[Development of the skeletal system in utero].

The skeletal system develops from mesenchyme originated from the mesodermal germ layer and neural crest. In view of developmental bone biology, the skeletal system can be divided into four parts : skull, limbs, vertebrae/the ventral column, and ribs/sternum. Bone formation takes place in two ways. In most bones including axial (vertebral column and ribs) and appendicular (limbs) skeletons, a cartilage model first forms and is finally replaced with bone, which is called endochondral ossification. In contrast, most flat bones, such as the majority of bones of the skull, form directly from mesenchymal cells without the prior formation of cartilage ; this type of osteogenesis is called intramembranous ossification. I here discuss development of the skeletal system focusing on its time line in utero.