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This type of tissue is found in the skeletal muscles and is responsible for the voluntary movements of the bones.
Hey everyone! This is Nicole from Kenhub and welcome to this histology tutorial, in which we'll be looking at what is probably the most recognized muscle tissue type in the body – the skeletal muscle tissue. So let's begin with some basic information about muscle tissue.
As you may already know, muscle tissue can be further divided into three different types – the first being skeletal muscle such as that seen in this micrograph. Skeletal muscle which is going to be the subject of this tutorial is also known as voluntary muscle as it is under the voluntary control of the somatic nervous system. The next type we're going to look at is the cardiac muscle which we can see here in this micrograph which as the name suggests is found in the heart. The highly synchronized muscle contraction of the cardiac muscle cells transforms the heart into a pump which sends blood pumping throughout the body. And finally, we have the smooth muscle, which is seen here in this micrograph and is the third type of muscle tissue. It's also known as involuntary muscle because its activity is neither initiated nor monitored consciously.
But, of course, this tutorial is about skeletal muscle so let's cover briefly what we're going to be talking about today. So, specifically, we'll be looking at the general properties of skeletal muscle tissue and then we'll move inward to explore the organization of the skeletal muscle tissue to really get an appreciation for how our skeletal muscles generate the forces that they do. We'll then look at connective tissue layers that are found in the skeletal muscle tissue. We'll then look specifically at myocytes or muscle cells and their associated structures and, finally, we'll take a closer look at the thin filaments present in skeletal muscle tissue. All of these will become clearer as we progress throughout the tutorial so let's begin.
It's worth quickly noting before we start that the majority of histological images we will look at today are stained with the hematoxylin and eosin stain. This stains the cell's cytoplasm a pinkish-red color as you can see and the nuclei a dark-bluish purple color and you can see both of these in the image.
So let's get started by taking a look at the general properties of skeletal muscle tissue. So most skeletal muscles are attached to bone by tendons which are bundles of collagen fibres that have incredibly high tensile strength. They have to be like this because when skeletal muscle contracts, it can produce a lot of force. Of course, a muscle is not just one giant muscle fibre. It's made up of many individual fibres, so let's look at the histological organization that allows the skeletal muscles to function in the way that they do.
Muscles like the biceps brachii here are like a bag full of individual small long muscle fibres, and these muscle fibres are grouped together in larger bundles called muscle bundles or fascicles which is the segment highlighted in green. When multiple muscle bundles are then grouped together, they form the muscle itself which would be the entirety of this image. Here we can see the full image of the cross-sectional view of the muscle fibres and bundles that make up the skeletal muscle tissue and as we mentioned before highlighted in green, we can see one of the many muscle bundles or the fascicles.
But how do these fibres and bundles stay together as one unit? Well that answer lies in the connective tissue layers that are found in the skeletal muscle tissue which we'll take a look at next.
Starting from the outside and working our way inwards, the connective tissue that encircles an entire skeletal muscle is called the epimysium – a segment of which is highlighted in green in this image. This outermost layer is dense irregular connective tissue and contains some elastic fibres as it has to be able to stretch and contract with the muscle tissue underneath.
Now if we move inward to the individual muscle bundles, we'll come across what is called the perimysium, which is a connective tissue layer that ensheathes the individual muscle bundles. In the image here, we can see a segment of the perimysium highlighted in green. Now think of the perimysium like a plastic wrap or cling film wrapping around the muscle bundle and keeping it together.
The next layer of connective tissue is known as the interfascicular connective tissue. This tissue fills the space between the fascicles wrapped in their own perimysium. This layer serves to further contribute to the structural integrity of the skeletal muscle. In reality, connective tissue here is packed a little bit closer together than what we can see in the image and when preparing histological slides, the process tends to shrink tissues and cells causing what appears to be void space.
Lastly, highlighted in green in the image just here, we can see the endomysium. This particular layer of connective tissue is loose connective tissue that fills the space between the muscle fibres and is composed of collagen fibres. This particular image shows the endomysium in a longitudinal section and, FYI, a collagen fibre looks like one of these squiggly fibres just here. In this image here, we can see the endomysium between muscle fibres within a muscle bundle.
One cell type found in connective tissue are fibroblasts which are of incredible importance as they are the cells that produce collagen fibres. Collagen is the most abundant fibre type we find in connective tissue and the image here shows the fibroblasts nuclei highlighted in green. We can find these structures between the more heavily stained muscle tissue within the connective tissue.
Surrounding all of these structures outside of the muscle tissue and on top of the epimysium is the perimuscular fat. Perimuscular fat is composed of adipocytes or fat cells that aid in the insulation and cushioning for the muscles of our body. Inside these cells, a vast storage of fat can be found but during slide preparation, the fat is removed. So its histological features have thinly walled cells with the nucleus found in the cell periphery.
Now that we have a basic understanding of the histological features and organization of skeletal muscle tissue, let's explore the even smaller parts of this tissue and look at the functional units of the skeletal muscle tissue which are the myocytes.
Here in this image, we can see a myocyte highlighted in green. Myocytes come together with other myocytes to form myofibres. These cells are tubular in shape and their nuclei appear on the periphery of a muscle fibre when viewed in cross-section. This image shows a cluster of myocytes and, highlighted in green, we can see the nuclei of the myocytes. As mentioned before, the nuclei of the myocytes are found in the periphery and pushed to the side of these myofibres which you can see in some of these longitudinal sections.
