Making the Thymus

When we look at the complicated organs in our bodies it’s natural to wonder how they could have arrived bit by bit. Working out how this might have happened is like trying to unravel the most complicated of puzzles – but that doesn’t mean it’s impossible.

The thymus is an organ located high up in your chest and between your lungs. Its main job is to produce the immune system employees, T lymphocytes. After building up quite a stock during our more youthful years it gradually diminishes as we get older. So how did it evolve?

Some new research has looked at this question.

The thymus first appears around 500 million years ago in primitive jawed vertebrates. Lymphocytes had been around previously but T lymphocytes needed new technology and the ability to adapt the immune response.

The development of the thymus from the tissues of the developing embryo needs the expression of the gene Foxn1. This gene also plays a part in the gathering of the cells that become T lymphocytes and in the process that leads to their specialisation. So it’s a key piece of the jigsaw. This new study looked at living examples of the animal groups in question to try and piece together the genetic development. They found that Foxn1 is first found in cartilaginous fish (and then in all jawed vertebrates) but it doesn’t just appear out of the blue. It has predecessors. Foxn4 and Foxn4b are more ancient forms. The diagram above shows the relationship in different groups and you can see how the family tree builds up. The paper goes into much more detail but you get the idea how detective biologists can work to solve what are really quite complicated problems.