Process of bone formation involving calcification of cartilage or connective tissue. Cartilage is a dense bone connective tissue, with many properties similar to bone (but it can‚eat be remodelled), and both rely on the secretion of a rigid extracellular matrix. It develops when mesenchyme cells on the proximal, central core of the limb bud transform into chondroblasts and condense, a transformation that is independent of condensation. Once condensed, the mesenchyme cells differentiate into cartilage. The chondroblasts begin to secrete an extensive extracellular matrix of Type II collagen fibrils embedded in a fibrous connective tiisue having a concentration of proteoglycan molecules. The cartilage that develops in an embryo is different than that which forms in the large skeletons of mature sharks and rays as it has a higher ratio of cells to matrix and is not calcified. In both ontogeny and phylogeny, cartilage precedes bone, and animals with completely cartilaginous skeletons (e.g., rays, sharks) have lost the ability to convert cartilage to bone, which perhaps is an adaptation as to an aquatic environment in which the effects of gravity are reduced as cartilage is more flexible and elastic than bone (as well as being lighter), even though it is rigid . As such, it can also be seen as an ontogenetic adaptation to the mammalian uterine environment. There are no mature terrestrial animals with a cartilaginous skeleton who have to deal with the full force of gravity
See Adaptation, Aponeuroses, Bone Morphogenetic Proteins (BMPs), Diaphysis, Chrondrification, Endochondral ossification, Epiphysis, Extracellular matrix, Mesenchyme, Musculoskeletal system, Ontogenetic adaptation, Ontogeny, Osteoclasts, Phylogeny