The human skull is not a single, solid bone at birth, but a collection of bony plates held together by flexible connections. These connections are known as cranial sutures, which are fibrous joints made of dense connective tissue. This structural arrangement allows the skull to be malleable, serving a significant purpose both during and after birth. This adaptation supports the rapid development and protection of the brain during the earliest stages of life.
Which Bones the Sagittal Suture Connects
The sagittal suture is a dense, fibrous joint running along the exact midline of the upper skull. This suture connects the left and right parietal bones to each other, which form the sides and roof of the skull. The sagittal suture extends from the coronal suture at the front to the lambdoid suture at the back. This arrangement creates a seam running from the anterior fontanelle, or “soft spot,” toward the posterior fontanelle.
The two edges of the parietal bones do not simply butt up against each other; instead, they are finely denticulated, featuring an irregular, zigzag pattern that interlocks tightly. This unique configuration provides mechanical strength and resilience to the skull while the joint remains flexible in early life. The meeting point of the sagittal and coronal sutures at the front of the skull is an anatomical landmark called the bregma.
Essential Functions of Cranial Sutures
The primary function of cranial sutures is to provide malleability to the skull, utilized in two specific physiological processes. First, the flexibility of these joints facilitates passage through the mother’s birth canal during delivery. The fibrous tissue allows the bony plates to slightly overlap and mold the shape of the head, temporarily reducing its diameter without causing injury to the underlying brain.
Second, the sutures act as expansion joints that accommodate the rapid growth of the brain after birth. During the first few years of life, the brain undergoes its most dramatic period of expansion, and the open sutures allow the cranial vault to enlarge evenly and symmetrically. The spaces where several sutures meet, such as the anterior and posterior fontanelles, are covered by a tough membrane and are clinically monitored.
When Sagittal Suture Fusion Occurs
The timeline for the ossification of the sagittal suture is much later than many other cranial joints. While some minor sutures close within the first year of life, the sagittal suture typically remains open throughout childhood and adolescence. Normal closure is generally not completed until late adolescence or early adulthood, with reported ages ranging up to around forty years old.
However, craniosynostosis occurs when one or more sutures prematurely fuse, often before or shortly after birth. Premature closure of the sagittal suture is the most common form of single-suture craniosynostosis. When the sagittal suture fuses too early, it prevents the skull from expanding sideways, forcing the head to grow long from front to back. This results in a distinctive, elongated, and narrow head shape known clinically as scaphocephaly, derived from the Greek word for “boat-shaped.”