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Bone

Bonesarerigidorgansthat constitute part of theendoskeletonofvertebrates.They support and protect the various organs of

the body, produceredandwhite blood cellsand storeminerals.Bone tissue is a type of denseconnective tissue.Bones come in

a variety of shapes and have a complex internal and externalstructure,are lightweight yet strong and hard, and serve

multiplefunctions.One of the types of tissue that makes up bone is the mineralizedosseous tissue,also called bone tissue, that

gives it rigidity and acoral-like three-dimensional internal structure. Other types of tissue found in bones

includemarrow,endosteum,periosteum,nerves,blood vesselsandcartilage.At birth, there are over 270 bones in an infant

human's body,[1]

but many of these fuse together as the child grows, leaving a total of 206 separate bones in a typical adult, not

counting numerous smallsesamoid bonesand ossicles. The largest bone in the human body is the femurand the smallest

bones among the 206 are theauditory ossicles.[2]

Functions[edit]

Bones have eleven main functions:

Mechanical[edit]

Protection bones can serve to protect internal organs, such as theskullprotecting thebrainor theribsprotecting

theheartandlungs.

Structure bones provide a frame to keep the body supported.

Movement bones provide leverage system for,skeletal muscles,tendons,ligamentsandjointsfunction together to

generate and transfer forces so that individual body parts or the whole body can be manipulated in three-dimensional

space. The interaction between bone and muscle is studied in biomechanics.

Sound transduction bones are important in the mechanical aspect of overshadowedhearing.

Synthetic[edit]

Blood production themarrow,located within themedullary cavityof long bones and interstices of cancellous bone,

produces blood cells in a process calledhematopoiesis.[3]

Metabolic[edit]

Mineral storage bones act as reserves of minerals important for the body, most notablycalciumandphosphorus.[citation

needed]

Growth factorstorage mineralized bone matrix stores important growth factors such asinsulin-like growth factors,transforming growth factor,bone morphogenetic proteinsand others.

[citation needed]

Fatstorage the yellow bone marrow acts as a storage reserve offatty acids.[citation needed]

Acid-basebalance bone buffers the blood against excessivepHchanges by absorbing or releasingalkaline salts.[citation

needed]

Detoxification bone tissues can also storeheavy metalsand other foreign elements, removing them from the blood and

reducing their effects on other tissues. These can later be gradually released forexcretion.[citation needed]

Endocrineorgan bone controlsphosphatemetabolism by releasingfibroblast growth factor23(FGF-23), which acts

onkidneysto reduce phosphatereabsorption.Bone cells also release a hormone calledosteocalcin,which contributes to

the regulation ofblood sugar(glucose)andfat deposition.Osteocalcin increases both theinsulinsecretion and sensitivity,

in addition to boosting the number ofinsulin-producing cellsand reducing stores of fat.[4]

Mechanical properties[edit]

