system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: How do bones and other organs know what shapes to grown into?
gpt
This topic may be too complicated for ELI5.  There are a lot of genes that produce growth factors, and many of these genes are expressed only at specific times and in specific places.  The location and timing are controlled by other genes making other transcription factors or inhibitors or whatever, also at very specific times and places.

Sometimes growth depends not just on a growth factor being produced somewhere, but on that factor diffusing outward through nearby tissues, producing a concentration gradient.  The gradient gives a sense of direction to the tissue growth.

Sometimes the shapes of growing organs are affected by the shapes of everything growing around them.  Sometimes the growth of one tissue layer is faster than another, which causes them both to fold or bulge or take other shapes.  Sometimes even the stretching and straining of tissues being pulled around by other growing parts triggers these tissues to make their own growth factors.

This is tangential to your question, but sometimes the shape doesn't even come from growing, it comes from dying.  Fingers work that way - your hand starts developing almost as a flipper, and the cells in between where your fingers should be undergo programmed cell death.