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):
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.
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.
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.
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.
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:
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?
© 2011 STCC Foundation Press