Bones and Organization of the Skull: Structure, Development, and Clinical Implications 13.0

Bones and Organization of the Skull

Overview of the Adult Skull

The human skull is a complex structure composed of cranial and facial bones, as well as associated bones and joints. It protects the brain, supports sensory organs, and provides attachment points for muscles.

• 8 Cranial bones: Form the cranium, encasing the brain.

• 14 Facial bones: Shape the face and support structures such as the nose and mouth.

• Associated bones: Hyoid bone (in the neck, not directly attached to other bones), and ossicles (6 small bones in the middle ear).

• Joints: Most skull bones are joined by sutures (immovable fibrous joints). The temporomandibular joint (TMJ) is the only diarthrosis (freely movable joint) in the skull.

• Teeth articulate with the bones of the jaw via dentoalveolar joints.

Facial Bones

Nasal Bones

These paired bones form the bridge of the nose and provide support for cartilage that shapes the lower portion of the nose.

Lacrimal Bones

Located in the medial wall of the orbit, these paired bones contain the lacrimal fossa, which allows passage of tears into the nasal cavity.

Inferior Nasal Conchae

Paired bones within the nasal cavity that form the lateral walls and help filter and humidify inhaled air.

Zygomatic Bones

Paired bones that form the cheekbones and the lateral wall of the orbit. They articulate with the temporal bone to form the zygomatic arch, an attachment site for chewing muscles.

Vomer Bone

Forms the inferior portion of the nasal septum, dividing the nasal cavity into right and left sides.

Maxillary Bones

Paired bones that make up the upper jaw, the inferior wall of the orbit, lateral walls of the nasal cavity, and the hard palate. They contain the upper teeth and articulate with the palatine bones.

Palatine Bones

Paired bones located on the inferior aspect of the skull, forming part of the hard palate and separating the nasal and oral cavities.

Mandible

The single bone forming the lower jaw, containing the lower teeth. It features the coronoid process (insertion for temporalis muscle), mandibular and mental foramina (passage for nerves), and a variable mandibular angle.

Joints of the Skull

Sutures

Sutures are immovable fibrous joints that connect most skull bones. Major sutures include:

• Coronal suture: Between frontal and parietal bones.

• Sagittal suture: Between the two parietal bones.

• Lambdoid suture: Between parietal and occipital bones.

• Squamous suture: Between parietal and temporal bones.

Temporomandibular Joint (TMJ)

The TMJ is a synovial joint between the mandibular condyle and the mandibular fossa of the temporal bone, allowing movement for chewing and speaking.

Dentoalveolar Joints

These are gomphosis joints where teeth are anchored into the alveolar processes of the maxilla and mandible.

Cranial Bones

Frontal Bone

Forms the anterior portion of the cranium and the superior part of the orbit. Articulates with parietal bones at the coronal suture and includes space for the ethmoid bone.

Parietal Bones

Paired bones forming the superior portion of the cranium. Articulate with:

• Frontal bone (coronal suture)

• Each other (sagittal suture)

• Occipital bone (lambdoid suture)

• Temporal bone (squamous suture)

Occipital Bone

Forms the posterior and inferior portions of the cranium. Features the foramen magnum (exit for spinal cord), occipital condyles (atlanto-occipital joint), and articulates with the temporal bone at the occipitomastoid suture.

Inca Bones (Sutural Bones)

Triangular bones found between the parietal and occipital bones at the lambdoid suture. Their size and shape vary among individuals.

Temporal Bones

Paired bones forming the lateral portion of the cranium. Consist of:

• Squamous portion: Articulates with the zygomatic bone.

• Petrous portion: Contains mastoid and styloid processes, internal and external acoustic meatus, jugular foramen, and carotid canal.

Ossicles (Middle Ear Bones)

The three smallest bones in the body (malleus, incus, stapes) are located in the petrous portion of the temporal bone and transmit sound waves from the eardrum to the inner ear.

Ethmoid Bone

Sits within the frontal bone, forming the medial portion of the orbit and lateral walls of the nasal cavity. Key structures include:

• Crista galli: Attaches brain meninges.

• Cribriform plates: Hold olfactory bulbs.

• Perpendicular plate: Forms the superior portion of the nasal septum.

