Bone Structure and Composition
- Bone is composed of a flexible matrix and bound minerals.
- The matrix is mostly elastic collagen fibers.
- Inorganic mineral salt, calcium phosphate, gives bones rigidity.
- Bones are constructed and remodeled by osteoblasts and osteoclasts.
- Cortical bone is denser and forms the hard exterior of bones.
Cortical Bone
- Cortical bone is the hard outer layer of bones.
- It accounts for 80% of the total bone mass.
- It supports the body, protects organs, and provides levers for movement.
- Cortical bone consists of microscopic columns called osteons.
- It is covered by a periosteum on the outer surface and an endosteum on the inner surface.
Trabecular Bone
- Trabecular bone is the internal tissue of skeletal bones.
- It is less dense and more flexible than cortical bone.
- Trabecular bone has a higher surface-area-to-volume ratio.
- It is found at the ends of long bones, near joints, and in the interior of vertebrae.
- Trabecular bone contains bone marrow and hematopoietic stem cells.
Bone Marrow
- Bone marrow is found in bones that hold cancellous tissue.
- In newborns, all such bones are filled with red marrow.
- As a child ages, the hematopoietic fraction decreases and marrow adipose tissue increases.
- In adults, red marrow is mostly found in the femur, ribs, vertebrae, and pelvic bones.
- Marrow contains hematopoietic stem cells and gives rise to blood cells.
Vascular Supply and Bone Cells
- Bone receives about 10% of cardiac output.
- Blood enters the endosteum and flows through the marrow.
- Blood oxygen tension in bone marrow is lower compared to arterial blood.
- Vascular supply in bone is important for nutrient and oxygen delivery.
- Small vessels in the cortex allow blood to exit the bone.
- Osteoblasts are mononucleate bone-forming cells.
- Osteoblasts synthesize osteoid, a protein mixture that mineralizes to become bone.
- Osteocytes are cells of mesenchymal origin that are trapped and surrounded by a bone matrix.
- Osteocytes communicate with other bone cells through gap junctions.
- Osteoclasts are large multinucleate cells responsible for bone resorption.
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and enable mobility. Bones come in a variety of shapes and sizes and have complex internal and external structures. They are lightweight yet strong and hard and serve multiple functions.
| Bone | |
|---|---|
 A bone dating from the Pleistocene Ice Age of an extinct species of elephant | |
 A scanning electronic micrograph of bone at 10,000× magnification | |
| Identifiers | |
| MeSH | D001842 |
| TA98 | A02.0.00.000 |
| TA2 | 366, 377 |
| TH | H3.01.00.0.00001 |
| FMA | 5018 |
| Anatomical terminology | |
Bone tissue (osseous tissue), which is also called bone in the uncountable sense of that word, is hard tissue, a type of specialised connective tissue. It has a honeycomb-like matrix internally, which helps to give the bone rigidity. Bone tissue is made up of different types of bone cells. Osteoblasts and osteocytes are involved in the formation and mineralisation of bone; osteoclasts are involved in the resorption of bone tissue. Modified (flattened) osteoblasts become the lining cells that form a protective layer on the bone surface. The mineralised matrix of bone tissue has an organic component of mainly collagen called ossein and an inorganic component of bone mineral made up of various salts. Bone tissue is mineralised tissue of two types, cortical bone and cancellous bone. Other types of tissue found in bones include bone marrow, endosteum, periosteum, nerves, blood vessels and cartilage.
In the human body at birth, there are approximately 300 bones present; many of these fuse together during development, leaving a total of 206 separate bones in the adult, not counting numerous small sesamoid bones. The largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear.
The Greek word for bone is ὀστέον ("osteon"), hence the many terms that use it as a prefix—such as osteopathy. In anatomical terminology, including the Terminologia Anatomica international standard, the word for a bone is os (for example, os breve, os longum, os sesamoideum).
