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Organ system histology
Skeletal muscle histology
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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.
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.
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