Cardiac muscle histology

Definitions & Key takeaways

The cardiac muscle is a specialized type of involuntary muscle tissue that makes up the bulk of the heart. Cardiac muscle cells, or cardiomyocytes, are elongated and spindle-shaped, with one to two centrally located nuclei. These cell fibers are arranged in a branching network and intercalated discs, which are junctions that allow these muscle cells to contract in synchronization. The cardiac muscle is capable of self-excitation, meaning it can generate electrical impulses that coordinately contract all of the heart muscles to pump blood. The cardiac muscle has many unique properties that allow it to function effectively. For example, cardiomyocytes have a high mitochondria density, providing them adequate energy to contract repeatedly. In addition, cardiac muscle cells have a large number of myofibrils, which are specialized protein filaments that allow for efficient contraction. Finally, the intercalation between cardiac muscle cells allows for rapid and coordinated contractions of the heart.

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Introduction0:00–1:01

Now, let's get a closer look. There are three types of muscles, cardiac skeletal and smooth muscle.
Each type has distinct functions as well as structural characteristics that can be identified. Histologically, cardiac muscle makes up the majority of tissue found in the wall of the heart.
Each mature cardiac muscle or cardiomyocyte is relatively short with a length. Approximately 85 to 120 micrometers long and a diameter.
Approximately 15 to 30 micrometers. Histologically, cardiac muscles have quite a few unique characteristics that make it easier to differentiate them from skeletal muscles.
Unlike skeletal muscles, cardiac muscle fibers are branched cells with only 1 to 2 centrally located nuclei. Also unique to cardiac muscles are the intercalated discs, which are the specialized junctions between neighboring cells that allows the cells to have synchronized contractions and pump blood out of the heart efficiently.
Let's take a look at a longitudinal section of cardiac muscle cells that was stained with hematoxylin and eosin or H and E for short, if we compare cardiac muscle cells to skeletal muscle cells, we can see that there are some key differences between the two muscle types.

Structure & function1:01–2:49

Skeletal muscle cells don't branch and have multiple nuclei that are peripherally located along the cell. Whereas cardiac muscle cells are branched and usually have just one or occasionally two centrally located nuclei.
If we look at a cross section of the cardiac muscle, it's easier to see that the nuclei are centrally located. Both cardiac and skeletal muscles have crossed striations that are perpendicular to the muscle fibers.
But they're much more prominent in skeletal muscles. Also unique to cardiac muscle cells are, as we mentioned, the intercalated discs that connect the neighboring cardiomyocytes with an H and E stain.
The intercalated discs are seen as thin, slightly darker lines that are perpendicular to the direction of the muscle cell.
In order to see the intercalated discs better. A section of tissue can be stained with a specialized stains such as phosphotungstic acid hematoxylin or PTA H for short.
Although both the cytoplasm and intercalated discs will stain purple. The intercalated discs will be a significantly darker purple.
In comparison to the surrounding cytoplasm, whether the section of cardiac muscle was stained with H and E or PTA H. Lipofuscin will be seen as the yellow brown pigment Granules found near some nuclei.
These Granules are residues of lysosomal digestion and they're considered to be a sign of wear and tear that accumulate in cardiac muscle tissue with age