Ossification

Ossification is the process of bone formation, in which connective tissues, such as cartilage are turned to bone or bone-like tissue. The ossified tissue is invaginated with blood vessels. These blood vessels bring minerals like calcium and deposit it in the ossifying tissue. Bone formation is a dynamic process, with cells called osteoblasts depositing minerals, and osteoclasts removing bone.^[1] This process, termed bone remodeling continues throughout life.^[2]

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Types of ossification

• Endochondral ossification: formation of bone by replacement of hyaline cartilage.
• Intramembranous ossification: formation of bone from mesenchyme, esp. flat bones found in the skull
• Heterotopic ossification: formation of bone in extraskeletal soft tissue, esp. in connective tissue or muscle tissue

Evolution

Several hypotheses have been proposed for how bone evolved as a structural element in vertebrates. One popular idea is that bone developed from tissues that evolved to store minerals. In this model, minerals such as calcium were stored in cartilage, and that bone was an exaptation from this ossified cartilage.^[3] However, other possibilities include bony tissue evolving as an osmotic barrier, or as a protective structure.

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