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Bones - Anatomy & Physiology (view source)

Revision as of 12:27, 5 January 2011

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<font color = 'red'> Aimee, please combine this section with [[Bone & Joint Development - Anatomy & Physiology]] page and provide link from this page to it. Any questions, please let me know. Thanks, ''Bara''</font color>

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~~'''Osteogenesis''' is the collective name for bone formation.~~

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'''Intramembranous Ossification'''

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'''Osteogenesis''' is the formation of [[Bones and Cartilage - Anatomy & Physiology#Types of Bone|bone]]. Bone forms from one of three lineages; the skull forms from the [[Neurogenesis - Anatomy & Physiology#Neural Crest|'''neural crest''']]; the limb skeleton forms from the '''lateral plate mesoderm'''; and the axial skeleton forms from the '''paraxial mesoderm (sclerotome)'''.

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There are two methods of osteogenesis;

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1. '''Intramembranous Ossification'''

Forms the '''flat''' bones of skull and mandible. There is no cartilaginous precursor, the mesenchyme forms bone directly. The mesenchyme condenses, differentiates to pre-osteoblasts, then to osteoblasts. The osteoblasts synthesize osteoid (collagen and proteoglycans) causing mineralization and bone spicule formation. Spicules produce spongy bone. The bone growth can be described as appositional growth, where bone is laid down on the exterior (vs interior) surface of the developing bone. During bone development there is an increasing association with blood vessels and bone marrow is formed by the mesenchyme, between the bone and the blood vessels.

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'''Endochondral Ossification'''

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2. '''Endochondral Ossification'''

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This is responsible for embryonic bone formation, as well as the growth in length. This occurs via a cartilaginous precursor. The mesenchyme condenses to form the bone outline. The increased cell density triggers core cells to differentiate to chondrocytes and begin secreting the cartilage matrix. Proliferation continues and the bone shape is formed, with a diaphysis (shaft) and epiphysis (bulbous ends). 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. After the chondrocytes have undergone hypertrophy, they die. '''Osteoblasts''' on the inner side of the perichondrium invade the cartilage matrix. They then secrete the extra - cellular matrix, characteristic of bone. 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.

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There is no perichondrium surrounding the epiphysis, this allows for expansion in length of the bone, but means that there are no surrounding osteoblasts 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.

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Cartilage is ''not replaced by bone'' in two areas:

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1. In between the diaphysis and epiphysis, a region called the '''epiphyseal growth plate (EPG)'''.

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2. At the ends of the epiphysis, between the joints. Acts as a '''shock absorber'''.

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This is responsible for embryonic bone formation, as well as the growth in length. This occurs via a cartilaginous precursor. The mesenchyme condenses to form the bone outline. Core cells differentiate to chondrocytes and begin secreting the cartilage matrix. Peripheral condensation forms the perichondrium. Interstitial (length) and appositional (width) growth can be noted. The central cells in the developing diaphysis mature and grow and the matrix surrounding the most mature chondrocytes calcifies. The perichondrium then differentiates to osteoblasts. Capillary invasion occurs to the central core of the bone, forming trabecular bone.

[[Image:Growth plate.jpg|right|thumb|100px|<small><center>Growth plate (Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]

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'''Epiphyseal Growth Plate'''- The growth plate is a transverse disc of cartilate between the two ossification centers, and allows longitudinal growth to continue to maturity. It then closes. The cartilage of the epiphyseal growth plate is divided into: (from right to left on the magnified image)

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1. Resting (reserve) zone

2. Proliferative zone

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