What is a Fontanelle Its Types Location and Function
Fontanelle or fontanel is a feature of the infant skull. It can also be referred to as a soft spot on the infant skull. In anatomical terms, fontanelles are soft membranous gaps or sutures between the cranial bones. This makes the calvaria of the infant or the fetus. Fontanelle mainly consists of the posterior fontanelle and anterior fontanelle. These membranous sutures expand and stretch and deform the neurocranium as the brain expands faster than the growth of the surrounding bones. In general terms, this is the fontanelle meaning.
A bulging fontanelle can be considered as an outward curving of an infant’s soft spot. This soft spot on baby head is the fontanelle. In this article, we will discuss the structure of the fontanelle along with its various other disorders.
Structure of the Fontanelle
The fontanelle mainly consists of the anterior and posterior fontanelle. In this section, we will look into the various parts of the bony bump on back of skull.
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Posterior Fontanelle
This is triangular. It is located at the junction of the sagittal suture and the lambdoid suture. The posterior fontanelle ossifies (closes) within 6-8 weeks after birth. The mesenchymal tissue turns into bone. This is known as intramembranous ossification. During birth, the posterior fontanelle remains quite small and a tough membrane covers the open area where the two parietal bones join the occipital bone. This happens at the lambda.
Anterior Fontanelle
The anterior fontanelle is a membrane-filled cavity located between two frontal and two parietal bones of the fetal skull. The shape of anterior fontanelle is like a diamond. It is situated between the coronal and the sagittal suture. The anterior fontanelle persists approximately until 18 months after the birth of the infant.
Sometimes there is a certain delay in the closing of the anterior fontanelle. It may be due to cleidocranial dysostosis. However other causes of delayed anterior fontanelle closure include achondroplasia, hypothyroidism, Down syndrome, increased intracranial pressure, and rickets. Palpating the anterior fontanelle is often used to examine infants. In some cases, the anterior fontanelle is late in closing (8-24 months) or may never close. This is all about the full anterior fontanelle.
Apart from the anterior and posterior fontanelle, two other fontanelles also exist and are quite significant. They are the mastoid fontanelle and sphenoidal fontanelle. The sphenoidal or the anterolateral fontanelle is located between the sphenoid, parietal, temporal, and frontal bones. However, the mastoid or the posterolateral fontanelle is located between the temporal, occipital, and parietal bones.
Fontanelle Fusion or Closing of The Fontanelles
The fontanelle fusion can be observed very distinctly in the case of humans. Generally, all the fontanelles are closed by five years of age. 38 percent of the fontanelles are fused by the end of the first year and 96 percent of the rest are fused by the end of the second year. Let’s look into fontanelle fusion in detail.
The posterior fontanelle normally fuses 2 to 3 months after birth;
The sphenoidal fontanelle fuses around 6 months after birth;
The mastoid fontanelle fuses next from 6 to 18 months after birth;
The anterior fontanelle is normally the last to close between 12–18 months.
This is all about fontanelle fusion.
Disorders of the Fontanelles
There are various disorders of the fontanelles like bulging fontanelle or a bony bump on the back of the skull. In normal cases, a bulging fontanelle depicts an increase in cranial pressure. This can happen due to encephalitis, hydrocephalus, meningitis, or Active Head Trauma (AHT). Sunken or depressed fontanelles may happen due to dehydration and malnutrition. The bony bump on the back of the skull is sometimes known as an occipital bun or inion hook. The enlarged fontanelle may happen due to Down syndrome, IUGR, or premature birth. Apart from all of these, one should be careful while handling the soft spots on the baby head as it can be dangerous if not treated with care.
Interesting Facts About Fontanelles
During birth, fontanelles enable the bony plates of the skull to flex, allowing the child's head to pass through the birth canal.
The late fusion of Fontanelles and sutures allows the postnatal growth of cerebral volume.
Often pulsations are felt on a baby’s fontanelles. These pulsating soft spots are actually due to the beating of the baby’s heart.
FAQs on Fontanelle in the Newborn Skull
1. What is a fontanelle?
A fontanelle is a soft, membranous gap between the bones of a newborn baby’s skull. It occurs where the cranial bones have not yet fused, allowing flexibility during birth and brain growth. Fontanelles are made of tough connective tissue and are commonly called “soft spots” on a baby’s head.
2. Where are the fontanelles located in a newborn?
Fontanelles are located at the junctions of the cranial bones in an infant’s skull.
- Anterior fontanelle – between the frontal and parietal bones (top of the head)
- Posterior fontanelle – between the parietal and occipital bones (back of the head)
- Two sphenoidal (anterolateral) fontanelles
- Two mastoid (posterolateral) fontanelles
3. What is the function of fontanelles?
The main function of fontanelles is to allow flexibility of the skull during birth and enable rapid brain growth in infancy.
- Facilitate compression of the skull during childbirth
- Provide space for expansion of the growing brain
- Assist healthcare providers in assessing hydration and intracranial pressure
4. When do fontanelles close?
Fontanelles close at different ages as the cranial bones fuse through a process called ossification.
- Posterior fontanelle: usually closes by 2–3 months of age
- Anterior fontanelle: typically closes between 12–18 months
- Lateral fontanelles: close within the first year
5. Why is the anterior fontanelle larger than the posterior fontanelle?
The anterior fontanelle is larger because it accommodates greater brain growth in the frontal and parietal regions during early infancy. It is diamond-shaped and remains open longer than the smaller triangular posterior fontanelle. This extended openness supports rapid cerebral development in the first year of life.
6. What happens if a fontanelle closes too early?
If a fontanelle closes too early, it may indicate craniosynostosis, a condition where skull sutures fuse prematurely. This can:
- Restrict normal brain growth
- Cause abnormal head shape
- Increase intracranial pressure in severe cases
7. What does a sunken fontanelle indicate?
A sunken fontanelle commonly indicates dehydration in an infant. It may occur when the baby has insufficient fluid due to vomiting, diarrhea, or poor feeding. Medical evaluation is recommended if the soft spot appears noticeably depressed.
8. What does a bulging fontanelle mean?
A bulging fontanelle may indicate increased intracranial pressure. Possible causes include infection (such as meningitis), hydrocephalus, or brain swelling. A temporarily bulging fontanelle can also occur when a baby cries, but persistent bulging requires urgent medical attention.
9. How many fontanelles does a baby have?
A newborn baby has six fontanelles in total. These include:
- One anterior fontanelle
- One posterior fontanelle
- Two sphenoidal (anterolateral) fontanelles
- Two mastoid (posterolateral) fontanelles
10. What is the difference between a fontanelle and a suture?
A fontanelle is a soft membranous gap between skull bones, while a suture is a fibrous joint connecting adjacent cranial bones.
- Fontanelles are wider spaces present in infants.
- Sutures are narrow fibrous joints that persist into adulthood.
- Fontanelles eventually close as sutures fuse through ossification.
