Final answer:
The frontal suture is absent and other sutures are closed in an adult human skull, as the frontal bone fuses during childhood and other sutures ossify later in life. Premature fusion of sutures, or craniosynostosis, can lead to abnormal skull and brain development.
Step-by-step explanation:
The frontal suture is typically absent and other sutures are closed in an adult human skull. During development, the frontal and maxillary bones have left and right halves that are joined by a suture, but by around the age of eight, these halves fuse to form a single bone, eliminating the frontal suture. Late in life, other sutures of the skull, such as the sagittal, coronal, and lambdoid sutures, also begin to ossify and fuse, with the suture line gradually disappearing.
The frontal suture is only present at birth and in early childhood as the frontal bone initially consists of two halves. The fusing of the cranial bones through the process called synostosis is a natural part of aging, as is the disappearance of the anterior fontanelle. However, if the sutures of the skull close too early—a condition known as craniosynostosis—it can lead to abnormal skull growth potentially impacting brain development. Primary craniosynostosis involves the premature fusion of a single cranial suture, whereas complex craniosynostosis includes several sutures, often leading to more severe deformities and possibly impeding brain growth and development.