A cranial suture is an immovable fibrous joint found exclusively in the skull, acting as a seam between the bony plates of the cranium. These joints are composed of a thin layer of dense fibrous connective tissue, primarily collagen, that binds the adjacent bones firmly together. Sutures are classified as synarthroses, meaning they allow for no movement in the adult skull. They are fundamental for connecting the separate bony elements of the head to form a cohesive, protective structure around the brain.
Classification of the Squamous Suture
The squamous suture is structurally categorized as a type of lap suture, also known by the anatomical term squamosal suture. This classification is based on the unique way the articulating bone edges meet, where one bone surface overlaps the other in a beveled, or scale-like, manner. The edges overlap like shingles on a roof, creating a strong, sloping connection.
The overlapping structure allows for a small degree of flexibility, which is necessary during developmental periods, while maintaining the cranium’s overall stability. As a fibrous joint, it ensures a secure articulation between the bones. The name “squamous” refers to the flat, scale-like portion of the temporal bone involved in this articulation.
Anatomical Location and Specific Bones Joined
The squamous suture is situated bilaterally on the lateral aspect of the skull. It forms a prominent, arching line that begins near the temple and extends backward over the ear.
More specifically, the suture unites the inferior border of the parietal bone with the superior margin of the squamous part of the temporal bone. This joint begins anteriorly at a junction called the pterion and then curves posteriorly. The location of the squamous suture along the side of the head is significant, as it contributes to the overall vertical height and lateral expansion of the neurocranium during growth.
Comparing the Primary Types of Cranial Sutures
The lap suture classification of the squamous joint provides a clear contrast to the two other main structural types of cranial sutures: serrate and plane sutures. Serrate sutures are characterized by their interlocking, saw-like edges, which create a zigzag pattern that provides maximum surface area contact for enhanced stability. The major seams of the skullcap, such as the sagittal, coronal, and lambdoid sutures, are classic examples of serrate joints.
Plane sutures are the simplest type, featuring edges that are relatively straight and smooth, meeting in a simple butt joint. These joints are found in the facial skeleton and base of the skull, such as the articulation between the palatine bones. The lap suture’s beveled overlap is structurally intermediate, offering a smooth yet robust articulation distinct from both serrate and plane joints.
Functional Role of Sutures in the Skull
Cranial sutures serve several mechanical and developmental purposes. In infants, the fibrous tissue allows the bony plates to shift and slightly overlap, facilitating passage through the birth canal. This temporary flexibility is immediately followed by the sutures acting as primary growth centers for the skull.
The open sutures accommodate the dramatic volumetric expansion of the brain during early childhood, which nearly triples in size by two and a half years of age. As the brain grows, the bone is deposited along the edges of the sutures, ensuring the cranium expands to match the underlying neural tissue.
Later in life, the process of ossification, known as synostosis, causes the fibrous tissue to be replaced by solid bone, resulting in the complete fusion of the sutures. Most sutures complete this fusion process by early adulthood, though the squamous suture is known to remain open later than many others, sometimes closing as late as age 60.