Each arch has components of ectoderm, mesoderm, endoderm, and neural crest. These arches form muscles, cartilaginous and osseous structures, nerves, blood vessels, and various organs of the head and neck. Five significant pharyngeal arches form in humans, starting rostral to caudal around days 19 to 21 of gestation. Neural crest cells form the rest of the neurocranium: the frontal bone, ethmoid bone, sphenoid bone, and squamous portion of the temporal bone, as well as the entirety of the viscerocranium. The sclerotome develops into the mesodermal portions of the skull (parietal bones, occipital bone, and petrous portion of the temporal bone). These cells then proliferate in a longitudinal fashion adjacent to the notochord (paraxial mesoderm) and eventually divide into various early connective tissue populations, including the sclerotome and myotome. Mesoderm begins to form in the third week of gestation after early mesenchymal cells have migrated through the primitive streak. Ossification and structural molding begin in the fetal phase (week seven onward). The skull develops alongside the rapid growth of the nervous system in the embryonic phase of development (weeks 1 to 8). The temporal bones derive from both the mesoderm and neural crest. The frontal, ethmoid, and sphenoid bones derive from the neural crest, while the parietal and occipital bones originate from the mesoderm. The critical anatomic landmarks of each fossa are listed below.Įmbryologically, the skull derives from ectodermal neural crest and mesoderm. Finally, the posterior cranial fossa forms from the occipital bone and two temporal bones. The middle cranial fossa forms from the sphenoid bone and two temporal bones. The anterior cranial fossa forms from the frontal bone, the sphenoid bone, and the ethmoid bone. There are three cranial fossae with various structural landmarks. The occipital bone is the most posterior aspect of the skull. The mastoid is a bony prominence that lies posterior to the auricle and has an associated sinus. The petrous portion houses the inner ear. The temporal bones subdivide into petrous, squamous, zygomatic, and mastoid parts. The bregma is the junction of the coronal and sagittal sutures, and lambda is the junction of the lambdoid and sagittal sutures. The frontal sinuses lie deep to the brow ridges. It lies superior to the nasion and between the superciliary ridges. The glabella is a key midline landmark of the frontal bone.
It is composed of the sphenoid and ethmoid bones (which have their associated air sinuses) and parts of the frontal, temporal, and occipital bones.Īnteriorly, the frontal bone forms the superior aspect of the orbits. Finally, the skull base allows the passage of various neurovascular structures. The asterion is the articulation of the parietal, temporal, and occipital bones. The pterion is the articulation of the frontal, parietal, temporal, and sphenoid bones just superior to the pinna. The sagittal suture lies along an anterior-posterior axis and is the articulation of the two parietal bones. The parietal bones articulate with the temporal bones inferiorly via the squamosal sutures and the occipital bone posteriorly via the lambdoid suture. The coronal suture is the transverse mid-anterior junction of the frontal bone and the two parietal bones. It is composed of the frontal bone, parietal bones, temporal bones, and occipital bone. The calvaria, the uppermost part of the skull, protects the cerebral cortex, cerebellum, and orbital contents. In addition, it supports the muscles of the face and scalp by providing muscular and tendinous attachments, protects neurovascular structures, and houses various sinuses to accommodate increases in pressure. The skull will harden and fuse through development to protect its inner contents: the cerebrum, cerebellum, brainstem, and orbits. The function of the skull is both structurally supportive and protective. The skull consists of 22 bones in most adult specimens, which come together via cranial sutures.
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