The frontal suture is the place where, in early childhood, the bilateral halves of the frontal bone in the skull meet in to form a vertical line in the middle of the forehead. These eventually fuse to form a single frontal bone, typically between six and eight years of age. The existence of the frontal suture allows the skull to compress to allow the infant’s head to fit through the birth canal during labor. As the brain finishes growing and the skull bones develop in the child’s first several years, the gaps between them close.
Including 22 bones in total, the human skull is made up of 14 facial bones and eight cranial bones. The cranium, or brain-enclosing portion of the skull, is shaped by the occipital bone in the lower rear section, the paired parietal bones covering the top rear part of the skull, the temporal bones on either side, and the frontal bone spanning the top front section. Bordered by the parietal bones above, the sphenoid bones of the temples laterally, and the nasal bone and eye sockets below, the frontal bone is bounded by 12 other bones. It is rounded above and on either side, and irregular below where it forms the top margin of each eye socket.
In the adult skull, what appears to be a jagged crack can be seen running up the center of the frontal bone from between the superciliary arches or eyebrows to approximately the height of the hairline. This is the fused remnant of the frontal suture. During fetal development, the frontal bone actually begins as one bone, the frontal suture developing prior to childbirth. By the time the baby is born, the suture is visible as a narrow gap filled with tiny tissue fibers. Known as Sharpey’s fibers, they are composed largely of collagen and hold the two halves of the frontal bone together while lending an elastic property to the suture, allowing the bones to move together and apart.
As the child grows, the bones begin to move together permanently, a process known as ossification. In some individuals this never occurs; the remaining frontal suture then becomes known as a metopic suture. This is not necessarily considered significant or dangerous, but the same cannot be said of the opposite condition. In some cases, the frontal bones fuse and frontal suture disappears too early, before the brain has finished growing in the skull. This prevents the skull from expanding further and places pressure on the brain, a condition known as trigonocephaly, although premature fusion of the frontal bone also can develop as a result of a brain that does not develop fully in the skull.