Uncommitted monocytes and preosteoclasts are rarely modeled as dynamic populations (thus, pOCL is a constant in Equation ( )). Osteoclasts are assumed to differentiate from a large pool of hematopoietic stem cells, so the uncommitted population is usually modeled as a fixed quantity. Although this assumption is reasonable for healthy bone remodeling, it loses validity when studying diseases where hematopoietic stem cell numbers are reduced . Preosteoclasts are usually omitted for simplification under the assumption that remodeling is already occurring, i.e., the activation stage is assumed to occur instantaneously (Figure ). However, this neglects the time needed to initiate this remodeling stage.