The primitive skeleton is cartilage, a solid avascular (without blood vessels) tissue in which individual cartilage-matrix secreting cells, or chondrocytes, occur. Chondrocytes do not have intercellular connections and are not coordinated in units. Cartilage is composed of a network of collagen type II held in tension by water-absorbing proteins, hydrophilic proteoglycans.[24] This is the adult skeleton in cartilaginous fishes such as sharks. It develops as the initial skeleton in more advanced classes of animals.
In air-breathing vertebrates, cartilage is replaced by cellular bone. A transitional tissue is mineralized cartilage. Cartilage mineralizes by massive expression of phosphate-producing enzymes, which cause high local concentrations of calcium and phosphate that precipitate.[24] This mineralized cartilage is not dense or strong. In the air breathing vertebrates it is used as a scaffold for formation of cellular bone made by osteoblasts, and then it is removed by osteoclasts, which specialize in degrading mineralized tissue.
Osteoblasts produce an advanced type of bone matrix consisting of dense, irregular crystals of hydroxyapatite, packed around the collagen ropes.[25] This is a strong composite material that allows the skeleton to be shaped mainly as hollow tubes. Reducing the long bones to tubes reduces weight while maintaining strength.
https://en.wikipedia.org/wiki/Osteoblast#Osteogenesis
No comments:
Post a Comment