Let's now have a look at two other structures associated with the myocyte, the first of which being the sarcolemma. This is simply the term given to the myocyte cell membrane and we can see the sarcolemma highlighted in green here. A fun fact about the sarcolemma but something that we cannot see in this image is that it has invaginations that go into the myocyte. Now, this special feature allows for adequate spread of an action potential throughout the muscle fibre to ensure synchronized and efficient contraction.
The other structure associated with myocytes is the sarcoplasm which is essentially the myocyte's cytoplasm. Highlighted in green here, the sarcoplasm fills myocytes and myofibres. It has a high concentration of calcium ions which allows for our skeletal muscle tissues to contract. Also found in the sarcoplasm is a high concentration of glycosomes storing glycogen for energy use and myoglobin which contains stores of oxygen. These features are all to facilitate adequate energy production for such energy demanding tissue.
Also found embedded in the sarcoplasm and contributing to the contractile function of this tissue are the myofibrils. Myocytes contain chains of these tubular structures which are made of the contractile units of the cell known as the sarcomeres. This image shows myofibril highlighted in green and can be imagined as tubes that are going into and out of the screen. Within the myofibril here are also various proteins that interact with each other to generate contractions and these proteins are organized into filaments that when viewed histologically, give rise to transverse striations in skeletal muscle tissue.
Next, let's take a look at another important component of skeletal muscle which are the different kinds of filaments. One of the most prominent features on a histological slide that should hint to you what type of muscle tissue you're looking at are the presence of striations such as what we can see in this micrograph. They are alternating bands of light and dark color that appear on these hematoxylin and eosin slides and this is due to the presence of both thick filaments of myosin and thin filaments of actin within the myocytes that are organized in a fashion that leads to these alternating colors.
Let's look a bit more closely at these filaments on the next slide. So, A-bands, are the name given to the thick filaments of myosin that we just mentioned. Myosin when stained with hematoxylin and eosin appears a darker color than its counterpart, actin, and this gives rise to dark stripes within the striations and the entire length of this stripe or thick filament is called the A-band, a few of which are highlighted in green just here.
The lighter stripes within the striations are known as the I-bands and they're composed entirely of thin filaments of actin. These filaments interact with the myosin in thick filaments to generate contractions.
And last but not least, let's discuss what z-lines are. Now, z-lines don't often appear obvious on histology slides and so we'll use this diagram here to show them as well as to help pull together what we've learned about the skeletal muscle filaments.
Now, if we recall, a sarcomere is a contractile unit of a skeletal muscle cell and the z-lines here indicate the sarcomere borders. Let's break down a sarcomere a little bit more.
Each sarcomere is composed of sliding protein filaments of actin and myosin and this diagram we can see the thick myosin filaments just here and this form the dark A-band as well as the thin actin filaments which form the lighter I-band. The sarcomeres create contractions in the muscle tissue when the thick myosin filaments slide between the actin filaments pulling the filaments together like so. And as you can see, the z-line also acts as an anchor point for the thin filaments that are helping in the contraction of the tissue.
So now that we've covered the features of healthy skeletal muscle tissue, let's take a look at the histopathology of skeletal muscles in a disorder known as muscular dystrophy.
Muscular dystrophy is a type of genetic disorder described by weakening and atrophy of muscular tissue. Under the microscope, this appears as disorganized and reduced number of myofibres. The atrophy or shrinking of muscle fibres in muscular dystrophy leads to the appearance of disorganized, centrally located nuclei in a histological image of skeletal muscle tissue. The breakdown of these muscle fibres leads to weakened atrophied muscles so patients with this disorder progressively lose their muscle mass and, unfortunately, eventually they lose their ability to walk, move and to breathe.
So this brings us to the end of our tutorial on skeletal muscle tissue. Let's quickly recap.
So first we took a look at the general organization of skeletal muscle tissue starting with the muscle fibres which together form muscle bundles or fascicles which in turn together form the whole muscle. We then discovered the several connective tissue layers that surround skeletal muscle and help it maintain its structural integrity. Starting from the outside in, we first looked at the epimysium which encircles the entire skeletal muscle. We next have the perimysium which ensheathes the individual muscle bundles and then we looked at the interfascicular connective tissue which fills the spaces between the muscle bundles.
The innermost layer of connective tissue is known as the endomysium and this fills the space between the muscle fibres and the other structures that are related to the connective tissue layers that we looked at are the fibroblasts which produce the collagen fibres so important to the structural integrity of the muscle.
Finally, we looked at perimuscular fat which serves to cushion the muscle. We then took a closer look at myocytes or muscle cells and their related structures, the first of which are being, of course, the myocyte nuclei followed by the sarcolemma which is the cell membrane of the myocyte, and then the sarcoplasm which is the cytoplasm of the myocyte. Finally, there are the myofibrils which are fibres composed of sarcomeres which are the contractile units of the skeletal muscle cell.
In the final section of the tutorial, we looked more closely at the filaments that contribute to the contraction of the skeletal muscle. First, there were the A-bands which appear dark and are composed of thick filaments of myosin. Secondly, we had the I-bands which appear lighter in contrast to the A-bands and are composed of thin filaments of actin. And finally, we looked at z-lines which act as anchor points for the thin actin filaments and indicate the borders between sarcomeres which are the contractile units of the cell.
And that concludes this tutorial. Thanks for watching and happy studying!