What causes the skull to grow? A guide to craniofacial development

The human skull is a marvel of biological engineering, starting not as a single piece but as multiple plates connected by flexible joints. The single most significant factor in this process is the rapid expansion of the brain, a key component in understanding what causes the skull to grow.

Quick Summary

Rapid infant brain expansion and the growth at flexible joints called sutures drive early skull enlargement. Although overall growth largely ceases in adulthood, subtle changes occur due to bone remodeling and aging, with some rare medical conditions causing true pathological growth later in life.

Key Points

• Brain Growth is the Primary Driver: Rapid infant brain expansion creates internal pressure that signals the skull's bony plates to grow at the sutures.

• Sutures and Fontanelles are Key: The fibrous joints (sutures) and soft spots (fontanelles) in an infant's skull provide the flexibility needed for growth and passaging through the birth canal.

• Two Types of Ossification: The skull's flat bones form directly via intramembranous ossification, while the cranial base uses cartilage replacement in endochondral ossification.

• Growth Stops in Adulthood: For most people, significant skull growth ceases once the sutures fuse in early adulthood, around 18-25 years of age.

• Subtle Changes Still Occur: Lifelong bone remodeling and hormonal shifts can cause subtle changes to the skull and facial bone structure, especially the jaw and eye sockets.

• Abnormal Causes of Enlargement: Rare conditions like acromegaly (excess growth hormone) and Paget's disease can cause pathological bone thickening and enlargement in adults.

The Dynamic Skull: More Than a Hard Shell

Contrary to a common misconception, the skull is not one solid piece of bone from birth. The human cranium is a complex structure composed of several separate plates of bone that remain flexible throughout infancy and childhood. This design is crucial for two primary reasons: to allow the baby's head to safely navigate the birth canal and to accommodate the tremendous growth of the brain in the early years of life. The skull's journey from multiple flexible pieces to a fused, rigid structure is a fascinating process orchestrated by internal pressures and hormonal signals.

The Role of Intramembranous Ossification

Most of the flat bones that form the cranial vault—the top and sides of the skull—develop through a process called intramembranous ossification. This is a direct process of bone formation, without a preceding cartilage model. Here is a simplified breakdown:

1. Mesenchymal Cell Condensation: First, specialized stem cells, known as mesenchymal cells, gather in clusters within the fibrous connective tissue where the bone will form.
2. Differentiation and Matrix Secretion: These cells differentiate into osteoblasts, the bone-building cells. The osteoblasts begin to secrete a collagen-rich, unmineralized substance called osteoid.
3. Calcification: Calcium and other mineral salts accumulate around the osteoid, causing it to harden and form bone tissue. The osteoblasts that become trapped in this new bone matrix mature into osteocytes.
4. Formation of Spongy Bone: As ossification centers grow and merge, they form a network of spongy, or cancellous, bone called trabeculae.
5. Formation of Compact Bone: Blood vessels and connective tissue fill the spaces in the spongy bone, and further layers of compact bone are laid down on the outer surfaces. The periosteum, a membrane covering the bone's exterior, is also formed.

The Importance of Cranial Sutures

In newborns, the flat bones of the skull are separated by soft, fibrous tissue joints called sutures. Where several sutures meet, there are larger soft spots known as fontanelles. These features are vital to answering the question of what causes the skull to grow, particularly during early life.

• Accommodating Brain Expansion: The primary driver of skull expansion is the growth of the brain. The soft sutures and fontanelles allow the cranial bones to move and grow apart as the brain rapidly increases in size. The dura mater, the tough membrane covering the brain, also signals the sutures to produce new bone at their edges.
• Directional Growth: The pattern of sutural growth determines the ultimate shape and size of the head. For example, growth at the sagittal suture (running down the middle of the skull) allows for an increase in the skull's width, while growth at the coronal suture (separating the frontal and parietal bones) affects length.
• Timed Fusion: Over time, these sutures and fontanelles gradually close and fuse completely in a process called synostosis. The posterior fontanelle typically closes within the first few months, while the larger anterior fontanelle takes around 18-24 months. Other sutures continue to slowly fuse into early adulthood.

Cranial Changes Beyond Childhood

Once the cranial sutures have fused, the skull's overall size is largely fixed. However, this does not mean the skull is completely static. Bone is a living tissue that undergoes continuous remodeling, a process of resorption (breakdown by osteoclasts) and formation (by osteoblasts) throughout life. This renewal process maintains bone health and can lead to subtle changes.

