Osteology of the Skull by DR. KENNEDY MUNISI, MD, MMed, MSc. PhD..pptx

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1. Osteology of the Skull: Introduction Osteology of the skull entails a number of component bones that constitute the formation of the human skull. What is the embryogenic background of the human skull?!

2. Osteology of the Skull: Embryogenesis Cells Source (s) Fate Mesenchyme/ mesenchymal cells Paraxial mesoderm ( somites & somitomeres ) the floor of the brain case a small portion of the occipital region, all voluntary muscles of the craniofacial region, the dermis and connective tissues in the dorsal region of the head, and the meninges caudal to the prosencephalon Lateral plate mesoderm The laryngeal cartilages (arytenoid and cricoid) and connective tissue in this region. Neural crest* form midfacial and pharyngeal arch skeletal structures all other tissues in these regions, including cartilage, bone, dentin, tendon, dermis, pia and arachnoid, sensory neurons, and glandular stroma Ectodermal placodes form neurons of the 5 th , 7 th , 9 th & 10 th cranial sensory ganglia + NC

3. continues Cell 1* Source Migration *Neural crest Neuroectoderm of; forebrain, midbrain, and hindbrain regions Ventrally into the pharyngeal arches and Rostrally around the forebrain and optic cup into the facial region

4. continues The skull begins to form prior to 12 th week of embryogenesis; A cartilaginous state begins to grow and is slowly replaced by bone in a process called intramembranous ossification This process occurs at different rates in two different regions of the skull; Calvarium (roof) : the frontal, occipital and parietal bones tend to grow slightly earlier Base : the sphenoid, ethmoid, occipital and temporal bones tend to grow slightly later By week 37, the cartilaginous frame for the skull is almost entirely replaced by bone, and only two relatively small cartilaginous structures remain: Anterior fontanelle : the soft region on the front of the infant’s head Posterior fontanelle : the soft region on the back of the infant’s head

5. continues The fontanelles undergo complete ossification within 9 – 18 months from birth. Clinical relevance: Fontanelle abnormalities: An enlarged anterior fontanelle can be due to endocrine pathologies like congenital hypothyroidism. A bulging anterior or posterior fontanelle may suggest raised intracranial pressure, which may occur in hydrocephalus. A sunken anterior or posterior fontanelle may suggest dehydration.

6. Human skull: Introduction The human skull serves the vital function of protecting the brain from the outside world, as well as supplying a rigid base for muscles and soft tissue structures to attach to. The bones of the skull are held rigidly in place by fibrous sutures. There are two different regions of the skull; Calvarium (roof) Base

7. Bones of the calvarium continues The calvarium, also known as the roof or skull cap, consists of three bones: Frontal bones Parietal bones Occipital bones These bones protect the brain superiorly, but also provide an anchor for important muscles of facial expression and eye movement. The parts of these bones that lie inferior to the brain are considered to be a part of the skull base. These bones of the skull cap contain two thin layers of cortical bone separated by diploë , which is synonymous with the spongy bone seen in long bones. Diploë provides structural integrity and transmits forces along the length of the bones of the skull cap

8. Frontal bones The frontal bones start as two paired bones. During childhood and adolescence, these two bones fuse into one. The fusion of these frontal bones forms a metopic suture , which may be visible in some skull specimens. There are three important parts of the frontal bone to know: Orbital plate of the frontal bone : forms the roof of the orbital cavity and separates the frontal lobe of the brain from the eye. Glabella : the brow ridge that serves as an attachment point for the frontalis muscle and superior portion of the orbicularis oculi Supraorbital notch : nerves supplying the forehead to emerge through this aperture.

9. Parietal bones The two parietal bones join in the sagittal line to form the sagittal suture. They are each joined anteriorly to the frontal bone by the coronal suture. Overlying the parietal bones is a thick connective tissue layer known as the galea aponeurotica . The galea joins the frontalis muscle belly anteriorly to the occipitalis muscle belly posteriorly. This is why raising your eyebrows can create the appearance that the back of the head is moving.

10. Occipital Bone The occipital bone forms a cup for the occipital lobe of the brain to sit in. The occipital bone is joined superiorly to the parietal bones by the lambdoidal suture. It contains an external occipital protuberance that can be felt on the back of your head. The foramen magnum housing the brainstem, is also a part of the occipital bone. Either side of the foramen magnum are the hypoglossal canals for cranial nerve XII. The clivus is an incline in the occipital bone that the ventral pons and medulla rest on. Anteriorly, the occipital bone joins to the sphenoid and temporal bones.

