What Are Fontanelles and Why Are They Only in Infants?
Fontanelles, colloquially known as soft spots, are areas of the infant's skull where the cranial bones have not yet fused together. These gaps are covered by a strong, fibrous membrane that protects the brain, and their existence is a vital aspect of early human development.
There are several reasons why newborns and young infants have fontanelles:
- Facilitating childbirth: During delivery, the flexible, unfused skull bones can overlap and compress, allowing the baby's head to safely pass through the narrow birth canal.
- Accommodating rapid brain growth: The brain grows most rapidly during the first two years of life. Fontanelles provide the necessary space for the brain to expand without being constricted by a rigid skull.
- Protecting against impact: The flexibility provided by the fontanelles and skull sutures helps to absorb minor impacts and protect the delicate brain from injury during a period when infants are learning to control their head movements.
The Process of Fontanelle Closure
Fontanelles are not permanent features. They close at different stages of early childhood as the fibrous tissue is gradually replaced by bone, a process called ossification.
The two most prominent fontanelles are:
- Posterior Fontanelle: Located at the back of the head, where the parietal bones and occipital bone meet. This fontanelle typically closes first, within the first 1 to 2 months of life.
- Anterior Fontanelle: Situated at the front top of the head, this is the largest of the soft spots. It closes later, typically sometime between 7 and 19 months of age.
Once the fontanelles close, the fibrous sutures that connect the skull bones remain. While still somewhat flexible in childhood, these sutures gradually ossify and become more rigid throughout adulthood.
Fontanelles vs. Sutures: A Comparison Table
It is important to distinguish between fontanelles, which disappear in infancy, and sutures, which persist throughout life, although they become less flexible with age.
| Feature | Location | Composition | Function | Fate in Adulthood |
|---|---|---|---|---|
| Fontanelles | Gaps between intersecting skull bones | Membranous connective tissue | Enable head molding during birth; accommodate rapid brain growth | Close and become solid bone by ~2 years |
| Sutures | Fibrous joints connecting skull bones | Fibrous connective tissue | Allow for expansion of the skull; eventually ossify completely | Remain as solid, immovable, interlocking joints |
What Happens to the Skull in Adulthood?
In a healthy adult, the fontanelles have fully ossified, and the skull is a solid, protective structure. The remnants of the fontanelles are marked by specific anatomical landmarks on the skull. The original anterior fontanelle location becomes the bregma, while the posterior fontanelle location becomes the lambda. The cranial sutures, which were once flexible to allow for growth, continue to ossify and become more rigid throughout the adult lifespan. Some sutures may not fully fuse until a person reaches their later years, with some, like the sagittal, closing in the 30s and 40s, and others, like the squamous suture, fusing as late as age 60.
Health Implications of Persistent Fontanelles in Adults
While a soft spot on an infant's head is a sign of normal development, its persistence into adulthood is abnormal and a cause for medical concern. A fontanelle that remains open in an adult can be a symptom of underlying health issues, including:
- Genetic Disorders: Certain rare genetic conditions, such as cleidocranial dysplasia, can interfere with bone development and prevent fontanelles from closing properly.
- Metabolic and Endocrine Issues: Disorders that affect bone growth, like congenital hypothyroidism or vitamin D deficiency, can also lead to delayed or incomplete fontanelle closure.
- Hydrocephalus: A buildup of cerebrospinal fluid in the brain can increase intracranial pressure, preventing the skull bones from fusing correctly.
In such cases, a healthcare provider will conduct a thorough examination, often involving imaging techniques like CT scans or MRIs, to diagnose the root cause and determine the appropriate treatment. A persistent open fontanelle can put an adult at risk for infections and head injuries.
Conclusion
To reiterate, healthy adults do not have fontanelles in their skulls. These soft spots are a temporary and normal feature of infancy, essential for both childbirth and early brain development. They gradually close as the fibrous membranes are replaced by solid bone, with the process completing by about age two. In place of the fontanelles and flexible sutures, the adult skull is a solid, fused structure designed for maximum protection. The persistence of a soft spot in adulthood is a rare condition that signals an underlying medical problem requiring professional attention. This transformation from a flexible infant skull to a rigid adult cranium is a remarkable example of human biological development and adaptation.
