Skeletal muscle histology

Definitions & Key takeaways

Skeletal muscles are composed of large, elongated, and cylindrical cells that are also called muscle fibers. Each fiber has multiple nuclei in the periphery, and the capillaries that supply the skeletal muscle are typically found at the corners of the muscle fibers. Within each muscle fiber, there are myofibrils, which are long, thin structures made up of repeating units called sarcomeres.

Sarcomeres are the basic functional units of muscle contraction and are made up of thick and thin filaments that slide past each other during muscle contraction. With high-power magnification and electron microscopy, we can identify some of the different parts of the sarcomere that form the striations, such as the A band, I band, and the Z discs that run down the middle of the I bands.

Skeletal muscle fibers are surrounded by a layer of connective tissue called the endomysium and are grouped into bundles called fascicles, also surrounded by another layer of connective tissue called the perimysium. The entire muscle is surrounded by a layer of connective tissue called the epimysium.

Watch video only
Now, let’s get a closer look! There are three types of muscles: skeletal, cardiac, and smooth muscle.
Each of them has distinct functions as well as structural characteristics that can be identified histologically.Let’s focus on skeletal muscles, which are composed of large, elongated, branching, and cylindrical cells with multiple nuclei that are located along the periphery.
These cells are often also called muscle fibers. In this longitudinal section of skeletal muscle, the muscle fibers are the narrow strands that are all arranged in the same direction.
The muscle fibers are also arranged in parallel bundles called fascicles.With longitudinal sections of skeletal muscle, the nuclei may not always look like they’re in the periphery, but with a transverse section, it’s much easier to visualize.
It’s also easier to identify the endomysium, which is the connective tissue that surrounds the polygonal muscle fibers. The perimysium is also easier to identify, since it’s an even thicker layer of connective tissue that surrounds the fascicles.Skeletal muscle has a rich network of capillaries, and if we zoom in further, we can see that the capillaries are typically seen at the corners of the polygonal muscle fibers.
Although it’s not always easy to see with H&E staining, the subtypes of skeletal muscle can sometimes be differentiated.
In this example, the Type I or slow twitch muscle fiber is distinguished by its smaller size and darker stain when compared to the neighboring Type II or fast twitch muscle fibers.The muscle fibers contain many myofibrils that are made up of contractile proteins called myofilaments.
The myofilaments consist of thin actin filaments and thick myosin filaments that are arranged in parallel and also form the basic unit of the striated muscles called a sarcomere.
The myofilaments mainly consist of thin actin filaments and thick myosin filaments. The alignment and structure of the sarcomeres result in perpendicular bands that can be seen in this image as striations that run vertically.
These striations can be seen in both skeletal and cardiac muscles.If we take a closer look at this image, we can even identify some of the individual parts of the sarcomere that form the striations.
The A band is the thick and dark band in the middle of the sarcomere. And the I band is the wide and light band which has a thin and dark Z disc that runs down the middle.
With electron microscopy, we’re able to obtain higher resolution images with more detail. As a result, we’re able to identify even more structures that make up the sarcomere.
The overall sarcomere spans the distance between the Z discs, which are seen as the dark vertical lines in this image. The A band is still a thick dark band in the middle of the sarcomere, except now we can also see the lighter H band and M line that’s found in the middle of the A band.
Electron microscopy is typically black and white, but this image has been colored to show the M line in purple. And finally, the pale region surrounding the z discs is the I band, which is the region that consists only of actin, and no overlapping myosin.If a skeletal muscle is not used for an extended period of time, it will gradually decrease in size or atrophy due to disuse.
This can happen when immobile for an extended period of time, but it can also be caused by medical conditions as well. For example, this can happen when a person needs an arm cast to help the body heal from a broken bone.
Atrophy can also occur when a muscle loses the nerve stimulation it normally receives for contraction, which can happen in conditions such as poliomyelitis or nerve damage from significant physical trauma.
Although muscle mass is decreased, the overall number of muscle fibers remain the same. The decrease in muscle mass is actually due to a decrease in myofilaments.
This cross-section of skeletal muscle is from an individual that had partial loss of nerve stimulation to his muscles. This resulted in atrophy of only some muscle fibers, leading to an abnormally wide range of muscle fiber diameters.
The atrophic muscle fibers can be identified in this image by their smaller diameter when compared to the larger and unaffected neighboring muscle fibers.Alright, as a quick recap… Skeletal muscles are composed of large, elongated, and cylindrical cells that are also called muscle fibers.
Each fiber has multiple nuclei in the periphery, and the capillaries that supply the skeletal muscle are typically found at the corners of the muscle fibers.
Within the muscle fibers are parallel myofilaments that are aligned with one another, creating perpendicular striations that can be seen under the microscope.
With high power magnification and electron microscopy, we can identify some of the different parts of the sarcomere that form the striations, such as the A band, I band, and the Z discs that run down the middle of the I bands.
Each basic unit of skeletal muscle is called a sarcomere, which spans the width from one Z disc to the next.