http://en.wikipedia.org/wiki/Stiffnesshttp://en.wikipedia.org/wiki/Stiffnesshttp://en.wikipedia.org/wiki/Organ_(anatomy)http://en.wikipedia.org/wiki/Organ_(anatomy)http://en.wikipedia.org/wiki/Organ_(anatomy)http://en.wikipedia.org/wiki/Endoskeletonhttp://en.wikipedia.org/wiki/Endoskeletonhttp://en.wikipedia.org/wiki/Endoskeletonhttp://en.wikipedia.org/wiki/Vertebratehttp://en.wikipedia.org/wiki/Vertebratehttp://en.wikipedia.org/wiki/Vertebratehttp://en.wikipedia.org/wiki/Red_blood_cellhttp://en.wikipedia.org/wiki/Red_blood_cellhttp://en.wikipedia.org/wiki/Red_blood_cellhttp://en.wikipedia.org/wiki/White_blood_cellhttp://en.wikipedia.org/wiki/White_blood_cellhttp://en.wikipedia.org/wiki/Mineralhttp://en.wikipedia.org/wiki/Mineralhttp://en.wikipedia.org/wiki/Mineralhttp://en.wikipedia.org/wiki/Connective_tissuehttp://en.wikipedia.org/wiki/Connective_tissuehttp://en.wikipedia.org/wiki/Connective_tissuehttp://en.wikipedia.org/wiki/Structurehttp://en.wikipedia.org/wiki/Structurehttp://en.wikipedia.org/wiki/Structurehttp://en.wikipedia.org/wiki/Function_(biology)http://en.wikipedia.org/wiki/Function_(biology)http://en.wikipedia.org/wiki/Function_(biology)http://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Coralhttp://en.wikipedia.org/wiki/Coralhttp://en.wikipedia.org/wiki/Coralhttp://en.wikipedia.org/wiki/Bone_marrowhttp://en.wikipedia.org/wiki/Bone_marrowhttp://en.wikipedia.org/wiki/Bone_marrowhttp://en.wikipedia.org/wiki/Endosteumhttp://en.wikipedia.org/wiki/Endosteumhttp://en.wikipedia.org/wiki/Endosteumhttp://en.wikipedia.org/wiki/Periosteumhttp://en.wikipedia.org/wiki/Periosteumhttp://en.wikipedia.org/wiki/Periosteumhttp://en.wikipedia.org/wiki/Nervehttp://en.wikipedia.org/wiki/Nervehttp://en.wikipedia.org/wiki/Nervehttp://en.wikipedia.org/wiki/Blood_vesselhttp://en.wikipedia.org/wiki/Blood_vesselhttp://en.wikipedia.org/wiki/Blood_vesselhttp://en.wikipedia.org/wiki/Cartilagehttp://en.wikipedia.org/wiki/Cartilagehttp://en.wikipedia.org/wiki/Cartilagehttp://en.wikipedia.org/wiki/Bone#cite_note-1http://en.wikipedia.org/wiki/Bone#cite_note-1http://en.wikipedia.org/wiki/Sesamoid_boneshttp://en.wikipedia.org/wiki/Sesamoid_boneshttp://en.wikipedia.org/wiki/Sesamoid_boneshttp://en.wikipedia.org/wiki/Femurhttp://en.wikipedia.org/wiki/Femurhttp://en.wikipedia.org/wiki/Femurhttp://en.wikipedia.org/wiki/Auditory_ossiclehttp://en.wikipedia.org/wiki/Auditory_ossiclehttp://en.wikipedia.org/wiki/Bone#cite_note-2http://en.wikipedia.org/wiki/Bone#cite_note-2http://en.wikipedia.org/wiki/Bone#cite_note-2http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=2http://en.wikipedia.org/wiki/Skullhttp://en.wikipedia.org/wiki/Skullhttp://en.wikipedia.org/wiki/Skullhttp://en.wikipedia.org/wiki/Brainhttp://en.wikipedia.org/wiki/Brainhttp://en.wikipedia.org/wiki/Brainhttp://en.wikipedia.org/wiki/Ribshttp://en.wikipedia.org/wiki/Ribshttp://en.wikipedia.org/wiki/Ribshttp://en.wikipedia.org/wiki/Hearthttp://en.wikipedia.org/wiki/Hearthttp://en.wikipedia.org/wiki/Hearthttp://en.wikipedia.org/wiki/Lungshttp://en.wikipedia.org/wiki/Lungshttp://en.wikipedia.org/wiki/Lungshttp://en.wikipedia.org/wiki/Skeletal_musclehttp://en.wikipedia.org/wiki/Skeletal_musclehttp://en.wikipedia.org/wiki/Skeletal_musclehttp://en.wikipedia.org/wiki/Tendonshttp://en.wikipedia.org/wiki/Tendonshttp://en.wikipedia.org/wiki/Tendonshttp://en.wikipedia.org/wiki/Ligamenthttp://en.wikipedia.org/wiki/Ligamenthttp://en.wikipedia.org/wiki/Ligamenthttp://en.wikipedia.org/wiki/Jointhttp://en.wikipedia.org/wiki/Jointhttp://en.wikipedia.org/wiki/Jointhttp://en.wikipedia.org/wiki/Biomechanicshttp://en.wikipedia.org/wiki/Biomechanicshttp://en.wikipedia.org/wiki/Biomechanicshttp://en.wikipedia.org/wiki/Hearing_(sense)http://en.wikipedia.org/wiki/Hearing_(sense)http://en.wikipedia.org/wiki/Hearing_(sense)http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=3http://en.wikipedia.org/wiki/Bone_marrowhttp://en.wikipedia.org/wiki/Bone_marrowhttp://en.wikipedia.org/wiki/Bone_marrowhttp://en.wikipedia.org/wiki/Medullary_cavityhttp://en.wikipedia.org/wiki/Medullary_cavityhttp://en.wikipedia.org/wiki/Medullary_cavityhttp://en.wikipedia.org/wiki/Hematopoiesishttp://en.wikipedia.org/wiki/Hematopoiesishttp://en.wikipedia.org/wiki/Bone#cite_note-3http://en.wikipedia.org/wiki/Bone#cite_note-3http://en.wikipedia.org/wiki/Bone#cite_note-3http://en.wikipedia.org/w/index.php?title=Bone&action=edit&secti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The primary tissue of bone,osseous tissue,is a relativelyhardand lightweightcomposite material.It is mostly made up of a

composite material incorporating the mineralcalcium phosphatein the chemical arrangement termed

calciumhydroxylapatite(this is theosseous tissuethat gives bones their rigidity) and collagen, an elastic protein which improves

fracture resistance. It has relatively highcompressive strengthof about 170MPa(1800kgf/cm)[5]

but poortensile strengthof

104121MPaand very lowshear stressstrength (51.6MPa),[6]

meaning it resists pushing forces well, but not pulling or

torsional forces. While bone is essentiallybrittle,it does have a significant degree ofelasticity,contributed chiefly bycollagen.

All bones consist of livingcellsembedded in the mineralized organic matrix that makes up the osseous tissue.

Structure[edit]

Bone structure[edit]

An illustration

Bone is not a uniformly solid material, but rather has some spaces between its hard elements.[citation needed]

Compact (cortical) bone[edit]

The hard outer layer of bones is composed ofcompact bonetissue, so-called due to its minimal gaps and spaces. Its porosity is530%.

