Bone loss can also be seen when bone formation rates are low, even though bone resorption rates are normal. Factors that affect BMU origination rates Origination is affected by mechanical loading, microdamage, or factors in the marrow. A host of hormones and cytokines which have attracted the attention of molecular biologists probably exert most of their influence at this step. Some of these depend on estrogen, which is why menopause is associated with an increase in origination of BMUs. The osteoclastic activity can be enhanced by estrogen deficiency, acidosis, corticosteroids and vitamin D metabolites.
Osteoclastic activity is reduced by calcitonin and bisphosphonates.
Once the osteoblasts have deposited a collagen matrix, it must be mineralized. This activity is also controlled by the osteoblasts. If mineralization is inhibited, patients will develop osteomalacia, which can mimic osteoporosis. This is seen with inadequate levels of calcium or phosphate, or with toxins, and is discussed later in this chapter. This will result in increased bone density even though the bone is still porous.