Bone is the hardest of all the connective tissues,because the organic fibrillar ECM(Extra Cellular Matrix) is impregnated withinorganic materials,primarily hydroxyapatite. The organic materila,primarily typeI collagen, gives bone its flexibility,whereas the inorganic material gives bone its compressive strength.
The cellular components consists of fibro blast,fibrocytes,osteoblasts,osteocytes,osteoclasts abd osteoprogenitor cells that can differentiate into osteoblasts.
The Fibroblasts and Fibrocytes produces typeI collagen and othe ECM components.
The Osteoblasts are the primary bone-forming cells that are responsible not only for the synthesis of bone but also for its deposition and mineralization. Osteoblasts also secrete procollagen (precursor of typeI collagen) into the surrounding matrix. When osteoblasts cease their bone-making activity,they are called osteocytes.
Osteoclasts, monocyte-derived large polymorphous cells with multiple nuclei, are responsible for bone resorption.
There are numerous terms used to describe the complex architecture of the highly calcified ECM in bones.The inner most layer is called Cancellous(also trabecular or spongy) bone, and the outer layer is called compact, or cortical bone. In cancellous bone, the calcified tissue forms thin plates called trabeculae that are laid down in response to stresses placed on the bone.
The cancellous bone is covered by a thin layer of dense compact bone called cortical bone, which is laid down in concentric layers.
The fibrous layer covering all the bones is the periosteum. This membrane covers the entire bone except the articular surfaces.The periosteum is well vascularized and contains many capillaries that provide nourishment for the bones.The periosteum contains an osteogenic layer that contains cells that are precursors to osteoblasts and osteoclasts,and it thus acts as a reservoir for cells neend for growth and repair. damage to the periosteum acts as a result of trauma or surgery will decrease the healing capacity of the bone.
At microscopic level,bothe cortical and cancellous bone may contain 2 distinct types of bone architecture: Woven bone and Lamellar bone.In woven (primary) bone,collagen fibres are irregularly arranged to form a pattern of alternating coarse and fine fibers that resembles woven material.
Lamellar bone requires a framework to form, is organized into parallel layer,and is older bone that constitutes most of the adult skeleton.
Bone remodels throughout life, as it responds to external force(or loads), such as the pull of tendon and weight of the body during functional activities.This change in form to match function is known as Wolff's law.
An imbalance between bone synthesis and resorption, in which osteoclasts break down or absorb the bone at a faster rate than the osteoblasts can remodel or rebuilt the bone, results in a condition called Osteoporosis.
In osteoporosis, the bones have a decreased mineral density(mass per unit volume) in comparison with normal bone and thus weaker bone(more susceptible to fracture) than bones with normal density.
