscles.
  • intercalated disc -
  • nodule - (of semilunar valve) small fibrous nodules located in the middle of the flaps, the nodules of the semilunar valve come closely together to fill the triangular opening.
  • Purkinje fibres -


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Cite this page: Hill, M.A. (2024, May 18) Embryology Cardiovascular System - Heart Histology. Retrieved from

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G