Development of the axial skeleton
Definitions & Key takeaways
The axial skeleton consists of the bones that run along the body's central axis - from the head to the tail, and it includes the skull, spine, and rib cage. The axial skeleton begins to develop very early in embryonic development, soon after gastrulation, meaning the period when the trilaminar disc with ectoderm, mesoderm, and endoderm layers is formed. Most axial skeleton bones develop from the mesoderm layer, except for the skull, which develops 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.
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.
These flat bones surround the brain - you can think of a protective helmet: anteriorly, there’s the frontal bones, above - the parietal bones, posteriorly, a part of the parietal bones and the occipital bone, and laterally, a special little spot where the parietal, occipital, and temporal bones meet - like three old friends.
Then there’s the cartilaginous neurocranium – or the chondrocranium, which forms through endochondral ossification and gives rise to the base of the skull – or the bony floor underneath the brain.
The chondrocranium accounts for the rest of the occipital and the temporal bones, as well as the sphenoid and ethmoid bone.
The viscerocranium arises from the first pharyngeal arches - which are the first of five paired bars of mesoderm that arch towards one another along the midline of the embryo.
These arches have a core of mesenchymal cells which originate as neural crest cells. As far as facial bones go, the first pharyngeal arch forms the upper and lower jawbone, as well as the cheekbones and the temples of the skull.
Just to go back to our helmet analogy - the viscerocranium would make up the faceguard protecting our mouth and nose from damage.
And this faceguard is slightly curved, to remind you that the bones come from the first pharyngeal arch. When a baby’s born, the skull bones are not fused and it looks a bit like a jigsaw puzzle where the pieces haven’t quite snapped together.
Between the bone plates, there are long, narrow, fibrous sutures that hold the skull together. Wide openings called fontanelles exist at the points where more than two bones come into contact: there’s 6 of them in total, 2 on either side, and two larger ones on top - the anterior fontanelle, between the parietal and frontal bones, and the posterior fontanelle between the occipital and parietal bones.
These sutures and fontanelles provide the skull with a tiny amount of flexibility, which makes it easier for the large head to squeeze through the vaginal canal during childbirth.
After the baby is born, the fontanelles close up one by one - the last one to close is the anterior fontanelle, and it’s often referred to as the soft spot on a baby’s head.
It closes at around 18 months. During week 4 of development the spinal vertebrae develop, and it happens when the somites undergo a process called resegmentation.
First, each somite separates in two populations of cells: a rostral, or top population, and a caudal, or bottom group. Next, these two populations of cell separate and the bottom population of one somite fuses with the top population of the ensuing somite.
This gives rise to new segments of tissue formed by cell populations deriving from two separate, adjacent somites. The sclerotome cells in these segments surround the notochord and spinal cord they transform into mesenchymal cells and give rise to the vertebrae through endochondral ossification.
As the vertebrae take shape, the neural tube develops into the spinal cord, from which spinal nerves emerge through the spaces between the vertebrae.
In front of the spinal cord, however, the bodies of the vertebrae grow and make the notochord degenerate for the most part.
However, between the vertebrae, the notochord persists as the nucleus pulposus of the intervertebral discs. During week 5, small clusters of mesenchymal cells called costal processes emerge from the lateral sides of each vertebrae.
In the thoracic region, these processes grow really long and form the ribs. Finally, the sternum develops from the parietal mesoderm layer in the anterior body wall of the embryo.
Here, the parietal mesoderm organizes as a pair of vertically-oriented cartilaginous bars on either side of the midline, which eventually fuse and differentiate into the three parts of the sternum: the broad manubrium at the top, the main body of the sternum, and the small xiphoid process at the bottom.
All right, as a quick recap. The axial skeleton arises from mesodermal and neural crest cells, which turn into mesenchymal cells.
Mesenchymal cells then give rise to bone, either through endochondral ossification, or through intramembranous ossification.
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