[7]This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an

adultskeleton.Compact bone may also be referred to as dense bone.[citation needed]

Trabecular (cancellous or spongy) bone[edit]

Filling the interior of the bone is thetrabecular bonetissue (an open cellporousnetwork also called cancellous or spongy bone),

which is composed of a network of rod- and plate-like elements that make the overall organ lighter and allow room for blood

vessels and marrow. Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface

area of compact bone. Its porosity is 3090%.[7]

If, for any reason, there is an alteration in the strain the cancellous is subjected

to, there is a rearrangement of the trabeculae. The microscopic difference between compact and cancellous bone is that

compact bone consists of haversian sites andosteons,while cancellous bones do not. Also, bone surrounds blood in the

compact bone, while blood surrounds bone in the cancellous bone.[citation needed]

Cellular structure[edit]

There are several types of cells constituting the typical bone:

Osteoblastsare mononucleate bone-forming cells that descend fromosteoprogenitorcells. They are located on the surface

of osteoid seams and make aproteinmixture known asosteoid,which mineralizes to become bone. The osteoid seam is a

narrow region of newly formed organic matrix, not yet mineralized, located on the surface of a bone. Osteoid is primarily

composed of Type Icollagen.Osteoblasts also manufacturehormones,such asprostaglandins,to act on the bone itself.

They robustly producealkaline phosphatase,anenzymethat has a role in the mineralisation of bone, as well as

http://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Rockwell_scalehttp://en.wikipedia.org/wiki/Rockwell_scalehttp://en.wikipedia.org/wiki/Rockwell_scalehttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Calcium_phosphatehttp://en.wikipedia.org/wiki/Calcium_phosphatehttp://en.wikipedia.org/wiki/Calcium_phosphatehttp://en.wikipedia.org/wiki/Hydroxylapatitehttp://en.wikipedia.org/wiki/Hydroxylapatitehttp://en.wikipedia.org/wiki/Hydroxylapatitehttp://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Compressive_strengthhttp://en.wikipedia.org/wiki/Compressive_strengthhttp://en.wikipedia.org/wiki/Compressive_strengthhttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Kgfhttp://en.wikipedia.org/wiki/Kgfhttp://en.wikipedia.org/wiki/Kgfhttp://en.wikipedia.org/wiki/Bone#cite_note-Schmidt-Nielsen-5http://en.wikipedia.org/wiki/Bone#cite_note-Schmidt-Nielsen-5http://en.wikipedia.org/wiki/Bone#cite_note-Schmidt-Nielsen-5http://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Shear_stresshttp://en.wikipedia.org/wiki/Shear_stresshttp://en.wikipedia.org/wiki/Shear_stresshttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Bone#cite_note-6http://en.wikipedia.org/wiki/Bone#cite_note-6http://en.wikipedia.org/wiki/Bone#cite_note-6http://en.wikipedia.org/wiki/Brittlenesshttp://en.wikipedia.org/wiki/Brittlenesshttp://en.wikipedia.org/wiki/Brittlenesshttp://en.wikipedia.org/wiki/Elasticity_(physics)http://en.wikipedia.org/wiki/Elasticity_(physics)http://en.wikipedia.org/wiki/Elasticity_(physics)http://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=8http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=8http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=8http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=9http://en.wikipedia.org/wiki/Cortical_bonehttp://en.wikipedia.org/wiki/Cortical_bonehttp://en.wikipedia.org/wiki/Cortical_bonehttp://en.wikipedia.org/wiki/Bone#cite_note-Hall.2C_Susan_Pg._88-7http://en.wikipedia.org/wiki/Bone#cite_note-Hall.2C_Susan_Pg._88-7http://en.wikipedia.org/wiki/Bone#cite_note-Hall.2C_Susan_Pg._88-7http://en.wikipedia.org/wiki/Skeletonhttp://en.wikipedia.org/wiki/Skeletonhttp://en.wikipedia.org/wiki/Skeletonhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=10http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=10http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=10http://en.wikipedia.org/wiki/Cancellous_bonehttp://en.wikipedia.org/wiki/Cancellous_bonehttp://en.wikipedia.org/wiki/Cancellous_bonehttp://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Bone#cite_note-Hall.2C_Susan_Pg._88-7http://en.wikipedia.org/wiki/Bone#cite_note-Hall.2C_Susan_Pg._88-7http://en.wikipedia.org/wiki/Bone#cite_note-Hall.2C_Susan_Pg._88-7http://en.wikipedia.org/wiki/Osteonshttp://en.wikipedia.org/wiki/Osteonshttp://en.wikipedia.org/wiki/Osteonshttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=12http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=12http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=12http://en.wikipedia.org/wiki/Osteoblasthttp://en.wikipedia.org/wiki/Osteoblasthttp://en.wikipedia.org/wiki/Osteoprogenitorhttp://en.wikipedia.org/wiki/Osteoprogenitorhttp://en.wikipedia.org/wiki/Osteoprogenitorhttp://en.wikipedia.org/wiki/Proteinhttp://en.wikipedia.org/wiki/Proteinhttp://en.wikipedia.org/wiki/Proteinhttp://en.wikipedia.org/wiki/Osteoidhttp://en.wikipedia.org/wiki/Osteoidhttp://en.wikipedia.org/wiki/Osteoidhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Hormonehttp://en.wikipedia.org/wiki/Hormonehttp://en.wikipedia.org/wiki/Hormonehttp://en.wikipedia.org/wiki/Prostaglandinhttp://en.wikipedia.org/wiki/Prostaglandinhttp://en.wikipedia.org/wiki/Prostaglandinhttp://en.wikipedia.org/wiki/Alkaline_phosphatasehttp://en.wikipedia.org/wiki/Alkaline_phosphatasehttp://en.wikipedia.org/wiki/Alkaline_phosphatasehttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/File:Illu_compact_spongy_bone.jpghttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Alkaline_phosphatasehttp://en.wikipedia.org/wiki/Prostaglandinhttp://en.wikipedia.org/wiki/Hormonehttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Osteoidhttp://en.wikipedia.org/wiki/Proteinhttp://en.wikipedia.org/wiki/Osteoprogenitorhttp://en.wikipedia.org/wiki/Osteoblasthttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=12http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Osteonshttp://en.wikipedia.org/wiki/Bone#cite_note-Hall.2C_Susan_Pg._88-7http://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Cancellous_bonehttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=10http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Skeletonhttp://en.wikipedia.org/wiki/Bone#cite_note-Hall.2C_Susan_Pg._88-7http://en.wikipedia.org/wiki/Cortical_bonehttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=9http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=8http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=6http://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Elasticity_(physics)http://en.wikipedia.org/wiki/Brittlenesshttp://en.wikipedia.org/wiki/Bone#cite_note-6http://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Shear_stresshttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Tensile_strengthhttp://en.wikipedia.org/wiki/Bone#cite_note-Schmidt-Nielsen-5http://en.wikipedia.org/wiki/Kgfhttp://en.wikipedia.org/wiki/Pascal_(unit)http://en.wikipedia.org/wiki/Compressive_strengthhttp://en.wikipedia.org/wiki/Osseous_tissuehttp://en.wikipedia.org/wiki/Hydroxylapatitehttp://en.wikipedia.org/wiki/Calcium_phosphatehttp://en.wikipedia.org/wiki/Composite_materialhttp://en.wikipedia.org/wiki/Rockwell_scalehttp://en.wikipedia.org/wiki/Osseous_tissue