11. Bones of the skull base The bones of the skull base include: Part of the frontal bone Part of the occipital bone Temporal bone Sphenoid bone Ethmoid bone

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13. continues Frontal bone: The orbital plate of the frontal bone is technically part of the skull base, as it lies below the brain. It forms the anterior cranial fossa for the frontal lobe. Occipital bone: The part of the occipital bone that is horizontal and creates the posterior cranial fossa for the occipital lobe is part of the skull base. The clivus is technically part of the skull base. The foramen magnum is considered part of the skull base Temporal bone: The temporal bone is a complex anatomical structure that transmits many veins, arteries and nerves into and out of the skull. The temporal bone consists of 4 main parts: squamous, petrous, tympanic & mastoid .

14. continues Parts of temporal bone and their features; Squamous Shaped like a fish scale (for which it is named) Contributes to the temporal fossa Petrous Pyramid or wedge-shaped Densest bone in the body Contains the internal auditory meatus for cranial nerves VII and VIII Tympanic Curved plate below the squamous portion Originates as a separate bone (the tympanic bone) which fuses with the temporal bone Contains the external auditory meatus Mastoid Named after its breast-like appearance Contains the mastoid and styloid processes

15. Sphenoid bone Literally translating to “wedge-like,” the sphenoid bone is named after its appearance. It is majorly referred to as being shaped like a butterfly, moth or bat with its wings spread. There are several important structures to know: Greater wings: form the floor of the middle cranial fossa for the temporal lobe to sit in Sella turcica: the “Turkish chair” that contains the pituitary gland Orbital surface: forming the posterior wall of the orbit

16. Ethmoid bone The ethmoid bone separates the nasal cavity from the brain. It forms part the perpendicular plate of the osseous nasal septum. It also contains small foramen that allow the olfactory nerve to move from within the skull to the top of the nasal cavity to sample air particles. This part of the ethmoid bone is called the cribriform plate

17. Bones of the face There are 14 individual bones of the face that support soft tissue structures to determine the appearance of the face. These bones fuse to form part of the orbital , nasal and oral cavities , as well as some of the sinuses of the face.

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19. continues Nasal bones Lie between the maxillae Form the top, osseous part of the nose Inferior nasal conchae Two bones in the nasal cavity that increase air turbulence and surface area for humidification Maxillae Form the upper jaw Lacrimal bones Forming the medial wall of the orbit House the lacrimal sac Zygomatic bone Form the inferolateral floor of the orbital cavity Form the cheekbones and zygomatic arch Palatine bones Posteriorly-located in the oral cavity The soft palate and uvula hang from the palatine bones

20. continues Vomer Form the lower part of the osseous nasal septum Mandible Form the lower jaw Joined to the temporal bone at the temporomandibular joint (TMJ)

21. Foramina of the skull The foramina (singular: foramen) of the skull are numerous and it is easy to get lost identifying them. Olfactory foramina Located in the ethmoid bone Transmits the olfactory nerves Optic canal Located in the sphenoid bone Transmits the optic nerve Superior orbital fissure Located in the sphenoid bone Transmits cranial nerves III, IV, V1, VI, and several vascular structures Foramen rotundum Located in the sphenoid bone Transmits V2

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23. continues Foramen ovale Located in the sphenoid bone Transmits V3 and portion minor (and several vascular structures) Foramen spinosum Located in the sphenoid bone Transmits the middle meningeal vessels Internal acoustic meatus Located in the temporal bone Transmits cranial nerves VII (within the skull bone) and VIII Stylomastoid foramen Located in the temporal bone Transmits cranial nerve VII (exit) Carotid canal Located in the temporal bone Transmits the internal carotid artery and plexus Jugular foramen Located in the temporal bone anteriorly and the occipital bone posteriorly Transmits the internal jugular vein and cranial nerves IX, X, XI

24. continues Foramen lacerum Located between the sphenoid, temporal and occipital bones Does not transmit anything normally Foramen magnum Located in the occipital bone Transmits the brainstem and cranial nerve XI into the skull Hypoglossal canal Located in the occipital bone Transmits cranial nerve XII

25. Sutures of the skull

26. Clinical anatomy Endocrine dysfunction: acromegaly and osteopetrosis It can lead to overgrowth of bones. It narrows the foramen responsible for nerve and blood vessel transmission. It Reduces visual acuity, and deafness can be consequences of such diseases. Skull fractures Regions of the skull where multiple bones join together can be relatively weak and predispose that region to fracture. One of the weakest points on the human skull is the ‘temple’ region, also known as the pterion . Pterion is formed by the junction of the frontal, parietal and sphenoid bones

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