Structural Class: Fibrous Joints: Study with Video Lessons, Practice Problems & Examples

Fibrous joints, including sutures, gomphoses, and syndesmoses, connect bones through dense connective tissue. Sutures, found in the skull, are synarthroses, allowing no movement. Gomphoses anchor teeth in their sockets, also classified as synarthroses, permitting slight movement during chewing. Syndesmoses, connecting bones with longer fibers, are amphiarthroses, allowing limited movement. The periodontal ligament in gomphoses and interosseous membranes in syndesmoses provide structural stability while accommodating necessary mobility, highlighting the balance between stability and movement in joint function.

Introduction to Fibrous Joints

In this video, we're going to talk about the 1st structural class of joint in our lesson, which are the fibrous joints. As their name implies, the skeletal elements are going to be bound by collagen fibers of dense connective tissue. Recall, this is a type of fibrous connective tissue, or a type of connective tissue proper. The exact type of dense connective tissue used in these fibrous joints depends on the specific type of joint and its function. However, it's very often the case that dense irregular connective tissue is used in these fibrous joints.

We'll be able to see that as we move forward in our course and talk more about these fibrous joints. Recall that collagen fibers provide a lot of strength and the strength of these collagen fibers really makes these fibrous joints structurally stable. But recall that the stability of a joint comes at the cost of the mobility of the joint. These fibrous joints, although structurally stable, are going to have limited movement. And because they have limited movement, the fibrous joints are always going to be classified either as synarthroses, which are still or stationary joints that essentially allow for no movement and are generally classified as immovable joints.

Or the fibrous joints could be classified as amphiarthroses, which allow for a little movement or an average amount of movement. This is why we have the symbols for synarthroses and amphiarthroses here. And again, the symbol for synarthroses does not have that movement arrow going around it, which means that it allows for essentially no movement. Whereas the amphiarthroses symbol has one movement arrow going around it, which can remind you that it allows for just a little movement, or an average amount of movement. There are actually 3 types of fibrous joints, and they are sutures, gomphoses, and syndesmoses.

Moving forward in our course, we're going to talk about each of these 3 types of fibrous joints in their own separate videos, starting with the suture.

Sutures

In this video, we're going to talk about the first type of fibrous joint in our lesson, which are the sutures. And so, sutures are fibrous joints of dense irregular connective tissue, which recalls a type of fibrous connective tissue or connective tissue proper. And these sutures are found only joining the bones of the skull, and so sutures are only found in the skull. Now, recall that dense irregular connective tissue is characterized by having these really densely packed protein fibers in the extracellular matrix with an irregular or random arrangement that actually allows it to resist forces in multiple directions, which is really important for the skull to be able to do in order to protect our brains. Now, recall that dense irregular connective tissue has lots of collagen protein fibers, and those collagen protein fibers provide a lot of strength. And the strength of those collagen protein fibers allows the sutures to provide a lot of structural stability and support to the skull. And so, again, that's really important for the skull to have in order to protect our brains effectively. However, recall that the structural stability of a joint comes at the cost of mobility of the joint. And so, a suture provides structural stability to the skull, but it essentially allows for no movement. And therefore, these sutures are always going to be classified functionally as synarthroses, which is why we have the symbol for synarthroses here. Now, again, notice that the symbol for synarthroses does not have any movement arrows going around it. Which can remind us that these synarthroses are going to be still or stationary joints that essentially allow for no movement and are generally considered immovable joints. Now, many of the sutures of the skull actually have wavy irregular interlocking edges, and those wavy irregular interlocking edges can add a lot of strength to the skull, which can help to prevent fracturing of the bones of the skull, which again is very important to protect the brain. Now, although sutures are always classified as synarthrosis, it is important to note that sutures are actually designed to allow for skull expansion as the brain grows during youth. And so, of course, brain growth during youth is going to be a very slow process. And so the movement of the sutures during youth is also very, very slow. And therefore, even though there is technically small amounts of movement, they are still classified as synarthrosis because generally we don't really detect that movement, on a regular basis, or it's hard to detect that movement on a regular basis. Now, it's also important to note that the dense irregular connective tissue of a suture will actually ossify over time. Meaning that it forms bone or is replaced with new bone tissue. And again, this happens over time in adults. Now, the specific timing of the closure of a suture and the ossification of a suture will actually vary among individuals because it's influenced by many different factors including genetic factors and your overall health. However, usually by middle age of adults, the sutures of the skull or most of the sutures of the skull have ossified. Now, the ossification of the sutures in the skull actually fuses the suture bones into a single bone forming what is known as a synostosis or a bony joint. And so, the root "ost" in synostosis is a root that means bone. And so, again, the dense irregular connective tissue of the sutures will be replaced with bone over time. And this is a natural process. And, once those bones have completely fused together to form a synostosis or a bony joint, then of course, there will be no movement and it will be also a synarthrosis. And so, let's take a look at our image down below where we can start to piece together some of these ideas. And notice on the left-hand side of

