Bone development
Development of bone (known as ossification) proceeds by the laying down of dense connective tissue and its replacement by a trabecular network forming a primary spongiosum. The two main forms of growth are intramembrous and endochondral. Intramembrous formation occurs in the embryo, and is concerned with flat 'membranous' bones . Here, undifferentiated mesenchyme condenses to form vascularised connective tissue. Cells deposit extracellular material and simultaneously become osteoblasts, which further continue bone development.
Most bones, including the vertebrae and the long bones of the limbs, form by endochondral ossification. A 'primary centre of ossification' develops within a template or precursor structure made of cartilage. A new periosteal bone collar develops. Blood vessels invade, increasing oxygen availability. Osteoprogenitors invade and differentiate in to osteoblasts; these deposit bone tissue on calcified cartilage. The centre grows and extends towards the ends of the bones. However it never reaches the ends but stops short at epiphyseal growth plates. Beyond these, secondary centres of ossification develop and a similar process happens. Secondary centres allow the bone to continue to grow in length throughout childhood. Plate closure denotes the end of bone growth and occurs in adults (though at different times for different bones). If these plates close too early, achondroplasia results - with severely underdeveloped bones. This can be corrected at diagnosis with the distribution of growth hormones.
Remodeling is the process of resorption followed by replacement of bone with little change in shape and occurs throughout a persons life. Its purpose is the release of calcium and the repair of micro-damaged bones (from everyday stress).
Bones can be connected to muscles via ligaments.
The science of the interaction of bone and muscle is called biomechanics. The science of bones is called osteology.
Some illnesses afflict human bones, for example osteoporosis and cancer. The joints can be affected by arthritis.
Cut and polished bone from a variety of animals is sometimes used as material for jewelry and other crafts.
Terminology