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manymatrix proteins.Osteoblasts are the immature bone cells, and eventually become entrapped in the bone matrix to

become osteocytes- the mature bone cell.[citation needed]

Bone lining cells are essentially inactive osteoblasts. They cover all of the available bone surface and function as a barrier

for certainions.[citation needed]

Osteocytesoriginate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that

they themselves produce. The spaces they occupy are known as lacunae.Osteocytes have many processes that reach out

to meet osteoblasts and other osteocytes probably for the purposes of communication. Their functions include, to varying

degrees: formation of bone; matrix maintenance; and calciumhomeostasis.They have also been shown to act as

mechano-sensory receptors regulating the bone's response to stress and mechanical load. They are mature bone

cells.[citation needed]

Osteoclastsare the cells responsible forbone resorption,thus they break down bone. New bone is then formed by the

osteoblasts (remodeling of bone to reduce its volume). Osteoclasts are large, multinucleated cells located on bone surfaces

in what are called Howship's lacunae or resorption pits. These lacunae, or resorption pits, are left behind after the

breakdown of the bone surface. Because the osteoclasts are derived from a monocytestem-celllineage, they are equipped

withphagocytic-like mechanisms similar to circulatingmacrophages.Osteoclasts mature and/or migrate to discrete bone

surfaces. Upon arrival, active enzymes, such astartrate resistant acid phosphatase,aresecretedagainst the mineral

substrate.[citation needed]

Molecular structure[edit]

Matrix[edit]

The majority of bone is made of the bone matrix. It is composed primarily of inorganic hydroxyapatiteand organiccollagen.

Bone is formed by the hardening of this matrix around entrapped cells. When these cells become entrapped from osteoblasts

they become osteocytes.[citation needed]

Inorganic[edit]

The inorganic composition of bone (bone mineral)is formed from carbonatedhydroxyapatite[9][10]

(Ca10(PO4)6(OH)2) with lower

crystallinity.[9][11]

The matrix is initially laid down as unmineralised osteoid (manufactured by osteoblasts). Mineralisation involves

osteoblasts secretingvesiclescontainingalkaline phosphatase.This cleaves the phosphate groups and acts as the foci for

calcium and phosphate deposition. The vesicles then rupture and act as a centre for crystals to grow on. More particularly, bone

mineral is formed from globular and plate structures,[11][12]

distributed among the collagen fibrils of bone and forming yet larger

structure.[citation needed]

Organic[edit]

The organic part of matrix is mainly composed of Type Icollagen.This is synthesised intracellularly as tropocollagen and then

exported, formingfibrils.The organic part is also composed of various growth factors, the functions of which are not fully known.

Factors present includeglycosaminoglycans,osteocalcin,osteonectin,bone sialo protein,osteopontinand Cell Attachment

Factor.[citation needed]

Woven vs. lamellar bone[edit]

Two types of bone can be identified microscopically according to the pattern of collagen forming the osteoid (collagenous

support tissue of type I collagen embedded in glycosaminoglycan gel):

Woven bone, which is characterized by haphazard organization of collagen fibers and is mechanically weak [citation needed]

