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physiology of bone formation

 Physiology of Bone Formation/Ossification

Process by which bone forms or develop is known Ossification (osteogenesis). Human embryo composed of either hyaline cartilage or fibrous membrane (formed by embryonic connective tissue, mesenchyme).

Bone develop embryo in two ways;

1. Intramembranous ossification

2. Endochondral ossification


1. Intramembranous ossification

When bone tissue  develop directly from mesenchyme (embryonic connective) tissue, process is called intramembranous ossification.

Intramembranous ossification begins at approximately 5th week of development with formation of mesenchymal membrane.

Ossification of membrane begin -- Approximately 8th weak of development and completed by approximately 2 year of age.


Site of intramembranous ossification

Such as; flat bones of skull, mandible, clavicle


Process of Intramembranous Ossification

Mesenchymal stem cells aggregated and form osteoblast by differentiation.


1. Development of ossification center 

These bone forming oesteoblast are going to come together and form ossification center. 

Oesteoblasts begin to secrete osteoid (substance, hardening bone, inmineralized bone)

Peripheral mesenchymal cell continue to differentiate.

2. Calcification

Osteoblasts secrete osteoid inward toward ossification center.

Oesteoblasts become trapped in osteoid, causing differentiation into osteocytes.


Osteoid calcifies and hardens after several days.

Osteoid continues to be deposite, assembles, in random manner around embryonic blood vessels.

3. Formation of Trabeculae

Finely waven trabeculae forms blod vessels grow into spaces between trabeculae.

Mesenchyme begins to differentiate in periosteum.

Lamellar (compact) bone replaces woven bone at outer edge 

Layered

Internal spongy bone remains

Vascular tissue within Trabeculae spaces forms red marrow.

Oesteoblasts remains on bone surface to remodel when needed development of periosteum (outer covering of bone).


2. Endochondral ossification

When bone tissue develops by replacing hyaline cartilage, process is known endochondral ossification.

This type of ossification begins with formation of cartilage end of 4th week of development in utero (in womb).


Site of endochondral ossification

Bone of arms, legs, base of skull

Uses hyaline cartilage as blue print for ossification.


Process of Endochondral ossification

1. Development of cartilage model

At site where bone is going to form, mesenchymal cells become chondroblast that produce cartilage metrix 

Chondroblast become surrounded by cartilage matrix, they become chondrocytes.

Now they consists of hyaline cartilage.

Membrane called perichondrium that surrounds cartilage model, occur in hyaline cartilage of epiphyseal plate.


2. Growth of Cartilage Model

Cartilage Model increases in size by;

1. Interstitial growth

Increase length of growing bone.

2. Appositional growth

Middle growth, increase diaphysis.

1. Interstitial growth

Interstitial growth occurs in hyaline cartilage of epiphyseal plate. Increase length of growing bone.

Interstitial growth is lengthening of bone resulting from growth of cartilage and it's replacement with bone tissue.


2. Appositional growth

Appositional growth in increase in diameter of bone by addition of bony tissue at surface of bone.

Happen when osteoblast in periosteum deposit new bone matrix and oestoclast, on inner surface break down bone, osteoblast which produce thicker and stronger bone.

3. Development of primary ossification center

By end of 2nd and 3rd month of parental development primary center of ossification is establish in central part of each developing bone. 

In long bone primary centers occurs in diaphysis/ shaft and in irregular bones primary centers occurs usually in the body of bone.

Cartilaginous matrix mineralized and begins to disintegrate 

Capillaries of periosteum grow into disintegrating calcified cartilage.

Connective tissue surrounds blood vessel bring in osteoblasts, osteoclasts from periosteum.


A vasular connective tissue bud from periosteum that enters cartilage of developing long bone and contributes to formation of center for ossification known periosteal buds, penetrates the area left by disintegrating cartilages.

In center, osteoblast brings to deposit bone matrix, forming spongy bone trabeculae.

Oesteoclasts break down newly formed spongy bone trabeculae, leaving, a cavity, medullary cavity in center.

Medullary cavity, fills with red bone marrow.

Now diaphysis develop bone callar under periosteum and increasing amount of marrow in center of shaft and support to growing bone and help it retain it's shape.


4. Development of Epiphysis

In long bone diaphysis is primary ossification center and epiphysis is site of secondary ossification centers.

After birth, chondrocytes in epiphysis begin to mature and enlarge.

Blood vessel and osteogenic cells from periosteum enter epiphysis where cells develop into osteoblasts.

Center of this activity in epiphysis called secondary ossification center.

Here spongy remains in inferior of epiphysis and no medullary cavity is formed.

Some cartilage remain on out edge of each epiphysis to form articular cartilage necessary for smooth operation at joint.

Epiphyseal plate also remains throughout growth period.

It provide framework for development of cancellous bone tissue inside metaphysis.

In mature bone, cancellous and compact bone are fully developed and epiphyseal plate has become epiphyseal line.


The only cartilage present is articular cartilage at end of bone.

All original perichondrium that surrounded cartilage made has become periosteum.

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