Osteoclasts are cells which degrade bone, undermining its structure, creating the porosity and loss of bone strength called osteoporosis.

Osteoblasts, on the other hand, are bone-building cells. As we age, the number of osteoblasts decreases and osteoclasts increase in number. This is especially true of post-menopausal women, as a decline in estrogen levels has been shown to lead to an increase in osteoclasts.