The beginning

The fetal skeletal system develops from mesenchymal cells and the neural ridge.

By the end of the third week of gestation, the paraxial mesoderm columns are segmented into mesodermal blocks called somites.

The cells of the neural crest migrate to the pharyngeal arches and form the bones and connective tissue of the craniofacial structures. Occipital somites and somtomers also help form the cranial vault and the base of the skull.

The somatic leaf of the lateral mesoderm intervenes in the formation of the bones of the extremities, the sternum and the pelvic and scapular waists.

Bones initially appear as condensations of mesenchymal cells that will constitute bone models. This condensation will delimit the beginning of the selective gene activity that precedes cell differentiation.

The flat bones develop in the mesenchyme inside pre-existing membranous sheaths, the mesenchymal models of the bones of the extremities are transformed into cartilage bone models, which will then carry out an endochondral ossification.

The cartilage appears during the fifth week. It arises from the condensation of mesenchyma, cartilage-forming cells, secrete collagen fibrils and the fundamental substance of the matrix. These fibers are then deposited in the matrix. Cartilage is divided into three types:

• Hyaline cartilage: wider distribution (e.g. in joints)
• Fibrocartilage (as in the intervertebral discs)
• Elastic cartilage (in the auditory pavilion)

Joints begin with the appearance of interzonal mesenchyma during the sixth week. Three types of joints are formed:

-fibrous: in which the interzonal mesenchyma is replaced by fibrous tissue (cranial sutures).
-Cartilaginous joints: where the interzonal mesenchyma is replaced by cartilage.

• synovial joints (such as in the knee).