Skull sutures are fibrous joints composed of dense connective tissue that connect the bony plates of the skull. Unlike most joints in the body, sutures are immovable in adults, creating a protective shell for the brain. They are particularly noteworthy in infants and children because they remain flexible and unfused during the early stages of life. This temporary flexibility is necessary for both the birth process and the rapid growth of the brain.
Anatomy and Major Sutures of the Skull
The human skull is made up of multiple bony plates, including the frontal, two parietal, occipital, and temporal bones, all held together by sutures. Sutures are distinct from the bones themselves, appearing as wavy lines or seams where the bony plates interlock. Before they fuse, the tissue within the suture is fibrous, allowing for a degree of movement.
The skull features several sutures, but four are considered major: the coronal, sagittal, lambdoid, and squamosal sutures. Where several sutures meet, particularly in infants, they create membranous gaps known as fontanelles, or “soft spots”.
The major sutures include:
- The coronal suture connects the frontal bone at the forehead with the two parietal bones.
- The sagittal suture runs down the midline of the head, joining the two parietal bones.
- The lambdoid suture connects the two parietal bones to the occipital bone, which forms the base of the skull.
- The squamosal suture is a horizontal seam joining the parietal bone with the temporal bone above the ear.
The Critical Role of Sutures in Development
Open, flexible sutures serve two specific functions in early life. The first is facilitating the passage of the baby’s head through the narrow birth canal during delivery.
The fibrous tissue allows the bony plates to temporarily overlap and compress, a process called molding, which reduces the head’s overall diameter. This overlap helps prevent injury during birth. After delivery, the skull bones return to their normal positions, and the head shape normalizes.
The second function of sutures is to accommodate the rapid growth of the brain after birth. The brain nearly doubles in volume during the first year of life and reaches about 80% of its adult size by the age of two. The sutures act as expansion joints, growing new bone tissue at their edges to enlarge the skull vault, providing the necessary space for brain development. Without this expansion, the growing brain would face restriction.
When Sutures Close
The flexibility of skull sutures is temporary, as they are programmed to eventually close and fuse through ossification. This process involves the fibrous connective tissue being replaced by solid bone, permanently joining the adjacent skull plates. The timing of this fusion varies widely among the different sutures, often extending from infancy into adulthood.
The metopic suture, which divides the frontal bone, is typically the first to close, usually fusing completely between three and nine months of age. The major cranial vault sutures—coronal, sagittal, and lambdoid—remain open much longer for continued brain growth. The sagittal suture may begin to fuse around age 22, while the coronal and lambdoid sutures may not fully fuse until the mid-twenties or later.
If one or more sutures fuse too early, the condition is known as craniosynostosis. Since the skull can no longer expand in the direction of the fused suture, the brain’s growth is forced into other directions, resulting in an abnormally shaped head. This premature fusion can happen before birth or in the first few months of life.