Cranial sutures are specialized, fibrous joints found between the flat, bony plates that make up the skull. These joints function as immovable connections that hold the cranial bones securely together. The skull is composed of 22 bones, and most are connected by a suture, meaning the total count of individual suture lines is numerous. For structural and functional purposes, the focus is generally placed on the four major sutures that form the dome of the cranium. These connective tissues are integral to the structure of the head, protecting the brain.
The Four Major Cranial Sutures
The primary focus when discussing the number of sutures in the skull is on the four largest, most functionally significant joints that connect the main bones of the cranial vault. These four major sutures are named for their location and the bones they join, providing a map of the skull’s structure.
The Coronal Suture runs horizontally across the top of the head, joining the frontal bone with the two parietal bones. Running vertically along the midline of the skull, the Sagittal Suture connects the two parietal bones to each other. This joint extends from the Coronal Suture to the Lambdoid Suture.
Positioned at the posterior aspect of the skull, the Lambdoid Suture connects the two parietal bones with the occipital bone, which forms the base and rear of the cranium. The Squamosal Suture is a slightly overlapping, beveled joint that runs along the side of the head, connecting the parietal bone with the temporal bone on each side. These four sutures define the main structural framework of the upper skull.
Functions of Sutures in Infancy and Adulthood
The presence of open, flexible sutures is related to the growth requirements of the developing human brain during infancy. The fibrous joints allow the bony plates of the skull to slightly overlap, a process called molding, which temporarily reduces the head’s diameter during vaginal delivery. This flexibility is important for the infant’s passage through the birth canal without damaging the brain.
After birth, open sutures accommodate the brain’s rapid increase in volume, which grows most quickly during the first two years of life. The sutures, alongside the fontanelles (soft spots where multiple sutures meet), allow the skull to expand uniformly. The largest soft spot, the anterior fontanelle, is located where the coronal and sagittal sutures meet and remains open for 7 to 19 months.
This flexibility ensures the growing brain does not encounter restrictive pressure. As development slows, the process of fusion, or synostosis, begins, where the fibrous tissue gradually turns into bone. While some minor sutures fuse early, the major cranial sutures begin to close in early adulthood, often between the late twenties and fifties. For example, the sagittal suture often fuses around age 22, and the coronal suture around age 24, permanently locking the skull bones into a solid structure.
Premature Fusion: Understanding Craniosynostosis
The biological timing of suture fusion is precise. When one or more sutures close too early, a condition known as Craniosynostosis occurs. This premature fusion, happening before the brain has finished growing, restricts the skull’s expansion perpendicular to the affected suture. Since the brain continues to grow, the skull compensates by growing excessively parallel to the fused suture, resulting in an abnormally shaped head.
The consequences of Craniosynostosis vary depending on which suture is affected, but the condition occurs in approximately one out of every 2,000 live births. If only a single suture is prematurely closed, the primary result is often a distinct, abnormal head shape, such as the elongated skull seen when the sagittal suture fuses early. If multiple sutures close prematurely, the restricted growth can cause increased intracranial pressure, which may interfere with brain development.
The inability of the skull to expand properly can lead to complications such as developmental delays or vision problems due to pressure on the brain. When Craniosynostosis is diagnosed, treatment often involves surgical intervention to reopen the fused suture or reshape the skull. This allows the brain the necessary space to grow and aims to correct the head’s shape.