http://en.wikipedia.org/wiki/Extracellular_matrixhttp://en.wikipedia.org/wiki/Extracellular_matrixhttp://en.wikipedia.org/wiki/Extracellular_matrixhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Osteocytehttp://en.wikipedia.org/wiki/Osteocytehttp://en.wikipedia.org/wiki/Lacuna_(histology)http://en.wikipedia.org/wiki/Lacuna_(histology)http://en.wikipedia.org/wiki/Lacuna_(histology)http://en.wikipedia.org/wiki/Homeostasishttp://en.wikipedia.org/wiki/Homeostasishttp://en.wikipedia.org/wiki/Homeostasishttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Osteoclasthttp://en.wikipedia.org/wiki/Osteoclasthttp://en.wikipedia.org/wiki/Bone_resorptionhttp://en.wikipedia.org/wiki/Bone_resorptionhttp://en.wikipedia.org/wiki/Bone_resorptionhttp://en.wikipedia.org/wiki/Monocytehttp://en.wikipedia.org/wiki/Monocytehttp://en.wikipedia.org/wiki/Stem_cellhttp://en.wikipedia.org/wiki/Stem_cellhttp://en.wikipedia.org/wiki/Stem_cellhttp://en.wikipedia.org/wiki/Phagocytosishttp://en.wikipedia.org/wiki/Phagocytosishttp://en.wikipedia.org/wiki/Phagocytosishttp://en.wikipedia.org/wiki/Macrophageshttp://en.wikipedia.org/wiki/Macrophageshttp://en.wikipedia.org/wiki/Macrophageshttp://en.wikipedia.org/wiki/Tartrate_resistant_acid_phosphatasehttp://en.wikipedia.org/wiki/Tartrate_resistant_acid_phosphatasehttp://en.wikipedia.org/wiki/Tartrate_resistant_acid_phosphatasehttp://en.wikipedia.org/wiki/Secretionhttp://en.wikipedia.org/wiki/Secretionhttp://en.wikipedia.org/wiki/Secretionhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=13http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=13http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=13http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=14http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=14http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=14http://en.wikipedia.org/wiki/Hydroxyapatitehttp://en.wikipedia.org/wiki/Hydroxyapatitehttp://en.wikipedia.org/wiki/Hydroxyapatitehttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=15http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=15http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=15http://en.wikipedia.org/wiki/Bone_mineralhttp://en.wikipedia.org/wiki/Bone_mineralhttp://en.wikipedia.org/wiki/Bone_mineralhttp://en.wikipedia.org/wiki/Hydroxyapatitehttp://en.wikipedia.org/wiki/Hydroxyapatitehttp://en.wikipedia.org/wiki/Hydroxyapatitehttp://en.wikipedia.org/wiki/Bone#cite_note-10http://en.wikipedia.org/wiki/Bone#cite_note-10http://en.wikipedia.org/wiki/Bone#cite_note-Legros.2C_R._1987-9http://en.wikipedia.org/wiki/Bone#cite_note-Legros.2C_R._1987-9http://en.wikipedia.org/wiki/Bone#cite_note-Legros.2C_R._1987-9http://en.wikipedia.org/wiki/Vesicle_(biology)http://en.wikipedia.org/wiki/Vesicle_(biology)http://en.wikipedia.org/wiki/Vesicle_(biology)http://en.wikipedia.org/wiki/Alkaline_phosphatasehttp://en.wikipedia.org/wiki/Alkaline_phosphatasehttp://en.wikipedia.org/wiki/Alkaline_phosphatasehttp://en.wikipedia.org/wiki/Bone#cite_note-r1-11http://en.wikipedia.org/wiki/Bone#cite_note-r1-11http://en.wikipedia.org/wiki/Bone#cite_note-r1-11http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=16http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=16http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=16http://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Fibrilhttp://en.wikipedia.org/wiki/Fibrilhttp://en.wikipedia.org/wiki/Fibrilhttp://en.wikipedia.org/wiki/Glycosaminoglycanhttp://en.wikipedia.org/wiki/Glycosaminoglycanhttp://en.wikipedia.org/wiki/Osteocalcinhttp://en.wikipedia.org/wiki/Osteocalcinhttp://en.wikipedia.org/wiki/Osteocalcinhttp://en.wikipedia.org/wiki/Osteonectinhttp://en.wikipedia.org/wiki/Osteonectinhttp://en.wikipedia.org/wiki/Osteonectinhttp://en.wikipedia.org/wiki/Bone_sialo_proteinhttp://en.wikipedia.org/wiki/Bone_sialo_proteinhttp://en.wikipedia.org/wiki/Bone_sialo_proteinhttp://en.wikipedia.org/wiki/Osteopontinhttp://en.wikipedia.org/wiki/Osteopontinhttp://en.wikipedia.org/wiki/Osteopontinhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=17http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=17http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=17http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=17http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Osteopontinhttp://en.wikipedia.org/wiki/Bone_sialo_proteinhttp://en.wikipedia.org/wiki/Osteonectinhttp://en.wikipedia.org/wiki/Osteocalcinhttp://en.wikipedia.org/wiki/Glycosaminoglycanhttp://en.wikipedia.org/wiki/Fibrilhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=16http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Bone#cite_note-r1-11http://en.wikipedia.org/wiki/Bone#cite_note-r1-11http://en.wikipedia.org/wiki/Alkaline_phosphatasehttp://en.wikipedia.org/wiki/Vesicle_(biology)http://en.wikipedia.org/wiki/Bone#cite_note-Legros.2C_R._1987-9http://en.wikipedia.org/wiki/Bone#cite_note-Legros.2C_R._1987-9http://en.wikipedia.org/wiki/Bone#cite_note-10http://en.wikipedia.org/wiki/Hydroxyapatitehttp://en.wikipedia.org/wiki/Hydroxyapatitehttp://en.wikipedia.org/wiki/Bone_mineralhttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=15http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Collagenhttp://en.wikipedia.org/wiki/Hydroxyapatitehttp://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=14http://en.wikipedia.org/w/index.php?title=Bone&action=edit&section=13http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Secretionhttp://en.wikipedia.org/wiki/Tartrate_resistant_acid_phosphatasehttp://en.wikipedia.org/wiki/Macrophageshttp://en.wikipedia.org/wiki/Phagocytosishttp://en.wikipedia.org/wiki/Stem_cellhttp://en.wikipedia.org/wiki/Monocytehttp://en.wikipedia.org/wiki/Bone_resorptionhttp://en.wikipedia.org/wiki/Osteoclasthttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Homeostasishttp://en.wikipedia.org/wiki/Lacuna_(histology)http://en.wikipedia.org/wiki/Osteocytehttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Ionhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Extracellular_matrix

