HOW BONES MODEL AND REMODEL THEMSELVES

Anatomy of the human bone

Bone structure

Bone is composed of osteoid matrix (unmineralized, organic portion of the bone matrix that forms prior to the formation of compact bone),  hydroxyapatite  crystal (natural form of calcium apatite)   and water.

Bones also contain  proteins, lipids (fats) and specialized bone cells. [1]

The type 1 collagen bone matrix gives bone elasticity, flexibility and tensile strength.

The role of  proteins in bone  include strengthening the collagen structure and regulating its mineralization.

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BONE TRANSFORMATION

Throughout life, the dynamic skeleton is constructed and renewed  by two processes:

Bone modelling  and  Remodeling.

Both processes involve bone resorption ( break down and removal of old bone ) and bone formation.

During early childhood, both bone modelling and remodeling  take place.

During childhood, the predominant process is the shaping and  formation of new bone ( bone modeling ) while in adulthood,  the replacement or renewal of old bone( bone remodeling ) is the predominant process except when a fracture occurs in which case, there is a rapid increase in bone formation. [2]

Bone modeling

Bone modelling is the process by which bones change shape or size in response to physiologic influences or mechanical forces that are encountered by the skeleton like exercises.

The bone lengthens and modifies by modeling, a process in which either bone breakdown or bone formation occurs on a given bone surface.

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During the process of modelling, bone resorption ( break down ) and formation occur independently at different skeletal sites to bring about major changes in bone architecture.

In the bone modeling process, bone is removed from one  site and new bone is formed at another.

For instance, bone is removed from the metaphysis on the periosteal surface ( a membrane that covers the outer surface of all bones ) , while new bone is formed on the inner endosteal surface ( The inside surface of bones which border the bone marrow cavity ) thus converting the shape of the epiphysis into the diaphysis and increasing the length of bone.

Bone modelling also leads to the growth in width  of the diaphysis of long bones in which case,  resorption occurs on the endosteal surface, while  bone formation occurs at the periosteal surface thus increasing the overall diameter of the bone.

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The majority of bone modelling is completed by skeletal maturity but modelling can still continue among  adults especially  in an adaptive response to mechanical loading and exercise or  in the case of the renal bone disease.

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Bone remodeling

Bone remodeling is a process by which the skeleton is continuously renewed to maintain the structural integrity or strength of bone.

Small zones of bone are broken down by osteoclasts and replaced by osteoblasts to ensure that structural integrity is maintained thereby replacing approximately 5 – 10% of the skeleton each year with the entire adult human skeleton replaced within 10 years.  [3]

During this process, old bone is broken down and released into the blood stream in form of calcium and phosphorus to maintain a steady state of  mineral concentration in the blood.

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Bone remodeling is a continuous process during which skeletal damage is repaired, accumulation of brittle hyper-mineralized bone is kept in check and mineral homeostasis is maintained  by releasing stores of calcium and phosphorus into the bloodstream.

The overall bone volume and structure remain unchanged during this process.

Bone cells

There are four main  types of cells that are active within bones.

1. Osteoblasts

These are the cells responsible for the manufacture and mineralization of new bone.

The process of bone formation begins with the osteoblasts manufacturing a protein matrix or mixture ( Osteoid ) which is made up of mainly collagen and other organic proteins.

The osteoblasts then deposit minerals like calcium into the protein matrix which are then solidified to manufacture bone.

After depositing the minerals, the osteoblasts are then buried within the bone matrix to become osteocytes.

Thus, before the organic matrix of bone or the young bone is mineralized, it is known as osteoid.

Hence, when osteoblasts produce osteoid at a higher rate, they can be stimulated to increase bone mass.

Manufacture of osteoid by osteoblasts is triggered by growth hormone release.
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2. Osteocytes

Osteocytes are simply osteoblasts that are no longer active. They form when osteoblasts become buried in the mineral matrix of bone.

They coordinate the actions of osteoblasts and osteoclasts via several mechanisms.

For instance, they squeeze out and release proteins that signal to osteoblasts, osteoclasts, and other bone-residing cells to respond to environmental changes.

Hence, they act as sensors when stress is applied to bones or in case of bone fractures, signaling osteoclasts to dissolve the bone.

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3. Chondrocytes

Cells that form cartilage including the growth plate cartilage and they also play a role when it comes to fracture repair.
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4. Osteoclasts.

The cells responsible for dissolving or breaking down old  bones.

They achieve this by secreting an acid that destroys the bone matrix.

This role played by osteoclasts is very vital for bone remodeling which involves continuously removing and replacing bone tissue for structured growth and in response to mechanical stress applied to bones.

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