Development of the axial skeleton

The fetal skeleton starts developing soon after gastrulation, which is when the trilaminar disc with ectoderm, mesoderm and endoderm layers are formed.

There are two parts to the skeleton - the axial skeleton, which includes the bones in the skull, the vertebrae, the rib cage, and the sternum, and the appendicular skeleton, comprising of the pelvic and shoulder girdle, as well as the bones in the limbs.

The bones in the axial skeleton mostly derive from the mesoderm layer, except for some bones in the skull which come from the ectoderm.

All the bones in the appendicular skeleton derive from the mesoderm.

During week 3, the embryo transitions from a flat organism to a more tubular creature, by folding along its longitudinal and lateral axes.

At the same time, a solid rod of mesoderm called the notochord forms on the midline of the embryo.

Above the notochord, the ectoderm invaginates to form the neural tube - an early precursor for the central nervous system.

This is the embryo’s first symmetry axis, and the mesoderm on either side of the neural tube differentiate in 3 distinct portions: immediately flanking the neural tube, there’s the paraxial mesoderm.

Next, there’s the intermediate mesoderm, and finally, the lateral plate mesoderm.

The intermediate mesoderm gives rise to the urinary and genital systems, while the paraxial mesoderm and lateral plate mesoderm work together to give rise to most of bones and muscles in our body.

The first step in skeletal development is when paraxial mesoderm segments into blocks of mesodermal tissue called somites, which are made up of lots of cube-shaped cells.

Next, the somites divide into three different cell populations: the sclerotome, which forms the vertebrae, the rib cage, and the lower part of the occipital bone, the dermatome, which forms the skin of the back, and the myotome, which forms the back, limb and intercostal muscles.