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Endochondral ossification is one of two types of bone formation (ossification) and is the process responsible for much of the bone growth in vertebrate skeletons, especially in long bones. As the name might suggest (endo - within, chondro - root for cartilage), endochondral ossification occurs by replacement of hyaline cartilage.
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Development of the cartilage model
During endochondral ossification in the developing fetus, mesenchymal cells aggregate to form a compact grouping of cells in a process called prechondrogenic condensation. Prechondrogenic condensation begins the process of endochondral ossification and is required for subsequent skeletal development. Following prechondrogenic condensation, the mesenchymal cells differentiate into chondrocytes and begin to produce a cartilaginous matrix. At the end of its life cycle, the chondrocyte calcifies its matrix before undergoing programmed cell death or apoptosis. At this point, blood vessels penetrate this calcified matrix, bringing in osteoblasts. The osteoblasts use the calcified cartilage matrix as a template to build bone, thus completing endochondral ossification.
Growth of the cartilage model
The cartilage model would grow in length by continuous cell division of chondrocytes, which is accompanied by further secretion of extracellular matrix. This is called interstitial growth. The process of appositional growth occurs when the cartilage model would also grow in thickness which is due to the addition of more extracellular matrix on the periphery cartilage surface, which is accompanied by new chondroblasts that develop from the perichondrium.
Primary center of ossification
The first site of ossification occurs in the primary center of ossification, which is in the middle of diaphysis (shaft). The following steps then occur:
Secondary center of ossification
Cartilage is retained in the epiphyseal plate, located between the diaphysis (the shaft) and the epiphysis (end) of the bone. These areas of cartilage are known as secondary centers of ossification. Cartilage cells undergo the same transformation as above. As growth progresses, the proliferation of cartilage cells in the epiphyseal plate slows and eventually stops. The continuous replacement of cartilage by bone results in the obliteration of the epiphyseal plate, termed the closure of the epiphysis. Only articular cartilage remains. Mineralisation of articular cartilage and its replacement by bone continues in the adult, though at a much reduced rate than in growing animals.
Appositional bone growth
The growth in diameter of bones around the diaphysis occurs by deposition of bone beneath the periosteum. Osteoclasts in the interior cavity continue to degrade bone until its ultimate thickness is achieved, at which point the rate of formation on the outside and degradation from the inside is constant.
During endochondral ossification, four distinct zones can be seen at the light-microscope level.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Endochondral_ossification". A list of authors is available in Wikipedia.|