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Bone & Cartilage Development - Anatomy & Physiology (view source)
Revision as of 17:05, 30 December 2010
, 17:05, 30 December 2010→Endochondrial Ossification
==Endochondrial Ossification==
==Endochondrial Ossification==
[[Bones and Cartilage - Anatomy & Physiology#Types of Bone|Bone]] formation via [[Bones and Cartilage - Anatomy & Physiology#Structure and Function of Cartilage|cartilage]] occurs in the following stages;
1. The proliferating limb mesenchyme condenses.
2. The increased cell density triggers mesenchymal cells to differentiate into chondrocytes. Proliferation continues and the bone shape is formed, with a diaphysis (shaft) and epiphysis (bulbous ends).
3. Chondrocytes in the middle of the diaphysis stop proliferating and undergo '''hypertrophy'''. The shaft is surrounded by the '''perichondrium''', a restrictive membrane. This means that the bone can only grow in length, not width. The epiphyses are pushed away from each other.
4. After the chondrocytes have undergone hypertrophy, they die.
5. '''Osteoblasts''' on the inner side of the perichondrium invade the cartilage matrix. They then secrete the extra - cellular matrix, characteristic of bone.
6. Ossification occurs from the outside - in. This is called primary ossification, or perichondrial ossification. Blood vessels invade the central region of the diaphysis to transport [[Bone Marrow - Anatomy & Physiology|bone marrow]] stem cells and osteoclasts. The osteoclasts break down the cartilage matrix to allow room for the bone marrow.
There is no perichondrium surrounding the eiphysis, this allows for expansion in length of the bone, but means that there are no surrounding oasteoblasts to invade. To achieve osteogenesis, blood vessels invade from the diaphysis and carry osteoblasts with them. In the epiphysis, osteoblasts initiate bone growth from the inside - out. This is called secondary ossification. In some mammals, secondary ossification occurs after birth. The bone becomes covered in a layer containing osteoblasts.
Cartilage is ''not replaced by bone'' in two areas:
1. In between the diaphysis and epiphysis, a region called the '''epiphyseal growth plate (EPG)'''.
2. At the ends of the epiphysis, between the joints. Acts as a '''shock absorber'''.
==Bone Growth==
==Bone Growth==