Structural Class: Fibrous Joints Example 1

So, here we have an example problem that says, use the words in the word bank to fill in the blanks and complete the sentences that you can see down below right here. And so, notice that we've got the word bank highlighted here and the words are ossify, immovable, skull, and protect. And so the first sentence here says that sutures allow for the blank to expand in youth. And so, of course, this first blank here must be the word skull. And so, it's true that sutures allow for the skull to expand in youth.

And since we've used skull here for this first blank, we can go ahead and cross it off the list in the word bank. Now, continuing the sentence here, it says, sutures allow for the skull to expand in youth as they do not completely blank and fuse cranial bones together until brain development and growth are completely finished. Now, of course, the second blank here is going to be the term ossify, which means to form new bone tissue. And so it is true that the sutures do not completely ossify until brain development and growth are completely finished. And so because we've used ossify here, we can cross it off the list in the word bank above.

Now, the next sentence says that their primary function is to blank the brain. Now, of course, the primary function of the skull is to protect the brain, and of course, the sutures which are a part of the skull are also going to help protect the brain. And so, this next blank here is going to be the word protect. And so, we can go ahead and cross off protect from the list above. And then the last sentence says that they are able to do this because they are not because they are completely blank joints.

And of course, the last term is immovable. And so recall that these sutures are going to be synarthroses, which means that they are considered immovable joints, so they do not allow for movement. And so, recall that there's an inverse relationship between the mobility of a joint and the stability of the joint. And so, because these sutures allow for no movement, they are immovable joints, this means that they allow for a great deal of stability and support, and that's exactly what the skull needs in order to protect our brain. And so, this here concludes this example, and I'll see you all in our next video.

Imagine you're a doctor, and a middle-aged patient comes to you with a medical condition that is causing their collagen fibers to become weak. What affect would this have on the sutures in their skull?

• A. Sutures are made of short collagen fibers, so their sutures would loosen and become fragile.
• B. There is never fibrous connective tissue in sutures so it wouldn't have an effect.
• C. At this patient's age their sutures will have ossified; therefore, this condition won't affect their sutures.

In a baby, the bones of the skull are joined by fibrous connective tissue. Over time, these bones fuse together to form ossified immovable joints. How would you classify these joints?

• A. Amphiarthroses in a young skull, synarthroses in an older skull.
• B. Fibrous joints in a young skull, synostoses in an older skull.
• C. Cartilaginous joints in a young skull, synostoses in an older skull.   
• D. Fibrous joints throughout life.

Which of the following scenarios is most concerning for a pediatric patient?

• A. The epiphyseal plate in their femur is not fully ossified.
• B. Their sutures are ossified.
• C. Their sutures are not fully closed.

Gomphosis (Pleural: Gomphoses)

In this video, we're going to talk about the second type of fibrous joint in our lesson, which is the gomphosis, and the plural form are the gomphoses. And so, gomphoses are fibrous joints found only bolting teeth to their bony sockets in our gums. In fact, the root gumpho, which is found in the term gomphoses, is a root that means bolt. And so, you can think these gomphoses are found in our gums. Now, it is important to recall from our previous lesson videos that teeth are technically not considered bones, since they have a different structure and composition than bones.

And so, these gomphoses are unique in that they are not connecting bone to bone, but instead, they are connecting bone to teeth. Now, because these gomphoses are bolting the teeth to their bony sockets in our gums, these gomphoses are going to be structurally stable joints since they again anchor our teeth to our jawbone, either the maxilla or the mandible. But recall that structural stability of a joint comes at the cost of mobility of the joint. And so although these gomphoses are structurally stable anchoring the teeth to the jawbone, they essentially allow for no movement under usual conditions, except under the chewing forces or mastication forces. And so the gomphosis is designed to allow our teeth to move very very subtly when we are chewing, in order to help transfer chewing forces from our teeth to our jawbone to help alleviate the pressure on our teeth and prevent our teeth from shattering when we bite into something hard.

