The function of the immovable joints is to support and protect organs and other internal structures. An example of a grouping of immovable joints serving a common purpose is demonstrated by the immovable joints that hold the bones of the cranium together, and which also provide protection and support for the brain. The areas of connection between the immovable joints in the cranium are called sutures, or fibrous joints, and they are held together by a layer of dense fibrous connective tissue comprised primarily of collagen.
Immovable joints do not move, nor do they have joint cavities in the manner of the movable joints. In the case of the fibrous joints of the cranium, many of the connective sutures are replaced through osseous fusion as age increases. During fetal development, the sutures between the fibrous joints of the cranium are wider, and allow for some movement during birth, but these joints later become rigid. These sutures will be fully ossified by the time middle age arrives, and they are then termed synostoses, or bony junctions.
Another example of an immovable joint is the connection between a tooth and the lower or upper jaw. The immovable joint between the root of a tooth and its socket in a mandible or maxilla is termed a gomphosis.