THE SKELETON By the end of the session, you should be able to: Identify different bone cells. Explain the microscopic structure of bone.

Components of Bone Bone is made up of a cells and extracellular matrix. The matrix is composed of organic and inorganic matter plus water. Bone cells are spread out within the matrix.

Bone Cells Bone tissue contains 4 types of cells: Osteoprogenitor cells Osteoblasts Osteocytes Osteoclasts

Osteoprogenitor Cells These cells undergo cell division and develop into osteoblasts. They are found lining the periosteum, endosteum and in canals in bone where blood vessels penetrate.

Osteoblasts These are bone building cells. They are incapable of cell division. Osteoblasts work in groups and secrete the organic components of bone, chiefly collagen and other proteins. This organic component of bone is called osteoid.

Osteoblasts In order to strengthen bone, inorganic matter in the form of mineral salts is added to the spaces between the collagen fibres. These crystallise and bone hardens. The main bone mineral is hydroxyapatite, which is made up of calcium phosphate.

Hydroxyapatite provides strength to the bone and helps to resist compression. Collagen fibres give bone tensile strength – this is resistance to being stretched or torn apart. The arrangement of collagen fibres and mineral salts in bone has been likened to reinforced concrete, where the collagen acts like the steel rods and the hydroxyapatite like the concrete. After mineral salts have crystallised and the bone has hardened, osteoblasts become trapped in the matrix they created and are then referred to as osteocytes.

Osteocytes These are mature bone cells that are trapped within the matrix of bone. They are the most abundant and longest living bone cell. They appear flattened and have cellular extensions (dendrites) to communicate with other bone cells. Their dendrites are in direct contact with neighbouring cells and small molecules can pass from one cell to another.

Functions of Osteocytes Osteocytes can detect micro-damage of bone. They release proteins that inhibit/stimulate/other bone cells – osteoblasts and osteoclasts. These in turn participate in bone remodelling and repair. Osteocytes also help to maintain mineral balance.

Osteoclasts These are large, multi-nucleated cells that are derived from monocytes. Osteoclasts settle on the surface of bone and secrete acids that dissolve bone minerals and enzymes that break down collagen and other proteins. The cell membrane in contact with the bone surface has many in- foldings and appears ruffled. Osteoclasts play an important role in development, growth, maintenance and repair of bone.

Build bone Destroy bone Maintain bone

Overview of Bone Cells

Types of Bone From our previous session, so you recall the 2 types of bone and where they are located in a long bone?

Compact Bone Can also be referred to as dense or cortical bone. Made up of units called osteons. Osteons have rings of concentric lamellae that surround a central Haversian canal. Haversian canals contain blood vessels, lymphatic vessels and nerves.

Compact Bone Osteocytes are located between the lamellae. They are found in fluid-filled spaces called lacunae. Their extensions (dendrites), extend into canaliculi. The canaliculi branch radially from lacunae. Canaliculi allow the transfer of oxygen, nutrients and wastes to and from the trapped osteocytes.

In addition to Haversian canals, compact bone contains perforating Volkmann’s canals. These allow vessels to traverse from the periosteum to Haversian canals, spongy bone/the medullary cavity.

Bone Remodelling