The formation of a human is extraordinary as it requires such a high level of organisation and accuracy to coordinate trillions of cells to create the same form time and time again. So how do we do it? In this article we look to provide you with a simplified overview of our current understanding of this extraordinary process.
There are 3 basic components to organised growth:
- Genes – these provide the basic plan for our development and drive cellular division.
- Cellular movement – this is a consequence of cellular division and thereby creates forces which mould the shape of our overall structure as well as stimulating surrounding cells.
- Chemical signals – these are released during stimulation and alter the function of the affected cell as well as the cells around them.
Let us delve in the detail of this process starting with the genes. These provide the basic information of what we are. They are the building plans of our form. And like all building plans they are modified as the project progresses. There is a greater degree of modification in the creation of a living being, as opposed to a house (for example) because in a living being everything is in a constant state of change. The cells (new or old) are moving, the surrounding fluid (amniotic fluid) is moving, the uterus is moving, etc. When building a house everything is still yet even then the completed article looks different to the initial plan. A further complexity is that cells that are being added are created from the existing cells. In the case of a brick wall the last placed brick does not generate the next brick.
It is this replication which generates the bulk of the movement and in so doing creates pulling, pushing, stretching and suction forces. It is this cellular movement which shapes the structure. For example, if the bone cells of an arm are replicating faster than the cartilage cells immediately in front of it, what will happen? The cartilage cells will get flattened and be forced to grow sideways. Hence why the bones have a flattened cartilage lining at the end of them.
The forces placed on the cells will not only influence their direction of movement but also what is chemically happening inside of it. Inside a cell we have many thousands of chemical reactions occurring. When the outer wall is deformed by a pushing force for example, it will change these reactions. The change may cause:
- The cell to change its function, i.e. turning from a bone making cell to a tendon making cell or
- The cell will release chemicals into the surrounding space.
These chemicals are called signalling molecules and they are absorbed by the surrounding cells. Which you guess it, affect their internal reactions. In this way, the cellular motion alters the direction of construction in the area. In a similar way to how a brick layer will signal to the plasterer that he is to take over. In our our bone-cartilage example as the bone cells push into the cartilage cells it causes them to flatten. The flattening force alters the internal reaction. Some of the signalling molecules would then activate a different part of the gene turning the cell into a synovial fluid producing cell. In addition some signalling molecules would be released into the outside and be absorbed by the surrounding bone making cells. This would affect their internal reactions in such a way that they stop replicating. Hence the bone stop growing, has a thin cartilage layer on its end and at the periphery are synovial producing fluid cells which lubricate the joint.
Through this complex interaction of gene expression, cellular motion and chemical signalling we replicate a human form time and time again.