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SKULL

Morphogenesis of the skull is divided into two parts: the neurocranium, which encloses the brain, and the viscerocranium, which supports the face, pharynx and larynx. The skull first appears as mesenchymal condensations which later either form membranes that ossify or first become cartilage that secondarily ossifies.

NEUROCRANIUM

The cartilaginous portion of the neurocranium (chondrocranium) forms the base of the skull. It is represented first by the following mesenchymal collections:

On each side of the cranial segment of the notochord constituting the parachordal condensation that blends caudally with the occipital sclerotomes.

Around the otocyst forming the otic capsule condensation.

Around the nasal pit forming the nasal capsule condensation.

VISCEROCRANIUM

The viscerocranium develops primarily from the branchial arch cartilages. These cartilages are represented first by mesenchymal condensations in the following regions:

First arch cartilage (Meckel’s)—medial to the mandibular nerve.

Second arch cartilage (Reichert’s)—medial to the facial nerve.

Laryngeal condensation—surrounding the primitive larynx and representing the caudal branchial arch cartilages.

POSTCRANIAL PART OF AXIAL SKELETON

The sclerotomes in each segment join around the notochord ventral to the neural tube.

Each sclerotome is composed of a caudal condensed part and a cranial less condensed part. The condensed part takes a position in the middle of each body segment with a spinal nerve on each side. The less condensed part is in the intersegmental position at the level of the dorsal intersegmental artery.

The sclerotomes from the occipital somites lose their segmentation and blend together around the notochord ventral to the myelencephalon. They fuse with the caudal part of the parachordal condensation and contribute to the formation of the occipital bone.

Caudal to the occipital region, cells from the cranial part of each sclerotome extend dorsally between the spinal ganglion and the myotome forming the neural arch.

Dense cellular condensations that develop from the twelve thoracic sclerotomes extend ventrolaterally as rib primordia.

Source: Atlas of Human Embryos.