Endochondral ossification is one of the two essential pathways by which bone tissue is produced during fetal development and bone repair of the mammalian skeletal system, the other pathway being intramembranous ossification. Both endochondral and intramembranous processes initiate from a precursor mesenchymal tissue, but their transformations into bone are different. In intramembranous ossification, mesenchymal tissue is directly converted into bone. On the other hand, endochondral ossification starts with mesenchymal tissue turning into an intermediate cartilage stage, which is eventually substituted by bone.
Endochondral ossification is responsible for development of most bones including long and short bones, the bones of the axial (ribs and vertebrae) and the appendicular skeleton (e.g. upper and lower limbs), the bones of the skull base (including the ethmoid and sphenoid bones) and the medial end of the clavicle. In addition, endochondral ossification is not exclusively confined to embryonic development; it also plays a crucial role in the healing of fractures.
{| class="wikitable"
|+ This hyaline cartilage template expands through both:
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! !! Interstitial growth !! Appositional growth
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| Cellular protagonists || Chondrocytes present within the existing cartilage. || Chondroblasts that develop from the perichondrium.
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| Mechanism || Chondrocytes proliferate and lay down matrix. || Chondroblasts differentiate into chondrocytes and lay down matrix.
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| Site of expansion || From within. || From the external surface of existing cartilage.
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| Outcome || Increase in length. || Increase in width and thickness.
|}
Primary center of ossification
upright=1.5|thumb|A schematic for [[long bone endochondral ossification.]]
In developing bones, ossification commences within the primary ossification center located in the center of the diaphysis (bone shaft),
<li>When chondrocytes die, matrix metalloproteinases result in catabolism of various components within the extracellular matrix and the physical boundaries between neighboring lacunae (the spaces housing chondrocytes) weaken. This can lead to the merging of these lacunae, creating larger empty spaces. relies heavily on endochondral ossification. In this type of healing, endochondral ossification occurs within the fracture gap and external to the periosteum. In contrast, intramembranous ossification takes place directly beneath the periosteum, adjacent to the broken bone's ends.
upright=1.5|thumb|A schematic of endochondral fracture, where B shows the location of both endochondral and intramembranous ossification.
Additional images
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File:Proximal tibia Masson Goldner Trikrom rabbit 600x growth zone.jpg|Masson Goldner trichrome stain of growth plate in a rabbit tibia.
File:Gray79.png|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.
File:Endochondral CCN.jpg|Process of endochondral ossification.
File:Gray80.png|Drawing of 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.
</gallery>