Bone & Cartilage Development - Anatomy & Physiology
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Osteogenesis is the formation of bone. Bone forms from one of three lineages; the skull forms from the neural crest; the limb skeleton forms from the lateral plate mesoderm; and the axial skeleton forms from the paraxial mesoderm (sclerotome).
There are two methods of osteogenesis:
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.
2. Endochondral Ossification
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 stem cells and osteoclasts. The osteoclasts break down the cartilage matrix to allow room for the bone marrow.
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.
Cartilage is not replaced by bone in two areas:
1. In between the diaphysis and epiphysis, a region called the epiphyseal growth plate (EGP).
2. At the ends of the epiphysis, between the joints. Acts as a shock absorber.
Also see Bone Growth
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