However, that movement is very very subtle and only really occurs under chewing types of forces. But generally, these gomphoses allow for essentially no movement and because that's the case, gomphoses are always going to be functionally classified as synarthroses, which is why we have the symbol for synarthroses right here. And so again, notice that the symbol for synarthroses does not have any movement arrows going around it, which can be helpful to remind you that these synarthroses are still or stationary joints that essentially allow for no movement and are generally considered immovable joints. Now, the periodontal ligament is also very important to the gomphoses. And so, it's the periodontal ligament that actually makes the gomphoses a fibrous joint.

And this is because the periodontal ligament is dense irregular connective tissue that essentially glues our teeth firmly into their bony sockets. And so, the root peri in periodontal is a root that means around. And the root odont or dent is a root that means tooth. And so, these periodontal ligaments are found around our teeth. Now, what's important to note is that, recall that dense irregular connective tissue is characterized by having really densely packed protein fibers in the extracellular matrix with an irregular or random protein fiber arrangement which allows it to resist forces in multiple directions.

And so when we are chewing, it is important for the gomphoses to resist forces in multiple directions. And so, many of the classic ligaments in our body usually are going to be made of dense regular connective tissue with parallel fiber arrangement because they usually only need to resist forces in one direction. But these periodontal ligaments, they need to be able to resist forces in multiple directions, which is why it's important they have dense irregular connective tissue. Now, it's also worthy of noting that in youth, the periodontal ligament actually deteriorates to allow our deciduous teeth or our baby teeth to fall out. And then, eventually those baby teeth are replaced with larger teeth that are more suitable for chewing and eating.

And so, let's take a look down below at our image where we can start to piece things together. So notice over here we have this guy and you can see his skeleton over here. And zooming in, you can actually see the gomphoses. You can see the tooth is right here in this position, and then you can see that the jawbone, the mandible is down below in this region. And so, the tooth is being anchored to the jawbone, in this gomphosis.

And what you'll also notice is that there are these little tiny periodontal ligaments that are all throughout here, essentially connecting the tooth to the bone and, essentially acting like glue. And so, what you'll notice is that over here on the right, we have this analogy for the gomphosis. And so what you'll see is that we have some, bolt, which recall the root gumpho is a root that means bolt. We have a bolt being bolted into some wood. And so here in this analogy, the wood would represent the jawbone, and the bolt itself would be the tooth.

And so the tooth is going to be the bolt. And then, what you'll notice is that over here, this person clearly did not have a lot of carpentry knowledge, but they used glue in addition to the bolt and the wood. And so, the glue that you see is really going to be the periodontal ligament. And again, the root peri is a root that means around and odont periodontal ligament, the root odont is a root that means tooth. And so these periodontal ligaments are found all around the teeth.

Essentially, gluing the teeth firmly to their bony socket. And so these gomphoses are found in your gums and so that can be another helpful reminder. And so this here concludes our brief lesson on the gomphosis or the gomphoses and we'll be able to get some practice applying these concepts and learn more about other types of fibrous joints as we move forward in our course. So, I'll see you all in our next video.

Structural Class: Fibrous Joints Example 2

So, here we have an example problem that asks, which of the following statements about the development of sutures and gomphosis is true? And we've got these 4 potential answer options down below. Now, option a says that they both start off rigid at birth and remain that way throughout life. Now, of course, we know that this is not going to be true, especially of sutures. Recall that the sutures of a young skull actually have bones that are more separated and over time, those sutures are going to develop into synostoses.

Where ossification of the dense irregular connective tissue occurs and that tissue is replaced with bone tissue to allow the bones of the skull to fuse together over time. And so, they don't start off rigid, they actually become more rigid over time. And so, for that reason, we can eliminate answer option a because it's not the best answer option. Now, option b says, they both start off less developed at birth and continuously become more rigid throughout life. Now, this is going to be true of sutures, but recall that with gomphoses, that at some point during youth, the gomphoses actually completely deteriorate, which allows the baby teeth to fall out.

