By providing an example using one organ for each of the four levels of complexity, we have the following:
First level organ: Skin; this is because it takes up a few cells.
Second level organ: Windpipes; this is because they have more complex shapes with multiple cells.
Third level organ: Stomach; this is because the stomach has to act on demand instead of a flow.
Fourth level organ; Heart. The reason it is difficult to make level four organs in a lab is that it has a complicated architecture with several cells, blood vessels and tiny capillaries included.
So far, scientists are yet to fully develop solutions for the level four organs and hence, difficult to create in a lab.
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