|Structure of collagen molecule from National Center for Biotechnology Information.
Mature bone is composed of proteins and minerals. Approximately 60% the weight of the bone is mineral, mainly calcium and phosphate. The rest is water and matrix, which is formed before the mineral is deposited, and can be considered the scaffolding for the bone. About 90% of the matrix proteins are collagen, which is the most abundant protein in the body. Collagen is very strong and forms bone, cartilage, skin, and tendons.
How you would construct a strong but light material from only the following 3 ingredients: short threads, plaster of paris, and glue? The best way would be to twist or braid the thread into strings, then glue the strings into ropes, then lay the ropes in a pattern and pour the plaster of paris over them, the way concrete is poured over steel rebar.
Collagen is formed as chains (short pieces of thread) which twists into triple helices (strings). These line up and are bonded together into ropes (fibrils). The fibrils then are arranged in layers, and mineral crystals will deposit between the layers.
The following flash movie demonstrates collagen synthesis to fibrils. Be like a transcription factor! Click on the DNA to begin.
Don't miss the "continue" buttons on the movie above!
Below is a scanning electron image(B) that shows bone collagen fibrils in both longitudinal and cross sections. The back-scattered electron image (C) shows the regular patterns of collagen in layers in bone, which is why normal bone is called lamellar bone. Images from Marian Young
|The following table gives information about some of the other bone matrix proteins that seem most important. For some of these, the gene has been deleted in special populations of mice (called "knockout mice") to try to understand the function of the protein.|
|NAME||COMMENTS||Effect of "knockout"|
|Fibronectin||Relatively abundant, may help regulate osteoblast differentiation||Lethal|
|Osteonectin||"Bone connector" may regulate mineralization||Osteoporosis|
|Thrombospondin||May inhibit bone cell precursors||Dense bones|
|Osteocalcin||Binds calcium||Bones seem normal|
|Matrix-gla-protein||Inhibits mineralization||Normal bones but calcified blood vessels|
|SIBLINGS - small integrin-binding ligand, N-linked glycoprotein family|
|Bone sialoprotein||Binds to integrins, may assist cancer cells||-|
|Osteopontin||Increases angiogenesis (makes new blood vessels) which enhances bone resorption in some situations||Resistance to PTH and removal of ovaries|
|Matrix extracellular protein||May induce a bone disease called osteomalacia||-|
|Proteoglycans - proteins with many attached sugars|
|Reference: Young, M. F.(2003). Bone matrix proteins: their function, regulation, and relationship to osteoporosis. Osteoporos Int 14 Suppl 3: S35-42.|