And so, they won't continuously become more rigid because at some point they must deteriorate and become less rigid. And so, option b is not going to be the best answer. Now, notice option c says, gomphoses, unlike sutures, break down completely during youth to allow shedding of baby teeth. And this is going to be the correct answer to this example problem. So, we can go ahead and highlight it and mark it as the correct answer.

Now, option d says that sutures, unlike gomphoses, become weak and unstable in old age, but this is not going to be true. Recall that sutures will actually develop into synostoses where the bones of the skull actually fuse together and that dense irregular connective tissue is replaced with osseous tissue or bone tissue. And so that actually makes the sutures become stronger in older age. And so, option d is not going to be the best answer. And so, again, option c is the correct answer to this example, and so I'll see you all in our next video.

In gomphoses, bones are connected to teeth via which of the following structures?

• A. Tendons.
• B. Periodontal ligament.
• C. Loose connective tissue.
• D. Osseus membrane.

Syndesmosis (Pleural: Syndesmoses)

In this video, we're going to talk about the 3rd type of fibrous joint in our lesson, which is the syndesmosis. In the plural form, they are called syndesmoses. These syndesmoses are fibrous joints connecting bones via bundles or ligaments that are made of dense irregular connective tissue. This type of fibrous connective tissue is characterized by densely packed protein fibers in the extracellular matrix with an irregular or random protein fiber arrangement, which allows them to resist forces in multiple directions. Now, the greater bone separation and the longer fibers of these syndesmoses actually allow for more mobility than the other two types of fibrous joints, the sutures and the gomphoses. Recall from our previous lesson videos that whereas the sutures and gomphoses were both classified as synarthroses, since they were still or stationary joints that essentially allowed for no movement and are generally considered immovable joints.

Whereas the syndesmoses, because of their greater bone separation and longer fibers allowing for more mobility, they are always going to be classified as amphiarthroses. Amphiarthroses allow for a little movement, or an average amount of movement, if you will. Notice the symbol for amphiarthroses has one movement arrow going around it, which can hopefully help remind you that these syndesmoses are amphiarthroses that have a little or an average amount of movement. Interosseous membranes are also closely associated with these syndesmoses.

These very broad sheets of dense connective tissue, either dense regular or dense irregular connective tissue, are found between certain bones. Let's take a look at our image below where we can start to piece some of these ideas together. Notice here in the middle we have a person and we're focusing on specific regions of their skeleton. On the left-hand side, we're focusing on the person's lower leg, and you can see the tibia here and the fibula over here. Zooming in on the distal portion of the tibia and fibula, we can see that there is a syndesmosis here connecting the tibia and fibula at the distal end.

This syndesmosis is a ligament made of dense irregular connective tissue. You'll also notice that in between the tibia and fibula is this broad sheet of dense connective tissue, the interosseous membrane. On the right-hand side, we're focusing on this person's forearm and you can see the ulna here and the radius over here. Notice that between the radius and the ulna is another interosseous membrane. However, the protein fibers in the interosseous membrane between the ulna and the radius are longer and broader fibers.

Whereas, the fibers in the interosseous membrane between the tibia and fibula are shorter fibers, and so it's not quite as broad. The shorter fibers between the tibia and fibula are not going to accommodate as much movement, allowing for less movement. Whereas the longer fibers that we can see here between the radius and the ulna are going to allow and accommodate more movement. This is partially why our forearms are able to have more movement and accommodate more movement than our lower leg.

This concludes our brief lesson on syndesmosis, and we'll be able to get some practice applying these concepts and learn more about other types of joints as we move forward in our course. I'll see you all in our next video.

Structural Class: Fibrous Joints Example 3

So, here we have an example problem that asks, which of the following statements is false? And we've got these 4 potential answer options down below. Now, option a says that syndesmoses are unique and that they are the only fibrous joint that are amphiarthroses. Recall from our previous lesson videos that we've only covered 3 types of fibrous joints, and those are sutures, gomphoses, and syndesmoses. Recall that the sutures and gomphoses are classified as synarthroses, meaning that they are still or stationary joints that essentially allow for no movement.

They are immovable joints. However, the syndesmoses are the only fibrous joint in our lesson that are considered amphiarthroses. Recall that the 'a' in amphiarthroses can remind us of the 'a' in "a little movement" or the 'a' in "average movement." And so, amphiarthroses allow for a little or slight movement. Therefore, option a is true, but because option a is true, it is not the false answer option that we are looking for. For that reason, we can eliminate answer option a.

