Bones in our body are living tissue. They have their own blood vessels and are made of living cells, which help them to grow and to repair themselves. As well, proteins, minerals and vitamins make up the bone.
We are born with about 300 soft bones. During childhood and adolescence, the cartilage grows and is slowly replaced by hard bone. Some of these bones later fuse together, so that the adult skeleton has 206 bones.
The major functions of bones are to:
- Provide structural support for the body
- Provide protection of vital organs
- Provide an environment for marrow (where blood cells are produced)
- Act as a storage area for minerals (such as calcium)
Bones are composed of two types of tissue:
1. A hard outer layer called cortical (compact) bone, which is strong, dense and tough.
2. A spongy inner layer called trabecular (cancellous) bone. This network of trabeculae is lighter and less dense than compact bone.
Bone is also composed of:
- Bone forming cells (osteoblasts and osteocytes)
- Bone resorbing cells (osteoclasts)
- Nonmineral matrix of collagen and noncollagenous proteins (osteoid)
- Inorganic mineral salts deposited within the matrix
*figures adapted from http://www.servier.com/slidekit/
Cells in our bones are responsible for bone production, maintenance and modeling:
- Osteoblasts: These cells are derived from mesenchymal stem cells and are responsible for bone matrix synthesis and its subsequent mineralization. In the adult skeleton, the majority of bone surfaces that are not undergoing formation or resorption (i.e. not being remodeled) are lined by bone lining cells.
- Osteocytes: These cells are osteoblasts that become incorporated within the newly formed osteoid, which eventually becomes calcified bone. Osteocytes situated deep in bone matrix maintain contact with newly incorporated osteocytes in osteoid, and with osteoblasts and bone lining cells on the bone surfaces, through an extensive network of cell processes (canaliculi). They are thought to be ideally situated to respond to changes in physical forces upon bone and to transduce messages to cells on the bone surface, directing them to initiate resorption or formation responses.
- Osteoclasts: These cells are large multinucleated cells, like macrophages, derived from the hematopoietic lineage. Osteoclasts function in the resorption of mineralized tissue and are found attached to the bone surface at sites of active bone resorption. Their characteristic feature is a ruffled edge where active resorption takes place with the secretion of bone-resorbing enzymes, which digest bone matrix.
Osteoid is comprised of type I collagen (~94%) and noncollagenous proteins. The hardness and rigidity of bone is due to the presence of mineral salt in the osteoid matrix, which is a crystalline complex of calcium and phosphate (hydroxyapatite). Calcified bone contains about 25% organic matrix (2-5% of which are cells), 5% water and 70% inorganic mineral (hydroxyapatite).
Types of bone
Two types of bone can be identified according to the pattern of collagen forming the osteoid:
- Woven bone is characterized by a haphazard organization of collagen fibers and is mechanically weak.
- Lamellar bone is characterized by a regular parallel alignment of collagen into sheets (lamellae) and is mechanically strong.
Woven bone is produced when osteoblasts produce osteoid rapidly. This occurs initially in all fetal bones, but the resulting woven bone is replaced by remodeling and the deposition of more resilient lamellar bone. In adults, woven bone is formed when there is very rapid new bone formation, as occurs in the repair of a fracture. Following a fracture, woven bone is remodeled and lamellar bone is deposited. Virtually all bone in the healthy mature adult is lamellar bone.
Bone development and growth
Osteogenesis (bone tissue formation) occurs by two processes:
- Intramembranous ossification involves the replacement of connective tissue membrane sheets with bone tissue and results in the formation of flat bones (e.g. skull, clavicle, mandible).
- Endochondral ossification involves the replacement of a hyaline cartilage model with bone tissue (e.g. femur, tibia, humerus, radius).
Long bones continue to grow in length and width throughout childhood and adolescence. Increase in length is due to continued endochondral bone formation at each end of the long bones. Increase in circumference of the bone shaft is achieved by formation of new bone on the outer surface of the cortical bone.
Modeling is when bone resorption and bone formation occur on separate surfaces (i.e. formation and resorption are not coupled). An example of this process is during long bone increases in length and diameter. Bone modeling occurs during birth to adulthood and is responsible for gain in skeletal mass and changes in skeletal form.
Remodeling is the replacement of old tissue by new bone tissue. This mainly occurs in the adult skeleton to maintain bone mass. This process involves the coupling of bone formation and bone resorption and consists of five phases:
1. Activation: preosteoclasts are stimulated and differentiate under the influence of cytokines and growth factors into mature active osteoclasts
2. Resorption: osteoclasts digest mineral matrix (old bone)
3. Reversal: end of resorption
4. Formation: osteoblasts synthesize new bone matrix
5. Quiescence: osteoblasts become resting bone lining cells on the newly formed bone surface
- What is Osteoporosis?
- Who's at Risk?
- Living with Osteoporosis