Most of the bones in the body are preceded by a temporary cartilaginous model of the same shape as the definite bone and the chief peculiarity of the method is the preliminary destruction of the cartilage and its replacement by bone, which develops exactly as in intramembranous ossification. So bones formed by this method are called replacement or substitution bones.The bones of the appendicular skeleton are all substitution bones.
In the hyaline cartilage, which is to be substituted by bone, the cartilage cells enlarge in the centre and become arranged in characteristic radial rows and some lime is deposited in their matrix. The cartilage cells and part of the matrix then disintegrate forming primary morrow cavities. This destruction is accompanied by a simultaneous invasion of this region by the vascular connective tissue, rich in cells derived from the perichondrium.
These cells perform the function of osteoblasts of depositing lime salts in the cartilage matrix and this results in the formation of bars of calcified matrix. Bone thus formed is spongy in nature consists of network of bony plates enclosing the primary marrow spaces containing blood vessels and connective tissue corpuscles and this constitutes the red marrow. In a progressive manner the cartilage undergoes invasion, destruction and replacement by spongy bone.
This spongy bone at the centre is destroyed and absorbed by giant cells called osteoclasts. Thus a large marrow cavity is created in the centre of the bone. Simultaneously the perichondrium at the periphery functions as periosteum and deposits, by the activity in the centre and periosteal bone at the periphery, its destruction and absorption by osteoclasts at the centre goes on, resulting in increased dimension of the marrow cavity and circumferential growth of bone.
The centre of ossification appearing in the shaft of a long bone of the foetus in the primary centre of ossification. Sometime between birth and puberty, centres of ossification appear at the epiphyses also. At the junction of the shaft and the epiphyses there are two plates of cartilages, one above and the other below termed the epiphyseal cartilages.
These supply more and more of cartilage for the shaft and the epiphyses for formation of new bone and this process results in an increase in length of the bone. When the bone has reached its adult, size, both in its length circumference, the epiphyseal cartilages are ossified and further growth stops.
Reconstruction takes place in periosteal bone to form the Haversian systems and the outer and inner circumferential lamellae. The osteoclasts destroy and remove periosteal bone forming Haversian canals. Osteoblasts arrange themselves around them and deposit their exoplasm in concentric layers which are later ossified by deposition of calcium salts.
Some of the osteoblasts are enqulfed in the lamellae and occupy lacunae and canaliculi. Others withdraw and arrange themselves at the periphery and deposit periosteal lamellae. The cells towards the medullary cavity form the endosteal lamellae. Thus through reconstruction, compact bone is formed outside enclosing cancellated or spongy bone inside, around the medullary cavity.
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