Moving on, option b says, syndesmoses are composed of loose connective tissue to allow for free movement. However, recall from our previous lesson videos that fibrous joints are usually made of dense connective tissue, usually dense irregular connective tissue, and that is the case for these syndesmoses as well. They are not made of loose connective tissue; they are made of dense irregular connective tissue. For that reason, this makes answer option b false.

And because it is false, this must mean that it is the false statement that we are looking for. So, answer option b is going to be the correct answer for this problem. We can mark it as correct. Now, let's take a look at options c and d, just to be sure. Option c says that syndesmoses are the only type of fibrous joint that can be found in the appendicular skeleton.

Recall that the other two types of fibrous joints, sutures, found in the skull, and gomphoses, found in our gums and mouth, are part of the axial skeleton. So, really, syndesmoses are the only type of fibrous joint in our lesson that can be found in the appendicular skeleton. For that reason, option c is true, and because it's true, it's not the false statement that we're looking for, so we can eliminate answer option c.

Option d says that the connecting fibers of syndesmoses are generally longer than those in sutures. Recall that in sutures, the bones of the skull are going to be really close together. So, really there's no need for the fibers to be very long if those bones are already really close together. Whereas in syndesmoses, the bones are further apart, and so longer fibers are needed, and also they tend to have more movement since they are amphiarthroses, and because they have more movement, they will need longer fibers as well. So, option d is also a true statement.

And again, because it is a true statement, it's not the false answer option that we're looking for, and we can cross it off. This here concludes this example problem, and I'll see you all in our next video.

Fibrous joints are all functionally classified as synarthroses because they don't allow for movement.

• A. True
• B. False

How does the length of ligaments in syndesmoses affect the joint?

• A. The longer the ligaments are, the more movable the joint.
• B. The shorter the ligaments are, the more movable the joint.
• C. The length of ligaments has no effect on the joint.

Here’s what students ask on this topic:

What are fibrous joints and how are they classified?

Fibrous joints are a type of joint where the bones are connected by dense connective tissue, primarily composed of collagen fibers. These joints are classified based on their movement capabilities into two main categories: synarthroses and amphiarthroses. Synarthroses are immovable joints, providing structural stability but no movement, such as sutures in the skull and gomphoses anchoring teeth. Amphiarthroses allow for limited movement and include syndesmoses, which connect bones with longer fibers, like the interosseous membrane between the radius and ulna. The classification highlights the balance between stability and mobility in joint function.

What are sutures and where are they found?

Sutures are a type of fibrous joint composed of dense irregular connective tissue, characterized by densely packed collagen fibers arranged irregularly. They are found exclusively in the skull, joining the bones to provide structural stability and protect the brain. Sutures are classified as synarthroses, meaning they allow no movement. However, during youth, sutures permit slow skull expansion to accommodate brain growth. Over time, sutures ossify, fusing the bones into a single unit called a synostosis, further enhancing stability and protection.

What is a gomphosis and what role does the periodontal ligament play?

A gomphosis is a type of fibrous joint that anchors teeth to their bony sockets in the jaw, either the maxilla or mandible. The periodontal ligament, composed of dense irregular connective tissue, plays a crucial role in this joint by firmly gluing the teeth into their sockets. This ligament allows the teeth to withstand forces in multiple directions, especially during chewing, by transferring pressure from the teeth to the jawbone. Gomphoses are classified as synarthroses, meaning they allow minimal movement, primarily under chewing forces.

What are syndesmoses and how do they differ from other fibrous joints?

Syndesmoses are a type of fibrous joint where bones are connected by longer bundles or ligaments of dense irregular connective tissue. Unlike sutures and gomphoses, which are synarthroses (immovable), syndesmoses are classified as amphiarthroses, allowing limited movement. This increased mobility is due to the greater bone separation and longer fibers. Examples include the interosseous membrane between the radius and ulna in the forearm and between the tibia and fibula in the lower leg. These membranes provide structural stability while accommodating necessary movement.

How does the ossification of sutures occur and what is its significance?

Ossification of sutures is the process where the dense irregular connective tissue in sutures gradually transforms into bone over time. This process typically occurs by middle age and results in the fusion of the skull bones into a single unit called a synostosis. The significance of this ossification is that it enhances the structural stability and protection of the brain by preventing fractures and further solidifying the skull. The timing of suture closure varies among individuals due to genetic and health factors.