What major physiological system's development is heavily dependent on the full gravitational load ($1g$) of Earth?

Answer

Skeletal structure

Human physiology is intricately tuned to $1g$, and the development of the entire biological system, including the skeletal structure, is profoundly affected by the absence of full gravitational load.

What major physiological system's development is heavily dependent on the full gravitational load ($1g$) of Earth?
reproductionMarsSpacehumanbirth