Age of Closure of Fontanelles / Sutures

Fontanelles are membranous areas that have not yet ossified in the developing cranial vault of neonatal and juvenile animals. Immature humans and apes have six fontanelles – two along the midline of the top of the vault (the anterior or bregmatic and the posterior or lambdoid fontanelles) and two on each side of the lateral vault (right and left sphenoidal fontanelles anteriorly and right and left mastoid fontanelles posteriorly). Cranial sutures are fibrous joints (synarthroses) between the bones of the vault or face. Both fontanelles and sutures are important for cranial vault growth (and accordingly, brain growth), as once they fully ossify no further expansion of the braincase is possible. In monkeys the fontanelles are nearly or completely closed at the time of birth, in apes the fontanelles are small but still patent at birth, whereas in humans the fontanelles are large in newborns. In humans the lateral fontanelles close soon after birth, the posterior fontanelle generally closes several months later, and the anterior fontanelle may remain open for three years (in one recent human sample, the anterior fontanelle was closed in most individuals by 31 months postnatally, in another sample most individuals older than 17 months exhibited closure of this fontanelle). In contrast, apes fuse the fontanelles soon after birth: in chimpanzees the anterior fontanelle is fully closed by 3 months of age. With respect to sutures, humans also show a delayed pattern of ossification relative to other apes. In humans the sutures remain patent (and capable of growth) until early adulthood (late in the third decade of life), whereas in apes the sutures begin to fuse in childhood. The large fontanelles of humans allow the neonate’s skull to compress at the time of delivery, and are thus part of an adaptive solution to the problem of giving birth to large-brained babies through a relatively narrow and rigid birth canal (in a pelvis adapted for bipedal locomotion). Also, the late fusion of the fontanelles and sutures permits a greater degree of postnatal growth of cerebral volume. In human infants the brain is only about 25% of its adult volume at birth, has only reached 50% of adult size by one year of age, and may continue growing until about 20 years of age. In chimpanzees, neonates have brain volumes that average about 40% of the adult volume, and have attained 80% of adult brain size by one year of age. Thus delayed fontanelle and suture closure is part of the human pattern of secondary altriciality.