Development of the skull

Embryological background

Lateral plate mesoderm found in the neck region, paraxial mesoderm and neural crest cells all contribute to the development and existence of the skull in its entirety.

The bones of the skull are formed in two different ways; intramembranous ossification and endochondral ossification are responsible for creating compact cortical bone or spongy bone. During the maturation of the skull, it is categorically divided into two main parts: the viscerocranium and the neurocranium. These two terms account for the bones of the face and the bones of both the cranial base and the cranial vault respectively. The cranial vault is further divided into membranous neurocranium and cartilaginous neurocranium.

Viscerocranium

The first part of the skull to be discussed is the viscerocranium or bones of the face. Below, they are grouped according to their derivative germ layer, their origins, their adult location and their way of ossifying.

All the bones in this category are derived from cells of the neural crest. Also, all of the following structures emerge from the first and second pharyngeal arches.

• The maxillary process from the first pharyngeal arch contributes to the formation of the maxilla, temporal bone, zygoma (zygomatic bone and arch), the palatine bone, lacrimal bone, nasal bone, vomer and the inferior nasal concha. All of these bones ossify via intramembranous ossification, with exception of the inferior nasal concha, which uses endochondral ossification.
• The mandibular process which also emerges from the first pharyngeal arch produces the mandible via both intramembranous and endochondral ossification, the sphenomandibular ligament which is an exception as it is never ossified and finally the malleus and incus which are both ossified in an endochondral manner.

Meanwhile, the second pharyngeal arch produces the styloid process, the stapes and the hyoid bone, all of which are ossified endochondrally. The final structure is not ossified at all and is known as the stylohyoid ligament.

Fontanelles

The second and last item worth mentioning about the viscerocranium is the skull fontanelles. These are the growth areas of the skull that are present since before birth up until certain ages in young children. Here is a list of the location of these areas and the time period in a child’s life in which they close:

• The anterior fontanelle known as bregma closes between four and twenty six months. It is a diamond-shaped space located between the paired frontal and parietal bones of the fetal/neonatal skull (also described as the junction of the coronal and sagittal sutures).
• The posterior fontanelle known as lambda closes much earlier than the anterior, at only one to two months after birth. It is located where the two parietal bones meet the occipital bone (also described as the junction of the sagittal and lambdoid sutures).
• The sphenoidal fontanelle (also known as the anterolateral fontanelle) closes between two and three months and is called pterion. It is located at the junction of the sphenoid, parietal, temporal, and frontal bones.
• The final fontanelle, which closes at twelve to eighteen months, takes its name from the area in which it is situated, the mastoid fontanelle, otherwise known as the asterion fontanelle. It is located at the junciton of the temporal, occipital, and parietal bones.

Neurocranium

As previously mentioned, the neurocranium is further separated into the membranous neurocranium and the cartilaginous neurocranium. The structures within these subgroups are arranged according to the germ layer from which they arose, the area of the neurocranium in which they are situated, the adult structure they eventually become and finally the way in which they were ossified.

• The membranous neurocranium consists of neural crest cells and paraxial mesoderm which form the main portion of the roof and lateral sides of the neurocranium. The neural crest cells create the adult frontal bone and squamous portion of the temporal bone. The paraxial mesoderm produces the intraparietal portion of the occipital bone and the parietal bone itself. All the structures undergo intramembranous ossification.
• The cartilaginous neurocranium is also made up of paraxial mesoderm and the cells of the neural crest. The neural crest maintains the development of the prechordal neurocranium anterior to the sella turcica. Meanwhile, the paraxial mesoderm is responsible for the growth of the chordal neurocranium, behind the sella turcica. The prechordal and chordal neurocranium mature into the ethmoid and sphenoid bones as well as the petrous and mastoid portion of the temporal bone and the occipital bone respectively. They all ossify via endochondral means.