In adulthood, several factors can influence the skull's appearance or cause actual, though usually minor, changes:

• Hormonal Influences: Puberty, driven by sex hormones and growth hormone (GH), significantly impacts the skeleton. For example, the jaw and brow ridges may become more prominent. While overall skull size is stable, hormonal fluctuations and imbalances in adulthood can influence facial bone structure. Conditions involving excess GH, such as acromegaly, can cause significant thickening of facial bones, hands, and feet, even long after normal growth has stopped.
• Aging and Bone Remodeling: As we age, the process of bone remodeling can become unbalanced, leading to changes in density and shape. Studies have shown subtle but statistically significant changes in skull and facial bone morphology, with eye sockets potentially enlarging and jaw angles changing. These changes contribute to the overall appearance of an aging face, including the perception of a changing head shape.
• Jaw and Dental Changes: The health and presence of teeth heavily influence the underlying jawbone. Tooth loss and subsequent bone density reduction in the mandible can alter the facial structure and change proportions.

Normal vs. Pathological Skull Growth

Here is a comparison of typical skull development and various abnormal conditions that can cause skull growth.

Feature	Normal Postnatal Growth	Craniosynostosis (Pathological)	Acromegaly (Pathological)	Paget's Disease (Pathological)
Cause	Primarily brain expansion signaling sutures to add new bone.	One or more cranial sutures fuse prematurely.	Excessive growth hormone (GH) production, usually from a pituitary tumor.	Disruption of normal bone remodeling, leading to rapid, disorganized bone formation.
Timing	Rapidly in infancy and childhood, stabilizing by early adulthood.	Before or shortly after birth, impacting early childhood.	Gradual onset in adulthood, developing over years.	Typically affects adults over 50 years old.
Mechanism	Compensatory growth at patent (open) sutures.	Fusion of sutures prevents growth in certain directions, forcing compensatory growth in others, leading to a misshapen head.	Thickening of bones and soft tissues, particularly in the face, hands, and feet.	Enlargement and misshaping of bones, which can affect the skull, spine, pelvis, and long bones.
Impact	Allows brain development and head shape normalization.	Can lead to increased intracranial pressure, developmental issues, and cosmetic deformities if untreated.	Characterized by a more prominent jaw, thickened brow ridge, and enlarged nose.	Symptoms include headaches, hearing loss (if cranial nerves are compressed), and potential risk of tumors.

Conclusion: A Lifetime of Bone Health

The most significant period of skull growth is during early childhood, driven by the expanding brain and the flexibility of the cranial sutures. Once these sutures fuse, true structural growth largely ceases. However, the skull and facial bones continue to undergo a lifelong process of remodeling, influenced by hormones and the natural aging process. Understanding these mechanisms helps differentiate between normal, subtle changes and pathological conditions that cause significant bone enlargement. For comprehensive information on bone health, consult resources like the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) here. For seniors, in particular, understanding the ongoing process of bone maintenance and factors that can lead to problems like Paget's disease is a crucial part of proactive health management.

Frequently Asked Questions

Does the skull grow larger in adulthood?

Significant overall skull growth stops when the cranial sutures fuse, typically between 18 and 25 years of age. However, bones undergo continuous, subtle remodeling throughout life, and some rare medical conditions can cause pathological enlargement.

What is the difference between skull growth in a baby versus an adult?

In babies, growth is rapid and driven by brain expansion, with new bone added at the flexible sutures. In adults, the sutures are fused, so there is no increase in overall cranial size, only subtle reshaping through constant bone remodeling.

Can genetics affect the shape and size of a person's skull?

Yes, genetics play a significant role. Studies have identified specific gene variations that influence head shape and size, contributing to the wide diversity seen in the general population.

What happens if a baby's skull sutures close too early?

This condition is called craniosynostosis. If a suture closes prematurely, the skull cannot expand normally in that area, forcing it to grow abnormally in other directions. This can lead to a misshapen head and, if severe, increased pressure on the brain.

How does aging change the appearance of the head and face?

Aging can cause changes in facial bone structure, such as larger eye sockets and altered jaw angles. These bony changes, along with the shifting of soft tissues, can make the head appear to be a different shape or size over time, even though the underlying cranial vault has not grown.

Can acromegaly cause the skull to grow?

Yes, acromegaly is a condition caused by excess growth hormone that can cause bones, particularly facial bones, to thicken and enlarge in adulthood. This can lead to a more prominent jaw, brow ridge, and larger features.

Is it normal to have a larger head circumference than average?

It can be normal for some individuals. A naturally large head size, often called benign familial macrocephaly, can run in families and is generally harmless. A doctor would use growth charts to determine if head circumference is within a normal range or requires further investigation.