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Which of the following components of bone is most responsible for compressive strength?
Type I collagen
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Proteoglycans, in addition to calcium hydroxyapatite [Ca10(PO4)6(OH)2], are most responsible for providing compressive strength.
Bone is composed of both organic and inorganic components. Inorganic components include calcium hydroxyapatite and osteocalcium phosphate. Organic components include collagen, proteoglycans, matrix proteins, cytokines and growth factors. While Type I collagen is responsible for providing the tensile strength of bone, proteoglycans and calcium hydroxyapatite [Ca10(PO4)6(OH)2] are most responsible for providing compressive strength. Proteoglycans contain a core protein with various numbers of covalently attached side chains of glycosaminoglycans. In addition to providing compressive strength, they are also responsible for binding growth factors and inhibiting mineralization.
Knothe et al. review the osteocyte. They discuss that osteocytes are the most abundant cells in bone, are actively involved in maintaining the bony matrix, and may act as mechanosensors.
Illustration A shows a proteoglycan aggregate, which can form when individual molecules link onto a chain of hyaluronic acid.
Answer 1: Type I collagen is responsible for the tensile strength of bone.
Answer 2: Osteocalcin is the most abundant non-collagenous protein in the matrix and promotes the mineralization and formation of bone.
Answer 4: Osteonectin is believed to have a role in regulating calcium or organizing mineral in matrix.
Answer 5: Osteopontin is a cell-binding protein.
Knothe Tate ML, Adamson JR, Tami AE, Bauer TW
Int. J. Biochem. Cell Biol.. 2004 Jan;36(1):1-8. PMID: 14592527 (Link to Abstract)
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All of the following are true regarding osteocalcin EXCEPT which of the following?
It is the most prevalent non-collagenous protein in bone
It is expressed by mature osteoblasts
It is considered a marker for osteoblast differentiation
It is a glycoprotein that binds calcium
Higher levels are correlated with increases in bone mineral density during osteoporosis treatment
Pure fact question from basic science. “Osteocalcin is the most prevalent noncollagenous protein in bone” (from Miller's Review). It is expressed by mature osteoblasts and is a marker of osteoblast differentiation. Osteocalcin is the most specific marker of the osteoblast phenotype and is expressed only in mature osteoblasts. During osteoporosis treatment, serum levels correlate with increases in bone mineral density. Osteonectin, not osteocalcin, is a glycoprotein that binds calcium.
Fukumoto S, Martin TJ
Trends Endocrinol. Metab.. 2009 Jul;20(5):230-6. PMID: 19546009 (Link to Abstract)
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Ca10(P04)6(OH)2 is the chemical formula of
The chemical structure of hydroxyapatite is Ca10(PO4)6(OH)2. Hydroxyapatite is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Up to 50% of bone by weight is made up of a modified form of hydroxyapatite. It is one of few materials that will support bone ingrowth and osseointegration when used in orthopaedic, dental and maxillofacial applications.
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All of the following statements are true of osteocalcin EXCEPT:
It is the most abundant noncollagenous protein of bone
It is secreted by osteoclasts
It is involved in mediating calcium homeostasis
It has been used as a biochemical marker of bone formation
It is part of the organic matrix of bone
Osteocalcin is the most abundant noncollagenous protein in bone. It is secreted by osteoblasts and plays a role in bone mineralization and calcium homeostasis. It is pro-osteoblastic, or bone-building, by nature. It has recently been used as a biochemical marker for bone formation in trials of anabolic drugs, such as forteo. Answer 2 is the false statement because osteocalcin is secreted by osteoblasts.
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