Bone, one of the heaviest organs in the body, is formed essentially in two components: one organic, composed of cells, vessels and nerves, and the other mineralized matrix, composed mainly of a water solution of protein (collagen) that becomes heavily embedded with crystals of calcium, known as hydroxyapatite crystals. The three main types of cells in bone are osteoblasts, osteoclasts and osteocytes.
Osteoblasts produce the collagen and generate the matrix, which then is filled with hydroxyapatite – or calcium – crystals. The calcium is deposited in layers called lamellae, and these are crossed by minute canals known as Haversian canals or osteons. It is their densely packed arrangement that gives compact bone its great strength. During our life, our bone is also resorbed, or digested, by the osteoclasts, which break bone down, digesting the matrix.
Special multi-cellular bone units, which act like patrols and travel across the surfaces of our bones with the osteoclasts, digest small amounts of the matrix; these are then replaced by the newly formed bone from the osteoblasts, which in turn become trapped as osteocytes for a number of years. In this way, our complete skeleton is renewed every 7 to 10 years.
These units also repair small cracks and defects that may appear in the bone, and they are responsible for remodeling the bone after a fracture.
Bones And Joints For Life
During our early years, the deposition of bone far exceeds its resorbtion, resulting in growth and in mineralization of the existing bone. During adult-hood, most people will achieve a stable balance, with the net amount of bone remaining constant. As we age, however, the balance shirts towards resorbtion, causing a reduction in the amount of bone and a loss of mineralization of the existing stock. The extremes of this process are called osteoporosis and osteomalacia respectively.
The reason this process is particularly important to women is that with menopause, the abrupt reduction in estrogen levels results in an increased resorption of bone, which quickly outpaces the formation of new bone and can lead to osteoporosis.
An asymptomatic condition, osteoporosis causes no pain or any other known symptom until a fracture ensues. These fractures can cause serious loss of function in the wrist, severe chronic back pain and incapacity or great difficulty in walking and mobility, leading in many cases to death in later life.
In fact, the number of deaths attributed to hip fractures in older people is very similar to that of heart attacks and cancer, with women being affected about 10 times more often than men.