Joint Tissue Types

Joints can be classified by the tissues that are involved in holding the bones together. In this manner, there are three major types of joints (the first two have additional subclasses):

Fibrous Joints

syndesmosis

suture

gomphosis
Cartilaginous Joints

synchondrosis

symphysis
Synovial Joints

I'll go over each one of these types of joints in more detail below. But you may already be able to imagine that "fibrous" joints are held together by fibrous connective tissue, "cartilaginous" joints have cartilage within them, and "synovial" joints are something altogether different.

Fibrous Joints:

These are joints where the bones are fastened together by dense (fibrous) connective tissue. This tough connection between bones prevents much movement at these joints. In fact, two of the three types of fibrous joints are immobile joints in adults.

You learned back in the connective tissue chapter that when dense connective tissue is used to hold bones together, we call that tissue a ligament. You will see that these fibrous joints all have tissue called "ligaments" holding the bones together.

Syndesmosis-- the only slightly mobile (amphiarthrotic ) fibrous joint. In these joints, ligaments hold bones together. It is because the ligaments offer a little flexibility that the bones can shift positions a bit against each other. The ligaments here are called interosseus ligaments (which, if you think about it, just means "between bones" ligaments).
    "syn-" means together
    You saw "desmo" once before in the term desmosome, which is a type of intercellular junction (see Figure 3.8). Desmosomes are not as tight as tight junctions... they are a bit looser.
    Therefore, think of the word syndesmosis as a close, but not tight, joining together of bones.
    This figure from your book shows an interosseous ligament in your ankle... don't worry about knowing the bones yet, since we haven't gotten to the appendicular skeleton in lab.

Suture-- In these joints, bones are tightly opposed, and the dense connective tissue (like the kind that originally existed embryonically as the membranes for intramembranous ossification) simply seems like a glue holding the bones firmly in place. You saw sutures in lab when you looked at the skull... the sutures are the lines that looked like "fusion" lines between the cranial bones. The ligaments here are called sutural ligaments. The image here from your book shows a close-up drawing of the squamous suture.

Gomphosis-- the joint found between teeth and the maxilla or mandible. The ligaments here are called periodontal ligaments ("around the teeth" ligaments). The way this differs from a suture is because the teeth have pointed processes that fit into the cup-like regions of the maxilla or mandible (these cup-like regions are called alveoli).

Cartilaginous Joints:

Here, instead of fibrous connective tissue, cartilage holds the joints together. You should remember that there are three types of cartilage... two of them are involved in these joints.

Synchondrosis-- hyaline cartilage connects the bones in this type of joint. This type of joint is immovable (synarthrotic), as you would expect since hyaline cartilage is rather tough. A good example of a synchondrosis is the epiphyseal disk... believe it or not, this is considered a joint between the epiphysis and the diaphysis.
You should be able to dissect this word, synchondrosis, apart without too much help. "syn-" is for together, and "chondro" is for cartilage. So these joints are a holding together of bones by cartilage. The only trick is to remember that this term is only used for hyaline cartilage joints.

Symphysis-- this type of joint is somewhat movable (amphiarthrotic). It is made up of fibrocartilage (coated in hyaline cartilage), rather than just hyaline cartilage. It is found between every vertebra (as constituting the intervertebral disks... commonly referred to as disks), and within the pelvic girdle. Read up on page 259 in your book about its composition (nucleus pulposis and annulus fibrosus).
You should think of the fibrocartilaginous disks that make up symphysis joints as a sort of natural shock absorber in our bodies. Note that in the photo above another place we find a symphysis joint is in our pelvic girdle. For childbirth, this fibrocartilaginous disk weakens and is more flexible to accomodate the movement of the child through the pelvic girdle. But the rest of the time, it is not so mobile.

Synovial Joints:

These joints are all freely mobile (diarthrotic). Synovial joints have more than just tissue between the bones of the joints-- they also have fluid (synovial fluid). The fluid is secreted into a space between the bones, helping to create a cushion and a smooth movement. You should have guessed by now that the fluid must be secreted by epithelial tissue which lines the space containing it. The epithelial tissue is the synovial membrane and it lines the cavity containing the synovial fluid.

An epithelial bag of fluid is certainly not strong enough to prevent itself from popping as the joint moves. Right? Therefore, you would expect that this bag would need to be surrounded and protected. This is what happens. It is carried out by the joint capsule, a dense connective tissue coating.

kneesidecut.JPG (38410 bytes) Within the joint capsule you find the synovial membrane. This membrane secretes the synovial fluid. In this image from your book, the synovial membrane is indicated, but the synovial fluid is not. The synovial fluid is simply the blue-colored region within the synovial membrane.

Lets go through this joint from the bone to the outside. The tip of each bone is covered by the articular cartilage that you learned about last week in Unit 5. Do you remember what type of cartilage makes up articular cartilage?

The synovial membrane attaches to the articular cartilage and forms a bridge between the bones of the joint. The contained space within this bridge is filled with synovial fluid.

Outside of the synovial membrane we find the joint capsule. Because this capsule is composed of dense connective tissue, and dense connective tissue (regular) can form ligaments and tendons, you'll see that this capsule typically serves as both protection for what's inside and as ligaments or tendons for the joint. So it doesn't usually have just one function-- it serves double-duty!

That's it for the basics of synovial joints. However there are two other structures that may be found at some synovial joints (but not at all). These two, and a description of them, are:

	meniscus (plural: menisci): This is a fibrocartilage cushion. Some synovial joints need extra cushioning, so areas of fibrocartilage exist within them. These joints are still synovial, not symphysis, joints; they just have this additional bit of fibrocartilage in them.
	bursa (plural: bursae): This is an extra packet of synovial fluid. Bursae are found in places where the joint might otherwise have too much friction. The packets of fluid increase the ability for these regions of a joint to nicely slide past one another.

I hope you now have an understanding of the structural make-up of the different types of joints. Remember-- this web page was meant to introduce you to how each joint is built (in other words, what tissues comprise each joint). It was not meant to make you an expert on where each type of joint is found and on details of how each joint works. So, go over the list at the top again, and see if you remember the types of tissues in and appearance of each of these joints. OK?

� 2006 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.