Skeletal/cranial development and axial skeleton

The skeletal system is made up of:

• 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.

Ossification

• desmogenous - bone tissue are created directly from mesenchyme,
• chondrogenic – a cartilaginous model of bone is created first.

Skull

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

 For more information see 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.