Endochondral ossification 1311158 224760575 2008-07-10T08:45:04Z 68.126.3.192 /* Primary center of ossification */ new picture [[Image:Gray79.png|thumb|279px|Section of fetal bone of cat. ir. Irruption of the subperiosteal tissue. p. Fibrous layer of the periosteum. o. Layer of osteoblasts. im. Subperiosteal bony deposit. (From Quain’s “Anatomy,” E. A. Schäfer.)]] '''Endochondral ossification''' is one of the two processes during [[fetal]] development of the [[mammal|mammalian]] [[skeleton|skeletal system]] in which [[bone tissue]] is created. It is also an essential process for the rudimentary formation of [[long bones]],<ref>[[Frank H. Netter|Netter, Frank H.]] (1987), ''Musculoskeletal system: anatomy, physiology, and metabolic disorders''. Summit, New Jersey: Ciba-Geigy Corporation ISBN 0914168886, p. 130: One exception is the [[clavicle]].</ref> increasing the [[Epiphyseal plate|length]] of long bones,<ref>Brighton, Carl T., Yoichi Sugioka, and Robert M. Hunt (1973), "Cytoplasmic structures of epiphyseal plate chondrocytes; quantitative evaluation using electron micrographs of rat costochondral junctions with specific reference to the fate of hypertrophic cells", ''Journal of Bone and Joint Surgery'', '''55-A''': 771-784</ref> and healing of [[bone healing|bone fractures]].<ref>Brighton, Carl T. and Robert M. Hunt (1986): "Histochemical localization of calcium in the fracture callus with potassium pyroantimonate: possible role of chondrocyte mitochondrial calcium in callus calcification", ''Journal of Bone and Joint Surgery'', '''68-A (5)''': 703-715</ref> Unlike [[intramembranous ossification]], which is the other process, endochondral ossification requires a pre-existing [[cartilage]] model. ==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== [[Image:Bone growth.png|450px|right]] 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: *'''Formation of periosteum''': Once vascularized, the [[perichondrium]] becomes the [[periosteum]]. The periosteum contains a layer of undifferentiated cells which later become [[osteoblast]]s. *'''Formation of bone collar''': The osteoblast secretes [[osteoid]] against the shaft of the cartilage model. This serves as support for the new bone. *'''Calcification of matrix''': [[Chondrocyte]]s in the primary center of ossification begin to grow (hypertrophy). They stop secreting [[collagen]] and other [[proteoglycan]]s and begin secreting [[alkaline phosphatase]], an enzyme essential for mineral deposition. Nutrients can no longer diffuse if the matrix becomes sufficiently calcified and the chondrocytes subsequently die. This creates cavities within the bone. *'''Invasion of periosteal bud''': A periosteal bud, which consists of [[blood vessel]]s, [[lymph vessel]]s and [[nerve]]s, invades the cavity left by the chondrocytes. The vascularization utlimately carries [[hemopoietic cell]]s, [[osteoblast]]s and [[osteoclast]]s inside the cavity. The hemopoietic cells will later form the [[bone marrow]]. *'''Formation of trabeculae''': Osteoblasts use the calcified matrix as a scaffold and begin to secrete [[osteoid]], which forms the bone [[trabecula]]. Osteoclasts break down spongy bone to form the medullary (bone marrow) cavity. ==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]]. [[Osteoclast]]s 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. ==Histology== [[Image:Gray80.png|thumb|Part of a longitudinal section of the developing femur of a rabbit. a. Flattened cartilage cells. b. Enlarged cartilage cells. c, d. Newly formed bone. e. Osteoblasts. f. Giant cells or osteoclasts. g, h. Shrunken cartilage cells. (From “Atlas of Histology,” Klein and Noble Smith.)]] During endochondral ossification, four distinct zones can be seen at the light-microscope level. #'''Zone of resting cartilage'''. This zone contains normal, resting [[hyaline cartilage]]. #'''Zone of proliferation'''. In this zone, [[chondrocyte]]s undergo rapid [[mitosis]], forming distinctive looking stacks. #'''Zone of maturation / hypertrophy'''. It is during this zone that the chondrocytes undergo hypertrophy (become enlarged). Chondrocytes contain large amounts of [[glycogen]] and begin to secrete [[alkaline phosphatase]]. #'''Zone of calcification'''. In this zone, [[chondrocyte]]s are either dying or dead, leaving cavities that will later become invaded by bone-forming cells. Chondrocytes here die when they can no longer receive nutrients or eliminate wastes via diffusion. This is because the calcified matrix is much less hydrated than hyaline cartilage. ==References== {{Reflist}} ==See also== *[[Intramembranous ossification]] {{Bone and cartilage}} [[Category:Developmental biology]] [[Category:Skeletal system]]