Composition of Bone

Bone is mainly composed of two components:

1. Matrix
2. Cells

MATRIX: -Large quantities of extracellular matrix loaded with Calcium-hydroxyapatite;
-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

• OSTEOBLASTS
  -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.
• OSTEOCYTES
  -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.
• OSTEOCLASTS
  -derived from circulating monocytes,
  -are specialized multinucleated macrophages,
  -located on the surface of bone,
  Function
  -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

Haversian system
-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.

This imparts a hard and dense texture to the compact bones.