Skeletal/cranial development and axial skeleton

The skeletal system is made up of: Ossification
 * paraaxial mesoderm - from the sclerotome → mesenchyme (at the end of the 4th week) - mesenchyme cells capable of migration → fibroblasts, chondroblasts, osteoblasts,
 * mesoderm of the lateral plate - skeleton of the limbs,
 * neural crest cells - part of the skull skeleton.
 * desmogenous - bone tissue are created directly from mesenchyme,
 * chondrogenic – a cartilaginous model of bone is created first.

Neurocranium
It surrounds the brain and sense organs,
 * division according to the method of ossification:
 * desmocranium,
 * neural crest cells + paraaxial mesoderm → mesenchyme surrounds the brain → spicules (islets of bone tissue) form in the mesenchyme → spicules expand radially,
 * chondrocranium,
 * each bone consists of several cartilaginous foundations → fusion → individual bones,
 * connections between bones → growth centers of the skull base,
 * prechordal chondrocranium = bones that are derivatives of the neural crest (in front of the sella turcica),
 * os ethmoidale, ala minor ossis sphenoidalis, ala major ossis sphenoidalis,
 * chordal chondrocranium = bones originating from occipital sclerotomes,
 * corpus ossis sphenoidalis, bassis ossis occipitalis, os petrosum.

Splanchnocranium
The facial part of the skull,
 * from gill arches:
 * 1st gill arch:
 * maxillary valva → premaxilla, maxilla, os zygomaticum, os palatinum, incus,
 * mandibular valance → Meckel's cartilage (the mandible desmogenously ossifies around it), malleus, lig. sphenomandibular,
 * 2nd gill arch → stapes, proc. styloid, lig. stylohyoideum, part of os hyoideum,
 * 3rd gill arch → rest of lingual.

Newborn skull

 * Bones separated by fibrous bands → they form sutures,
 * at the point of contact of several bones there are fibrous membranes = fontanelles,
 * the largest is the fonticulus anterior,
 * these fibrous structures allow the skull to change shape during birth.

Vertebrae

 * 4th week – sclerotome cells travel to the periphery of the spinal cord and chordae → connection with the bilateral sclerotome,
 * resegmentation – the caudal half of the sclerotome of each somite joins the cranial half of the next somite,
 * myotomes do not divide → muscles begin and attach to adjacent vertebrae → movement of the spine is enabled,
 * the chorda dorsalis forms the nucleus pulposus of the intervertebral discs.

Ribs and Sternum

 * Ribs – arising by lengthening the processus costales of the thoracic vertebrae → originate from sclerotomes,
 * sternum – from the mesenchyme of the somatopleura, from a paired cartilaginous model.