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Lamellar bone, which has a regular parallel alignment of collagen into sheets (lamellae) and is mechanically strong[citation

needed]

Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all fetalbones (but is later replaced

by more resilient lamellar bone). In adults woven bone is created afterfracturesor inPaget's disease.Woven bone is weaker,

with a smaller number of randomly oriented collagen fibers, but forms quickly; it is for this appearance of the fibrous matrix that

the bone is termed woven. It is soon replaced by lamellar bone, which is highly organized inconcentricsheets with a much

lower proportion of osteocytes to surrounding tissue. Lamellar bone, which makes its first appearance in thefetusduring the

third trimester,[13]

is stronger and filled with many collagen fibers parallel to other fibers in the same layer (these parallel columns

are called osteons). Incross-section,the fibers run in opposite directions in alternating layers, much like inplywood,assisting in

the bone's ability to resisttorsionforces. After a fracture, woven bone forms initially and is gradually replaced by lamellar bone

during a process known as "bony substitution." Compared to woven bone, lamellar bone formation takes place more slowly. The

orderly deposition of collagen fibers restricts the formation of osteoid to about 1 to 2mper day. Lamellar bone also requires a

relatively flat surface to lay the collagen fibers in parallel or concentric layers.[citation needed]

These terms arehistologic,in that a microscope is necessary to differentiate between the two.[citation needed]

Types[edit]

There are five types of bones in the human body: long, short, flat, irregular, and sesamoid.

Long bonesare characterized by a shaft, thediaphysis,that is much longer than it is wide. They are made up mostly

ofcompact bone,with lesser amounts ofmarrow,located within themedullary cavity,andspongy bone.Most bones of

thelimbs,including those of thefingersandtoes,are long bones. The exceptions are those of

thewrist,ankleandkneecap.[citation needed]

Short bonesare roughlycube-shaped, and have only a thin layer of compact bone surrounding a spongy interior. The

bones of the wrist and ankle are short bones, as are thesesamoid bones.[citation needed]

Flat bonesare thin and generally curved, with two parallel layers of compact bones sandwiching a layer of spongy bone.

Most of the bones of theskullare flat bones, as is thesternum.[citation needed]

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Sesamoid bonesare bones embedded in tendons. Since they act to hold the tendon further away from the joint, the angle

of the tendon is increased and thus the leverage of the muscle is increased. Examples of sesamoid bones are

thepatellaand thepisiform.[citation needed]

Irregular bonesdo not fit into the above categories. They consist of thin layers of compact bone surrounding a spongy

interior. As implied by the name, their shapes are irregular and complicated. Often this irregular shape is due to their many

centers of ossification or because they contain bony sinuses. The bones of the spine,Pelvis,and some bones of the skull

are irregular bones. Examples include theethmoidandsphenoidbones.[14]

Formation[edit]

The formation of bone during the fetal stage of development occurs by two processes:Intramembranous

ossificationandendochondral ossification.[citation needed]

Intramembranous ossification[edit]

Intramembranous ossification mainly occurs during formation of the flat bones of the skullbut also the mandible, maxilla, and

clavicles; the bone is formed from connective tissue such asmesenchymetissue rather than from cartilage. The steps in

intramembranous ossification are:[citation needed]

1. Development of ossification center

2. Calcification

3. Formation of trabeculae

4. Development of periosteum

Endochondral ossification[edit]

Endochondral ossification, on the other hand, occurs in long bones and most of the rest of the bones in the body; it involves an

initial hyaline cartilage that continues to grow. The steps in endochondral ossification are:[citation needed]

1. Development of cartilage model

2. Growth of cartilage model

3. Development of the primary ossification center

4. Development of the secondary ossification center

5. Formation of articular cartilage andepiphyseal plate

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Endochondral ossification begins with points in the cartilage called "primary ossification centers." They mostly appear during

fetal development, though a few short bones begin their primary ossification afterbirth.They are responsible for the formation of

the diaphyses of long bones, short bones and certain parts of irregular bones. Secondary ossification occurs after birth, and

forms theepiphysesof long bones and the extremities of irregular and flat bones. The diaphysis and both epiphyses of a long

bone are separated by a growing zone of cartilage (theepiphyseal plate). When the child reaches skeletal maturity (18 to 25

years of age), all of the cartilage is replaced by bone, fusing the diaphysis and both epiphyses together (epiphyseal

closure).[citation needed]

In the upper limbs, only the diaphyses of the long bones and scapula are ossified. The epiphyses, carpal bones, coracoid

process, medial border of the scapula, and acromion are still cartilaginous .[15]

The following steps are followed in the conversion

of cartilage to bone:

1. Zone of reserve cartilage. This region, farthest from the marrow cavity, consists of typical hyaline cartilage that as yet shows

no sign of transforming into bone.[16]

2. Zone of cell proliferation. A little closer to the marrow cavity, chondrocytes multiply and arrange themselves into longitudinal

columns of flattened lacunae.

