Bone

Bone is a special from of cartilage in which the collagen fibers are coated with a calcium phosphate salt. The great advantage of bone over chitin as a structural material is that it is strong without being brittle. To understand the properties of bone, first consider those of fiberglass. Fiberglass is composed of glass fibers embedded in epoxy glue. The individual fibers are rigid, giving great strenght, but they are brittle, like the cuticle of arthropods. The epoxy component of fiberglass, on the other hand, is flexible but weak. The composite, fiberglass, is both rigid and strong. When a fiber breaks because of stress and a crack stars to form, the crack runs into glue before it reaches another fiber. The glue distorts and reduces the concentration of the stress, and the adjacent fibers consequently are not exposed to the same high stress. In effect, the glue acts to spread the stress over many fiber.
The construction of bone is similar to that of fiberglass. Small, needle-shaped crystals of a calcium-containing mineral, hydroxyapatite, surround and impregnate collagen fibrils of bone. The fibrils are placed parallel to the axis of long bones and also parallel to the curved ends of bones in joints. As a result of the way these fiber are placed, no crack can penetrate far into bone without encountering a hard mass of hydroxyapatite crystals embedded in a collagenous matrix. Bone is more rigrd than collagen , just as fiberglass is more rigrd than epoxy. On the other hand, bone is more flexible and resistant to fracture than is hydroxyapatite-or chitin.
New bone is formed by cells called osteoblasts, which secrete the collagen fibers on which calcium is later deposited. Bone is laid down in thin, concentric layers called lamellae, like so many layers of paint on an old pipe. The lamellae are laid down a series of tubes around narrow channels called Haversian canals, which run parallel to the length of the bone. The Haversian canals are interconnected and contain blood vessels and nerve cells. The blood vessels provide a life line to living bone-forming cells, while the nerves control the diameter of the blood vessels and thus the flow through them. When bone is first formed in the embryo, osteoblasts use the cartilage skeleton as a template for bone formation. Later, new bone is formed along lines of stress.
Bone is formed in two stages: first, collagen is laid down in a matrix of fibrils along lines of stress, and then calcium minerals impregnate the fibrils. These minerals provide rigidity, while the collagen provides flexibility.
The bones of the vertebrate skeleton are composed of two kinds of tissue. The ends and interiors of long bones are composed of an open lattice of bone called spongy bone tissue, or marrow. Within this lattice framework, most of the body's red blood cells are formed. Surrounding the spongy bone tissue at the core of bones are concentric layers of compact bone tissue in which the collagen fibrils are laid down in a pattern that is far denser than the marrow. The compact bone tissue gives the bone the strength to withstand mechanical stress.