Development of bone:

· Derived from mesenchyme (undifferentiated cells) which is mesodermal in origin.

· Ossification is the process of development of bone from mesenchyme.

· Ossification starts from ossification centers and extend to the other soft tissue.

· Long bones have primary ossification center for diaphysis and secondary ossification center for epiphysis develop later on.

· For various processes of bone separate center may appear.

· Number of centers of each bone remains same in a particular species.

· Ossification takes place by two methods: intramembranous and intracartilagenous or endochondral method.

· Intramembranous ossification

At center of ossification mesenchymal cells differentiate into osteoblasts.
Osteoblasts form a meshwork of collagen fibers in between the cells.
Vascularization appears in the form of capillary network.
Other organic intercellular substances like mucoprotein, glycoprotein, mucopolysacchrides etc. are produced by osteoblasts.
Such organic non-calcified matrix is known as osteoid.
For ossification few osteoblasts are entrapped in surrounding matrix and are transformed into osteocytes.
Simultaneously other osteoblasts proliferate and get arranged in radiating manner from center, thus forming trabeculae inbetween the cells.
Trabeculae join each other to form cancellous bone.
The osteoblasts surrounding bony spicules deposite more bones to the free ends and sides and thus calcification is spread and bone becomes compact.
Some mesenchymal cells condense and form the periosteum.
Bones of cranial cavity and face ossify by this process.

· Intracartilagenous ossification: Most bones ossify by this process. It occurs in three stages.

· Stage I:

Mesenchymal tissue condenses to form a cartilage model with pericardium around.
Central chondroblasts (cartilage cells) proliferate by mitosis and are arranged in rows towards the ends.
Chondroblasts get matured, hypertrophied and produce alkaline phosphatase and precipitate calcium salt at the matrix.
Calcification causes death of the cartilage cells leading to formation of spaces also known as primary areolae.
This zone is known as primary ossification centre.
Simultaneously osteoblasts appear underneath the perichondrium and for subperiosteal layer around the primary ossification center.

· Stage II:

Subperiosteal layer is eroded by subperiosteal osteoclasts.
Periosteal buds containing osteoblsts, osteoclasts and blood vessels enter into primary ossification center.
Osteoclasts absorb the irregular calcified masses and form secondary large areolae.
Secondary areolae later become marrow cavity and are filled by bone marrow.

· Stage III:

True bone formation stage.
Osteoblasts form lamellated bone and subsequently many longitudinal grooves develop and enclose a small vessel.
Osteoblasts lead to formation of Haversian system by proliferation and differentiation into osteocytes.

· This whole process is repeated again and again and ossification extends longitudinally.

· At birth secondary ossification centers appear at the end of the cartilaginous model.

· Ossification at secondary ossification centers occur in the similar way in both the directions.

Growth of bone

· Flat bones of skull increase in size by continued ossification at margins of connective tissue.

· Growth in thickness in both cartilaginous and membranous bones is by further deposition of matrix at their outer surfaces.

· In long bones articular cartilage does not ossify throughout life.

· In growth phase epiphyseal cartilage remain between the epiphysis and diaphysis.

· Epiphyseal cartilage is replaced by bone after growth is complete.

· Width of bone increase by growth at subperiosteal membrane.

· Calcium, phosphorous, vitamin D, vitamin C, alkaline phosphate, parathyroid hormone, growth hormone of pituitary, thyroid hormone and vitamin A affect the development and growth of bone.

Back to Introduction

Different branches of Veterinary Anatomy, Planes of body, Descriptive Anatomical Terms, Osteology terms, Classification of bones, Composition of bones, Structure of bones, Development of bones, Skeleton

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