Veterinary Gross Anatomy

DEVELOPMENT OF BONE

• In the embryonic life, the future elements of the skeleton are derived from the mesenchyme, which is derived from the mesoderm.
• Ossification is the process of development of bone from the mesenchyme.
• The formation of bone during the fetal stage of development occurs by two processes called
  • intramembranous and
  • endochondral ossification.

Intramembranous ossification

• The flat bones of the skull are developed by ossification in a membranous layer formed by the condensation of mesenchymal tissue. Hence, this process is being termed as intramembranous method of ossification.The bones developed by this method are termed as membranous bones.
• The steps in intramembranous ossification are
  1. Development of ossification centre
  2. Calcification
  3. Formation of trabeculae
  4. Development of periosteum
• At a point (centres of ossification), osteoblasts are differentiated from mesenchymal cells. A meshwork of collagen fibres produced by the osteoblasts appears between the cells. It becomes vascularised by capillary network. The osteoblasts produce other organic intercellular substances like mucoprotein, glycoprotein, mucopolysaccharides etc. This organic non-calcified matrix is known as osteoid. Now the matrix is calcified by the osteoblasts. Few osteoblasts become entrapped by the surrounding matrix and are transformed into osteocytes. Simultaneously, other osteoblasts proliferate by division and are arranged in radiating manner from the centre. Thus trabeculae are formed between the cells. The trabeculae join each other to form cancellous bone. The osteoblasts surrounding the bony spicules deposit more bones to the free ends and sides and thus calcification is spread and the bone becomes compact. The periosteum is developed from the condensation of the mesenchyme.

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Intra cartilagenous ossification /endochondral method of ossification

• The bones of appendicular skeleton, vertebral column, ribs etc, are developed by intracartilagenous or endochondral method of ossification . In this method, a cartilage model of future bone is formed at first and then it is replaced by bone. Bones developed by this method are termed as cartilaginous bones. The process of ossification does not take place simultaneously all over the cartilaginous or membranous representative of a future bone. It begins at one or more points known as centres of ossification, from which it extends until the central soft tissue is replaced by bone. In a long bone, there is a primary centre of ossification for the diaphysis and secondary centres appear for the epiphysis. Additional centres may appear in some bones for the various processes present in the bone. The number of centres is different for the various bones in the body, but the number for each bone is constant in a particular species.
• This process occurs in stages.
  • Stage-I
    • At the beginning, a cartilage model is formed by the condensation of the mesenchymal tissue. Perichondrium appears around the cartilage. The cartilage cells (chondroblasts) at the mid section of the model proliferate by mitosis and are arranged in rows towards the ends. They mature and hypertrophied. The hypertropic cells produce alkaline phosphatase and precipitate calcium salt at the matrix. The surrounding calcification cause death of the cartilage cells and thereby form spaces-the primary areolae . This zone is known as primary ossification centre. At the same time osteoblasts appear at the inner layer of perichondrium and form subperiosteal collarbone around the primary ossification centre.
  • Stage-II
    • At this stage the collar bone is eroded by the increased activity of the subperiosteal steoclasts and peroisteal buds containing osteoblasts, osteoclasts and blood vessels enter into the primary ossification centre. The osteoclasts absorb the irregular calcified mass and form secondary large areolae . These secondary areolae lead to the formation of marrow cavity, which subsequently becomes filled up by the bone marrow.
  • Stage-III
    • It is known as stage of true bone formation. Here the osteoblasts appear and lay down lamellated bone. Subsequently a number of longitudinal grooves proliferate and enclose a small blood vessel, which already developed along the periosteum and convert the grooves into tunnels. The lining osteoblasts of the tunnel convert the tunnel into Haversian system by proliferation and differentiation into osteocytes.
    • The whole process is repeated again and again and the ossification extends longitudinally. Simultaneously new bones are formed under the periosteum by appositional method.
      Secondary ossification centres appears at the ends of the cartilagenous model at birth. This zone is called epiphysis. Ossification occurs in the similar way in both directions. 

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