Also referred to in anatomy as a sagittal suture, a parietal suture is the joint connecting the skull's parietal bones. These two bones form the top and rear portions of the cranium, the part of the skull that envelops and protects the brain. Unlike the joints found in the knees or elbows, the parietal suture is a fibrous joint, meaning that it does not contain cartilage or a sac of synovial fluid but rather a web of connective fibers holding the bones together. Also categorized as a synarthrosis, the parietal suture is not designed for movement but rather allows only the slightest amount of contraction and expansion between bones.
The portion of the skull known as the cranium consists of six bones that fit tightly together: the parietal, frontal, temporal, occipital, sphenoid, and ethmoid bones. Shaped like curved plates, these bones form the forehead, temples, and the back and base of the skull. The parietal bones are paired and make up either side of the top and back of the cranium; the parietal suture is the vertical joint between them. In contrast to most of the body’s joints, which involve the articulating ends of two long bones, the parietal suture is a long and continuous joint involving the shared medial borders of the parietal bones.
Beginning at the center of the top of the cranium where the parietal bones meet the frontal bone — the bone of the forehead — the parietal suture forms a midline running down the back of the skull. This is not a straight line, however, but an uneven joint resembling a crack in pavement. When the skull is still forming in utero, the individual bones are separated slightly, allowing for compression of the baby’s head during childbirth. After birth, these bones begin to harden and unite, their uneven edges gradually fitting together.
In the space between the parietal bones along the entire length of the parietal suture is a dense collection of collagen fibers. A type of connective tissue that is similar to ligaments and tendons, these fibers are named Sharpey’s fibers and serve opposing functions at the joint. On one hand, they are strong enough to hold the bones together, encasing the brain in a protective bony chamber. They are also elastic enough, however, to allow the skull to expand and contract as may be necessary to alleviate swelling on the brain following a concussion or other injury.