Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support and protect the various organs of the body, produce red and white blood cells, and store minerals. Bone tissue is a type of dense connective tissue. Although bone may appear superficially as a static tissue, it is actually very dynamic, undergoing constant remodeling throughout the life of the vertebrate organism and well beyond in initial formation of the bone. This occurs with the synchronized action of osteoclasts and osteoblasts, cells that resorb and deposit bone, respectively. Bone remodeling also occurs in response to trauma, such as that following accidental bone fracture or the placement of dental implants.
Formation
The formation of bone during the fetal stage of development occurs by two processes: intramembranous ossification and endochondral ossification.
Intramembranous ossification mainly occurs during the formation of the flat bones of the skull, but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as mesenchyme tissue rather than from cartilage. The steps in intramembranous ossification are:
- Development of ossification center
- Calcification
- Formation of trabeculae
- Development of periosteum
Endochondral ossification begins with points in the cartilage called "primary ossification centers. " They mostly appear during fetal development, though a few short bones begin their primary ossification after birth. They are responsible for the formation of the diaphyses of long bones, short bones, and certain parts of irregular bones.
Secondary ossification occurs after birth, and forms the epiphyses of long bones and the extremities of irregular and flat bones. The diaphysis and both epiphyses of a long bone are separated by a growing zone of cartilage (the epiphyseal plate). When the child reaches skeletal maturity (18 to 25 years of age), all of the cartilage is replaced by bone, fusing the diaphysis and both epiphyses together (epiphyseal closure).
Remodeling
Remodeling or bone turnover is the process of resorption followed by replacement of bone with little change in shape, and occurs throughout a person's life; it continues to occur beyond the initial development of bone. Osteoblasts and osteoclasts, coupled together via paracrine cell signalling, are referred to as bone remodeling unit. Approximately 10% of the skeletal mass of an adult is remodeled each year.
The bone remodeling period consists of the duration of the resorption, the osteoclastic reversal (the phase marked by shifting of resorption processes into formative processes), and the formation periods of bone growth and development. The bone remodeling period refers to the average total duration of a single cycle of bone remodeling at any point on a bone surface.
The purpose of remodeling is to regulate calcium homeostasis and repair micro-damaged bones (from everyday stress), but also to shape and sculpt the skeleton during growth. Repeated stress, such as weight-bearing exercise or bone healing, results in the bone thickening at the points of maximum stress (Wolff's law).
Osteoclasts and Osteoblasts
Bone tissue is removed by osteoclasts, and then new bone tissue is formed by osteoblasts. Both processes utilize cytokine (TGF-β, IGF) signalling.