Composition of Bone
Bone is mainly composed of two components:
-The bone-matrix has Organic component-OSTEOID(35%) & Mineral component(65%).
-Embedded in the matrix are variety of cells.
-Osteoid is made up chiefly of type I collagen & smaller amounts of Glycosaminoglycans & other proteins.
-The hardness is provided by inorganic moiety hydroxyapatite (Ca10[PO4]6[OH]2).
-Bone matrix is produced in two histologic forms- Woven & Lamellar.
Woven bone– rapidly produced, e.g. fetal development, fracture repair; haphazardly arranged collagen fibres with less structural integrity.
Lamellar bone– slowly produced, parallel collagen fibres, e.g. a normal adult long bone.
Cells of Bone
Bone contains three major types of specialized cells embedded within the bone matrix: Osteoblasts, Osteocytes & Osteoclasts
-derived from mesenchymal stem cells present under the periosteum in the developing bone & in the medullary cavity later in life.
-located on the surface of the matrix,
Function-(i)synthesize, transport & assemble bone matrix and regulate its mineralization, (ii)regulate Osteoclast-activation & inhibition.
-those osteoblasts which get incorporated into the bone matrix during its synthesis are now called Osteocytes.
-located within the bone, interconnected by an intricate network of cytoplasmic processes through tunnels called canaliculi.
Function-help to control calcium and phosphate levels, detect mechanical forces, & translate them into biologic activity:-mechanotransduction.
-derived from circulating monocytes,
-are specialized multinucleated macrophages,
-located on the surface of bone,
-are responsible for bone resorption.
-Cell surface integrins helps osteoclasts to attach to bone matrix & to create a sealed extracellular trench (resorption pit-Howship’s lacunae).
-They secrete acid & neutral proteases (predominantly matrix metalloproteases [MMPs]) into the pit, and these enzymes resorb the bone
Haversian system of the Bone
-It is the arrangement of a large number of parallel cylindrical-shaped structural units called Osteons in cortical/compact bones. Osteons tend to be aligned the same way that force is applied to the bone.
-Each OSTEON is a cylindrical unit with the following components:
- Haversian Canal-It is the long hollow central canal along the long axis of the bone. The canal contains blood vessels, lymph vessels & nerve fibres.
- Lamellae-The series of cylindrical plates/layers of bone-matrix are arranged around each central canal. These layers are called lamellae.
- Lacunae-Between the adjacent lamellae of the osteon are strings of small cavities arranged in the same circular fashion as the lamellae called lacunae, in each of which lies an osteocyte.
- Canaliculi– Lacunae communicate with each other via a network of tiny channels called canaliculi. It allows the circulation of interstitial fluid through the bone, & osteocytes directly contact with each other by extending the fine processes into them.
- Volkmann’s canal-Each central canal(Haversian canal) is linked with neighbouring canals by tunnels running at right angles between them,called Volkmann’s canals/perforating canals. Through the Volmann’s canal vessels of the central canals connect with each other & to the subperiosteal vessels.
- Interstitial lamellae-The remnants of previous concentric lamellae can be found between the osteons, called interstitial lamellae.