[16]

3. Zone of cell hypertrophy. Next, the chondrocytes cease to divide and begin to hypertrophy (enlarge), much like they do in the

primary ossification center of the fetus. The walls of the matrix between lacunae become very thin.[16]

4. Zone of calcification. Minerals are deposited in the matrix between the columns of lacunae and calcify the cartilage. These

are not the permanent mineral deposits of bone, but only a temporary support for the cartilage that would otherwise soon be

weakened by the breakdown of the enlarged lacunae.[16]

5. Zone of bone deposition. Within each column, the walls between the lacunae break down and the chondrocytes die. This

converts each column into a longitudinal channel, which is immediately invaded by blood vessels and marrow from the marrow

cavity. Osteoblasts line up along the walls of these channels and begin depositing concentric lamellae of matrix, while

osteoclasts dissolve the temporarily calcified cartilage.[16]

Bone marrow[edit]

Bone marrowcan be found in almost any bone that holdscancellous tissue.Innewborns,all such bones are filled exclusively

with red marrow, but as the child ages it is mostly replaced by yellow, or fattymarrow. In adults, red marrow is mostly found in

the marrow bones of the femur, the ribs, the vertebrae andpelvic bones.[citation needed]

Remodeling[edit]

Remodelingor bone turnoveris the process of resorption followed by replacement of bone with little change in shape and

occurs throughout a person's life. Osteoblasts and osteoclasts, coupled together viaparacrine cell signalling,are referred to as

bone remodeling unit. Approximately 10% of the skeletal mass of an adult is remodelled each year.[17]

Purpose[edit]

The purpose of remodeling is to regulatecalcium homeostasis,repairmicro-damaged bones(from everyday stress) but also to

shape and sculpt the skeleton during growth.[citation needed]

Calcium balance[edit]

The process of bone resorption by the osteoclasts releases stored calcium into the systemic circulation and is an important

process in regulating calcium balance. As bone formation actively fixescirculating calcium in its mineral form, removing it from

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the bloodstream, resorption actively unfixesit thereby increasing circulating calcium levels. These processes occur in tandem at

site-specific locations.[citation needed]

Bone volume[edit]

Bone volume is determined by the rates of bone formation and bone resorption. Recent research has suggested that certain

growth factors may work to locally alter bone formation by increasing osteoblast activity. Numerous bone-derived growth factors

have been isolated and classified via bone cultures. These factors include insulin-like growth factors I and II, transforming

growth factor-beta, fibroblast growth factor, platelet-derived growth factor, and bone morphogenetic proteins.[18]

Evidence

suggests that bone cells produce growth factors for extracellular storage in the bone matrix. The release of these growth factors

from the bone matrix could cause the proliferation of osteoblast precursors. Essentially, bone growth factors may act as

potential determinants of local bone formation.[18]

Research has suggested that trabecular bone volume in postemenopausal

osteoporosis may be determined by the relationship between the total bone forming surface and the percent of surface

resorption.[19]

Repair[edit]

Repeated stress, such as weight-bearingexerciseor bone healing, results in the bone thickening at the points of maximum

stress (Wolff's law). It has been hypothesized that this is a result of bone's piezoelectricproperties, which cause bone to

generate small electrical potentials under stress.[20]

Paracrine cell signalling[edit]

The action ofosteoblastsandosteoclastsare controlled by a number of chemicalfactorsthat either promote or inhibit the

activity of the bone remodeling cells, controlling the rate at which bone is made, destroyed, or changed in shape. The cells also

useparacrine signallingto control the activity of each other.[citation needed]

Osteoblast stimulation[edit]

Osteoblasts can be stimulated to increase bone mass through increased secretion ofosteoidand byinhibitingthe ability of

osteoclasts to break downosseous tissue.[citation needed]

Bone building through increased secretion of osteoid is stimulated by the secretion ofgrowth hormoneby thepituitary,thyroid

hormoneand the sex hormones (estrogensandandrogens). These hormones also promote increased secretion

ofosteoprotegerin.[21]

Osteoblasts can also be induced to secrete a number ofcytokinesthat promote reabsorbtion of bone by

stimulating osteoclast activity and differentiation from progenitor cells. Vitamin D,parathyroid hormoneand stimulation from

osteocytes induce osteoblasts to increase secretion of RANK-ligandandinterleukin 6,which cytokines then stimulate increased

reabsorption of bone by osteoclasts. These same compounds also increase secretion ofmacrophage colony-stimulating

factorby osteoblasts, which promotes the differentiation of progenitor cells into osteoclasts, and decrease secretion

ofosteoprotegerin.[citation needed]

Osteoclast inhibition[edit]

The rate at which osteoclasts resorb bone is inhibited bycalcitoninand osteoprotegerin. Calcitonin is produced byparafollicular

cellsin thethyroid gland,and can bind to receptors on osteoclasts to directly inhibit osteoclast activity. Osteoprotegerin is

secreted by osteoblasts and is able to bind RANK-L, inhibiting osteoclast stimulation.[21]

HORMONES INVOLVED IN BONE METABOLISM AND REMODELLING

The main hormones involved in bone metabolism and remodelling are parathyroid hormone (PTH), members

of the vitamin D family, oestrogens and calcitonin. Glucocorticoids and thyroid hormone also affect bone.

