A cranial suture is a joint found between the bones of the cranium, the rounded portion of the skull that houses the brain. A type of joint known in anatomy as a synarthrosis, the cranial suture allows little to no movement between bones, and what little is allowed typically occurs in the first couple of years of life as the skull is hardening. This joint may also be classified according to its structure as a fibrous joint, a joint type in which the bones are held together by a network of tiny fibers known as Sharpey’s fibers. These fibers permit a small degree of elasticity, so that in the case of brain swelling following injury the skull can expand slightly.
While there are more than 15 cranial sutures, the most substantial of these join the large bones of the skull: the frontal bone, the parietal bones, the temporal bones, and the occipital bone. Between the frontal bone in the forehead and the paired parietal bones covering the top of the skull is a large suture called the coronal suture. Beginning above one ear, the coronal suture extends across the top of the head just behind the hairline to the other ear. Like any cranial suture, it is not a straight line but rather jagged like a river on a map, the bones on either side having fused nearly together like two tectonic plates.
Another significant cranial suture divides the two parietal bones. This is known as the sagittal suture. Beginning at where the parietal bones meet at the midline of the coronal suture, the sagittal suture divides the skull into right and left halves. It then extends down the back of the skull to the base of the parietal bones, ending where they unite with the occipital bone. Similar sutures are found between the parietal bones and the occipital bone, between the parietal bones and the temporal bones, and between the temporal bones and the occipital bone, among others.
What makes these skull bones fit so tightly together like puzzle pieces is the presence of small collagen fibers within each cranial suture. Referred to as Sharpey’s fibers, these crisscrossing lengths of fibrous connective tissue bond the bones securely but also lend flexibility to the joint. In adulthood these bones rarely have to move relative to one another, but in the case of brain trauma, the skull may need to expand somewhat to relieve pressure on the brain. The flexibility of Sharpey’s fibers allows the individual skull bones to move outward.