The hard tissue component of the vertebrate skeleton. It is composed of two functionally important principal components physically blended together: an organic element (mainly collagen), 25% of the weight of the fully formed bone, and a mineral matrix (calcium, phosphate, and variable amounts of magnesium, sodium, carbonate, citrate, and fluoride), having a crystalline structure. It basically consists of many cylindrical units (osteons), each with a central canal, containing bone-forming cells (osteoblasts), blood vessels, and nerve filaments, surrounded by bony tissue. Long bones consist of a shaft (a hollow tube surrounded by compact bone) and the ends (a network of spongy bone). Growth in length occurs in the region between shaft and ends (the epiphyseal growth plate). Growth in width occurs by deposition of bone from the surface membrane (periosteum). When the growth plate disappears (between puberty and 25 years) growth in length ceases, as the shaft has fused with the ends. As bones grow, they increase in size and also change shape. Bone shape and dimensions are genetically determined, but are also influenced by hormonal, nutritional, mechanical, and neural factors. Calcium, phosphorus, and vitamins A, C, and D are all required for normal bone growth and development. Abnormal pressures on bone (from a tumour or aneurysm) may cause bone erosion. Bone deprived of a nerve supply and muscular paralysis results in poor bone development and atrophy.
For other uses, see Bone (disambiguation), including "Bones" which redirects here.
Functions
Long bones can be connected to skeletal muscles via tendons. The red bone marrow, located in the spongy bone matrix, allows for bones to produce red blood cells (hemopoiesis). Also, the bone marrow located at the bones diaphysis allow for white blood cells to be produced.
Structure
Bone is a relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxyapatite. Cortical bone accounts for 80% of the total bone mass of the adult skeleton. It has a relatively high surface area (about 10 times that of cortical bone) and it accounts for approximately 20% of the total bone mass. Lamellar bone forms only on calcified hyaline cartilage or bone. In contrast to woven bone, lamellar bone's collagen fibres, which are many, are oriented in parallel, accounting for its greater strength. Woven bone often forms initially and, as either growth or repair continues, it is replaced by lamellar bone. The central shaft of a long bone is called the diaphysis, and has a hollow middle—the medullary cavity filled with bone marrow. The extremities of the bone are called the epiphyses and are mostly cancellous bone covered by a relatively thin layer of compact bone. finger bones) have a similar structure to long bones, except that they are shorter. the skull and ribs) consist of two layers of compact bone with a zone of cancellous bone sandwiched between them.
Irregular bones are bones which do not conform to any of the previous forms (e.g.
All bones consist of living cells embedded in a mineralised organic matrix that makes up the main bone material.
Bone cells
Osteoblasts are typically viewed as bone forming cells.
Bone lining cells (BLCs) share a common lineage with osteogenic (bone forming) cells.
The process of bone resorption releases stored calcium into the systemic circulation and is an important process in regulating calcium balance.
Formation
The formation of bone during the fetal stage of development occurs by two methods: intramembranous and endochondral ossification. 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. In adults, red marrow is mostly found in the flat bones of the skull, the ribs, the vertebrae and pelvic bones.
Remodeling is the process of resorption followed by replacement of bone with little change in shape and occurs throughout a person's life. It has been hypothesized that this is a result of bone's piezoelectric properties, which cause bone to generate small electrical potentials under stress.
Bone pathologies
One of the most common bone illnesses is a bone fracture. Bones heal by natural processes, but untended and unsupported can lead to misgrown bone.
Exposed bone
Bone penetrating the skin and being exposed to the outside, apart from the effect of injury, occasionally is a normal feature:-
One very obvious case is deer's antlers.Terminology
Several terms are used to refer to features and components of bones:
| Process | A relatively large projection or prominent bump. |
| articulation | The region where adjacent bones contact each other—a joint. |
| articular process | A projection that contacts an adjacent bone. |
| eminence | A relatively small projection or bump. |
| tuberosity | A projection or bump with a roughened surface. |
| tubercle | A projection or bump with a roughened surface, generally smaller than a tuberosity. |
| trochanter | One of two specific tuberosities located on the femur. |
| spine | A relatively long, thin projection or bump. |
| suture | Articulation between cranial bones. |
| malleolus | One of two specific protuberances of bones in the ankle. |
| condyle | A large, rounded articular process. |
| epicondyle | A projection near to a condyle but not part of the joint. |
| line, ridge | A long, thin projection, often with a rough surface. |
| crest | A prominent ridge. |
| facet | A small, smooth articular surface. |
| foramen | An opening through a bone. |
| fossa | A broad, shallow depressed area. |
| canal | A long, tunnel-like foramen, usually a passage for notable nerves or blood vessels. |
| meatus | A short canal. |
| sinus | A cavity within a cranial bone. |
Several terms are used to refer to specific features of long bones:
| Diaphysis, shaft | The long, relatively straight main body of the bone; |
| epiphyses | The end regions of the bone; |
| epiphyseal plate, growth plate | The thin disc of hyaline cartilage between the diaphysis and epiphyses; |
| head | The proximal articular end of the bone. |
| neck | The region of bone between the head and the shaft. |
Several terms are used to describe the orientation of bones with respect to axes of the body, generally those anatomical terms of location, especially:
| Medial | The side of the bone toward the centre line of the body. |
| lateral | The side of the bone towards the outside line of the body. |
| proximal | The side or location of a bone toward the top of the skull. |
| distal | The side or location of a bone toward the bottom of the feet. |
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