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.

Meanwhile, lateral plate mesoderm splits into parietal mesoderm and visceral mesoderm layers.

The parietal mesoderm forms the early limb buds, the bones of the pelvic and shoulder girdle, and the sternum, while the visceral mesoderm helps form organs like the heart, lungs, and organs in the gastrointestinal tract.

So, the axial skeleton derives mainly from paraxial and lateral plate mesoderm cells.

But, in the head region, another group of cells derived from the ectoderm layer, called neural crest cells, contributes to the development of the skull.

Before they can develop into bone, all these different kinds of cells first transform into multipotent mesenchymal cells, through a process called epithelial to mesenchymal transition.

The resulting mesenchymal cells have special properties, such as the ability to migrate to different locations and give rise to different organs and tissues in our body - including bones.

Now, from here on, there’s two ways that fetal bones can form.

First, there’s endochondral ossification, in which case mesenchymal cells first differentiate into chondrocytes that build a hyaline cartilage model which then turns into bone.

When that happens, the center of this cartilage model is the primary ossification center, and blood vessels enter it, bringing in nutrients and osteoblast cells which help build bone - B for build, as well as osteoclast cells that collapse bone, C for collapse.

The osteoblasts replace the chondrocytes at the primary ossification center and start to replace the cartilage with bone.

As the bones grow thicker and more sturdy, osteoclasts start to chomp away in the middle of the bone, making it more porous - and this is how bone marrow appears.

Most of the bones in our body form through endochondral ossification, except for the clavicles, and bones in the skull like the parietal and frontal bones, as well as the maxilla, mandible, the nasal bone and parts of the temporal and occipital bones.

These exceptions form through intramembranous ossification, which is when mesenchymal cells differentiate into osteoblast cells which create the primary ossification center and start building bone without any cartilage model.

As before, blood vessels reach the center of the primary ossification center, which already has osteoblasts, and bring in nutrients.

Okay, now let’s start at the very top and take a look at the development of the skull.

The skull has two main parts: the neurocranium, which is the hard shell protecting the brain, and the viscerocranium, which makes up the structures underlying the face.

The neurocranium is itself divided into two parts: First, there’s the membranous neurocranium, which forms through intramembranous ossification, and makes up the flat bones of the skull.