Although the π terms in and have slight derivation differences and biological assumptions, the resulting models are functionally similar. Consider a cell type A that is formed by the differentiation of precursor cells pA. This differentiation process is activated by signaling factor X 1 and inhibited by signaling factor Y 1 . Apoptosis of A is activated by signaling factor X 2 and inhibited by signaling factor Y 2 . The general form for describing bone cell dynamics following the mass action kinetics approach is