Cardiovascular System - Heart Histology

Embryology - 11 Dec 2023        Expand to Translate
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Introduction

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

Histology

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.

Unlabeled Images

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Cardiac Layers

Endocardium

• 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.

Myocardium

• 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.

Epicardium

• 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.

Sarcomere

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.

Links: Sarcomere animation | Sarcomere histology

Intercalated Discs

Cardiac muscle showing intercalated disc (electron micrograph)
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	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).

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.

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

Terms

• 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 -

External Links

External Links Notice - The dynamic nature of the internet may mean that some of these listed links may no longer function. If the link no longer works search the web with the link text or name. Links to any external commercial sites are provided for information purposes only and should never be considered an endorsement. UNSW Embryology is provided as an educational resource with no clinical information or commercial affiliation.

Need to update links

• Blue Histology Cardiac Muscle
• UNSW Virtual Slides Science ANAT2241 Cardiovascular System (requires login for access).
• UIOWA Virtual Slidebox of Histology Heart

Glossary Links

Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link

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Cite this page: Hill, M.A. (2023, December 11) Embryology Cardiovascular System - Heart Histology. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Cardiovascular_System_-_Heart_Histology

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© Dr Mark Hill 2023, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G