Age of Fontanelles / Cranial Sutures Closure
Fontanelles are membranous areas that have not yet ossified in the developing cranial vault of neonatal and juvenile animals. Fontanelles allow for rapid stretching and deformation of the cranium as the brain expands faster than the surrounding bone can grow. 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.
Immature humans and apes have six primary 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 or anterolateral fontanelles and right and left mastoid or posterolateral fontanelles). The two midline fontanelles both participate in anteroposterior and medioateral brain growth. The fontanelles on the lateral vault permit superoinferior brain growth. Two additional fontanelles (metopic fontanelle and sagittal or third fontanelle) can also be present in humans. In monkeys the fontanelles are nearly or completely closed at the time of birth, in apes the fontanelles are small but still present at birth, whereas in humans the fontanelles are large in newborns.
In humans, the sequence of fontanelle closure is as follows: 1) posterior fontanelle generally closes 2-3 months after birth, 2) sphenoidal fontanelle is the next to close around 6 months after birth, 3) mastoid fontanelle closes next from 6-18 months after birth, and 4) the anterior fontanelle is generally the last to close between 1-3 years of age (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). If the sagittal fontanelle is present, it is usually located near the parietal notch and is present at birth in 50-80% of perinatal skulls. It is defined by the sixth prenatal month and is usually obliterated at birth or within a few months after birth. The sagittal fontanelle has been clinically associated with Down’s syndrome and other abnormalities. If the metopic fontanelle is present, it will obliterate between 2 to 4 years of age. In humans, all fontanelles are generally fused by the fifth year of life with 38% of fontanelles closed by the end of the first year and 96% of the fontanelles closed by the second year. 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 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. Fusion of sutures in humans has been used as an age indicator, but fusion of the cranial sutures is highly variable and has been shown to be an unreliable indicator of age because of the range of variation in the timing of obliteration.
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.
One pathological condition of particular interest in terms of fontanelle closure and brain development is craniosynostosis. Craniosynostosis is a condition in which the sutures and fontanelles prematurely fuse and result in a change in the growth pattern of the skull. In some cases this just results in a abnormally shaped head, while in other cases if brain development is disrupted there can be developmental impairments. It is a physical defect that might be caused by genetics or hormonal factors such as with exposure to high levels of thyroid hormone. In humans with craniosynostosis, wormian bones (extra sutural bones) are significantly more frequent to develop with premature suture closure on the order of 3.5 greater odds. It has been documented that chimpanzees do have the occurrence of extra sutural bones. No cases of great ape craniosynostosis could be found in the literature but it is possible that if wormian bones are a response to craniosynostosis that chimpanzees do possess the necessary response for the pathology.
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