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Health and Disease

The Human Skull: Adaptations of and Changes to the Foetal Skull

By Aryan Bhadra

Published 2:41 EST, Sat November 20th, 2021

Introduction

The Human Skull (commonly referred to as the Cranium) is a protective bone structure located around the brain. At first glance, it would seem the skull is simply a protective structure- however as we begin to look more closely at the Skull in foetuses’ and babies, and in older humans, we can begin to understand its changing purpose. In this article, we will be discovering how the seemingly simple protective bone structure has undergone a great journey of change.

Skull at Birth

At birth, all of the major bones of the skull have formed, however they are distinctly separated by connective tissues called the sutures and fontanelles. These tissues serve two purposes at birth. The first ensures that during delivery the skull can change shape(due to flexibility provided by sutures) to enable it to pass through the relatively narrow uterus. If the foetal skull were to not have this connective tissues-it would be extremely difficult to deliver a baby the traditional way and even if delivered- would result in immediate injury to the skull of the baby and uterus of the mother. The second function is to host growth of the brain during the first few months of a baby’s life, and more rapid growth in later years. This ensures that the child does not develop brain damage by the brain growing faster than the skull, causing continuous intracranial pressure on the cerebrum.

Looking more closely at the sutures and fontanelles at birth- the diagram below (Fig 1) shows the superior view of the skull. We can distinctly observe how the frontal bones and parietal bones are separated by the anterior fontanelles and posterior fontanelles respectively. This links back to our 2 previous functions -ensuring the growth of the brain and easier passing of the foetus down the birth canal.

Fig 1. Labeled superior view of the skull at birth. Relevant labels link back to functions of fontanelles and sutures between the bones. Image by Stanford Children’s Health.

Skull at Infancy

For the first18months as a baby learns basic movements, such as twisting the head, holding up its own head, rolling over, and sitting up. However, due to the obvious lack of developed motor skills- these movements are often spontaneous and uncontrolled. This arises an immediate risk of injury, hence the fontanelles and sutures that are in place during birth remain until about 18months. This ensures that there is sufficient protection from these minor impacts from learning movements. As a result, the skull does not change significantly during the first 18 months of a child’s life- maintaining the flexibility to facilitate growth and protect from minor injuries.

However, by the age of 2, the anterior fontanelle begins to close, reducing flexibility between the two frontal bones for the infant. The anterior fontanelle is the largest of the connective tissues, but also one of the most clinically significant tissues. It provides information about the state of health of babies- specifically to do with hydration and intracranial pressure. As a result once at the age of 2, the clinically and structurally significant structure within the skull disappears, removing a source of clinical information and cranial protection.

The Changes to the Mandible

We now shift our attention away from the changing tissue structure of the skull, and toward the changes to the Mandible.

The Mandible is known as the lower jaw and is the largest bone in the lower skull, responsible for supporting the lower teeth and holding them in place. It is an extremely strong bone and is capable of movement (unlike the upper jaw). The Mandible undergoes a series of changes from birth to old age- which we will explore below.

Fig 2. Gradual changes of the shape of the Mandible through the ages. Image by True Value Dental Institute- Mandible Development and Age changes.

At birth, the Mandible consists of 2 sections of bone, which become joined within 1-2days. The whole body of the Mandible elongates but there is a focus on the growth of the area behind the mental foramen. This is an area responsible for providing space for the growth of the first few teeth. Moreover, there is an increase in depth of the body of the bone in order to facilitate increased room for roots of the teeth. Hence in childhood, we see the growth of specific areas of the Mandible, mostly down to the growth of teeth and their roots in early ages.

At birth, the angle of the mandible is found to be obtuse at 140 degrees or more, whereas in adults it is at an angle of 110-120 degrees due to the presence of a full set of teeth. This naturally raises the volume and hence raises the chief part of the bone just above the oblique line. In old age the mandible returns to an obtuse angle due to the loss of teeth, therefore the chief part of the bone is below the oblique line, and the angle measures at about 140 degrees. Angles of the mandible is very significant in terms of skeletal maturity. Its changes are used by dentists very often to reach a conclusion about dental health and the overall development of the Mandible

Finally, the Mandible goes through great changes in height and length (as seen before in changes due to initial growth of teeth). We shall focus on the changes in old age in this context. As someone gets older they begin to lose their teeth, with the primary culprits for this being receding gums and deterioration of the jawbone. As the teeth are lost in old age- the alveolar process becomes absorbed. The alveolar process is a bone that hosts the tooth sockets on bones which facilitate the teeth ( Fig 3). As this bone becomes absorbed, the mandible reduces in height. Reduction in mandible height has various implications- the most notable of these being a reduction of jaw mobility and a distinct change in facial aesthetic.

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Fig 3. Detailed diagram showcasing the position of the Alveolar Process relative to a tooth. Image by Jaypee Digital- Textbook of Periodontics.

Summary

In this article, we have discovered that the human skull is not just a large bone surrounding the brain- but an intricate, dynamic structure with numerous adaptations and changes. Ranging from fontanelles and sutures to the versatile Mandible and alveolar process- the skull is packed with different functions and can be classified as the most important bone in our body- responsible for our safety, speech, and facial aesthetic through the ages.

Aryan Bhadra, Youth Medical Journal 2021

References

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https://www.health.harvard.edu/diseases-and-conditions/the-aging-mouth-and-how-to-keep-it-younger

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https://en.wikipedia.org/wiki/Mandible

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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4877539/

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https://medlineplus.gov/ency/article/002320.htm

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