Sphenoid Bone

A complex bone forming the middle of the cranium. Articulates with temporal, occipital, frontal, ethmoid, and parietal bones. Contains several foramina for cranial nerves and blood vessels, and the sella turcica (pituitary gland site).

• Pterygoid plates: Attachment for chewing muscles.

Bones and Structures of the Orbit

The orbit is a bony cavity housing the eye and is formed by several bones, including the frontal, sphenoid, ethmoid, lacrimal, maxilla, zygomatic, and palatine bones. It contains multiple foramina and fissures for the passage of cranial nerves and blood vessels.

Development of the Skull

Intramembranous Ossification

Forms the superior cranial bones and facial bones. Bone develops directly from mesenchymal tissue, beginning at the center and proceeding outward. Remaining tissue between bones forms fontanels, which later become sutures.

Endochondral Ossification

Forms the inferior cranial bones, creating a stable base to support the brain. Bone develops by replacing cartilage models.

Fontanels

Membranous gaps between cranial bones in infants, allowing for brain growth. Fontanels fuse to form sutures around age 2.

Clinical Implications

Cleft Palate

Occurs when bones and tissues of the lip or hard palate do not fuse correctly, requiring surgical intervention.

Skull Deformities

• Positional skull deformities: Caused by birth trauma or insufficient movement.

• Craniosynostosis: Premature fusion of cranial bones, affecting brain growth.

• Cleidocranial Dysplasia: Delayed closure of fontanels, cleft palate, and extra teeth.

Summary Table: Major Bones of the Skull

Bone	Type	Key Features	Articulations
Frontal	Cranial	Forehead, superior orbit	Parietal (coronal suture)
Parietal	Cranial	Superior cranium	Frontal, occipital, temporal, other parietal
Occipital	Cranial	Posterior/inferior cranium, foramen magnum	Parietal, temporal
Temporal	Cranial	Lateral cranium, mastoid/styloid processes	Parietal, occipital, zygomatic
Sphenoid	Cranial	Sella turcica, pterygoid plates	Frontal, parietal, temporal, occipital, ethmoid
Ethmoid	Cranial	Cribriform plate, perpendicular plate	Frontal, sphenoid, nasal, maxilla
Maxilla	Facial	Upper jaw, hard palate	Palatine, zygomatic, nasal, lacrimal
Mandible	Facial	Lower jaw, mental/mandibular foramen	Temporal (TMJ)
Zygomatic	Facial	Cheekbone, zygomatic arch	Temporal, maxilla
Nasal	Facial	Bridge of nose	Maxilla, ethmoid
Lacrimal	Facial	Lacrimal fossa	Maxilla, ethmoid
Palatine	Facial	Hard palate	Maxilla, sphenoid
Vomer	Facial	Inferior nasal septum	Ethmoid, maxilla, palatine
Inferior Nasal Concha	Facial	Lateral nasal cavity	Maxilla, ethmoid
Hyoid	Associated	U-shaped, supports tongue	None (does not articulate)
Malleus, Incus, Stapes	Associated	Middle ear, sound transmission	Temporal bone (petrous portion)

Key Terms and Definitions

• Suture: Immovable joint between skull bones.

• Fontanel: Soft membranous gap between cranial bones in infants.

• Diarthrosis: Freely movable joint (e.g., TMJ).

• Gomphosis: Peg-in-socket joint (e.g., teeth in jaw).

• Foramen: Opening in bone for nerves and vessels.

• Process: Projection from a bone for muscle attachment.

Summary of Skull Development

• Intramembranous ossification: Direct bone formation from mesenchyme; forms flat bones of the skull and clavicle.

• Endochondral ossification: Bone formation from cartilage models; forms base of the skull.

• Fontanels: Allow for brain growth; fuse to form sutures by age 2.

Clinical Relevance

• Cleft palate: Failure of palatine processes to fuse, leading to oral-nasal communication.

• Craniosynostosis: Premature suture closure, restricting brain growth.

• Cleidocranial dysplasia: Delayed fontanel closure, extra teeth, and cleft palate.

Example Application

Understanding the anatomy and development of the skull is essential for diagnosing and treating craniofacial abnormalities, planning surgical interventions, and interpreting radiological images.

Additional info: The notes include clinical implications and developmental biology, which are crucial for understanding congenital and acquired skull disorders.