Determine the expresions of the boundaries of cells in CAD

Hello,

I am learning to use Cylinrical Algebraic Decomposition (CAD) available through CylindricalDecomoposition[]. For instance,

CylindricalDecomposition[
 g - (u^4 + x1 u^2 + x2 u + 1) == 0, {x1, x2, g, u}]

The result provides regions where the function has a constant sign, but I wonder if there is a way to extract polynomials that represent the cells’ boundaries.

I am new to this. I hope this is not a silly question.
Regards
Jesus