In histology, osteoid is the unmineralized, organic portion of the bone matrix that forms prior to the maturation of bone tissue. Osteoblasts begin the process of forming bone tissue by secreting the osteoid as several specific proteins. When the osteoid becomes mineralized, it and the adjacent bone cells have developed into new bone tissue.
Osteoid makes up about fifty percent of bone volume and forty percent of bone weight. It is composed of fibers and ground substance. The predominant fiber-type is Type I collagen and comprises ninety percent of the osteoid. The ground substance is mostly made up of chondroitin sulfate and osteocalcin.
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