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PARATHYROID HORMONE

Parathyroid hormone, which consists of a single-chain polypeptide of 84 amino acids, is an important

physiological regulator of Ca2+metabolism. It acts on PTH receptors in various tissues (bone, kidney,

gastrointestinal tract) to maintain the plasma Ca2+concentration. It mobilises Ca2+from bone, promotes its

reabsorption by the kidney and stimulates the synthesis of calcitriol, which in turn increases Ca2+absorption

from the intestine and synergises with PTH in mobilising bone Ca2+(Figs 35.3and35.4). PTH promotes

phosphate excretion, and thus its net effect is to increase the concentration of Ca2+in the plasma and lower that

of phosphate.

The mobilisation of Ca2+from bone by PTH is mediated, at least in part, by stimulation of the recruitment

and activation of osteoclasts. Pathological oversecretion of PTH (hyperparathyroidism) inhibits osteoblast

activity (not shown inFig. 35.1). But given therapeutically in a low intermittent dose, PTH and fragments of

PTH paradoxically stimulate osteoblast activity and enhance bone formation.

Parathyroid hormone is synthesised in the cells of the parathyroid glands and stored in vesicles. The

principal factor controlling secretion is the concentration of ionised calcium in the plasma, low plasma

Ca2+stimulating secretion, high plasma Ca2+decreasing it by binding to and activating a Ca2+-sensing G-

protein-coupled surface receptor (seeCh. 3,Figure 35.3). (For reviews, see Stewart, 2004; Deal, 2009.)

VITAMIN D

Vitamin D (calciferol) consists of a group of lipophilic prehormones that are converted in the body into a

number of biologically active metabolites that function as true hormones, circulating in the blood and

regulating the activities of various cell types (see Reichel et al., 1989). Their main action, mediated by nuclear

receptors of the steroid receptor superfamily (seeCh. 3), is the maintenance of plasma Ca2+by increasing

Ca2+absorption in the intestine, mobilising Ca2+from bone and decreasing its renal excretion (seeFig. 35.3). In

humans, there are two sources of vitamin D:

1. Dietary ergocalciferol(D2), derived from ergosterol in plants.

2. Cholecalciferol(D3) generated in the skin from 7-dehydrocholesterol by the action of ultraviolet irradiation, the 7-

dehydrocholesterol having been formed from cholesterol in the wall of the intestine.

Cholecalciferol is converted to calcifediol(25-hydroxy-vitamin D3) in the liver, and this is converted to a series

of other metabolites of varying activity in the kidney, the most potent of which is calcitriol(1,25-dihydroxy-

vitamin D3); seeFig. 35.4).

The synthesis of calcitriol from calcifediol is regulated by PTH, and is also influenced by the phosphate

concentration in the plasma and by the calcitriol concentration itself through a negative feedback mechanism

(Fig. 35.4). Receptors for calcitriol are ubiquitous, and calcitriol is important in the functioning of many cell

types.

The main actions of calcitriol are the stimulation of absorption of Ca2+and phosphate in the intestine, and

the mobilisation of Ca2+from bone, but it also increases Ca2+reabsorption in the kidney tubules (Fig. 35.3). Its

effect on bone involves promotion of maturation of osteoclasts and indirect stimulation of their activity (Figs

35.1and35.3). It decreases collagen synthesis by osteoblasts. However, the effect on bone is complex and not

confined to mobilising Ca2+, because in clinical vitamin D deficiency (see below), in which the mineralisation of

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bone is impaired, administration of vitamin D restores bone formation. One explanation may lie in the fact that

calcitriol stimulates synthesis of osteocalcin, the Ca2+-binding protein of bone matrix.

OESTROGENS

During reproductive life in the female, oestrogens have an important role in maintenance of bone integrity,

acting on both osteoblasts and osteoclasts. They inhibit the cytokines that recruit osteoclasts and oppose the

bone-resorbing, Ca2+-mobilising action of PTH. They increase osteoblast proliferation, augment the production

of TGF-and bone morphogenic proteins, and inhibit apoptosis (seeCh. 5). Withdrawal of oestrogen, as

happens at the menopause, can (and usually does) lead to osteoporosis.Parathyroid, vitamin D and bone mineral homeostasis

The vitamin D family are true hormones; precursors are converted to calcifediol in the liver, then to the main hormone,calcitriol, in the kidney.

Calcitriol increases plasma Ca2+by mobilising it from bone, increasing its absorption in the intestine and decreasing itsexcretion by the kidney.

Parathyroid hormone (PTH) increases blood Ca2+by increasing calcitriol synthesis, mobilising Ca2+from bone and

reducing renal Ca2+excretion. (But, paradoxically, small doses of PTH given intermittently increase bone formation.) Calcitonin (secreted from the thyroid) reduces Ca2+resorption from bone by inhibiting osteoclast activity.

CALCITONIN

Calcitonin is a peptide hormone secreted by the specialised C cells found in the thyroid follicles (seeCh. 33).

The main action of calcitonin is on bone; it inhibits bone resorption by binding to a specific receptor on

osteoclasts, inhibiting their action. In the kidney, it decreases the reabsorption of both Ca2+and phosphate in

the proximal tubules. Its overall effect is to decrease the plasma Ca2+concentration (Fig. 35.3).

Secretion is determined mainly by the plasma Ca2+

concentration.

OTHER HORMONES

Physiological concentrations of glucocorticoids are required for osteoblast differentiation. Excessive

pharmacological concentrations inhibit bone formation by inhibiting osteoblast differentiation and activity,

and may stimulate osteoclast actionleading to osteoporosis, which is a feature of Cushings syndrome (Fig.

32.7)and an important adverse effect of glucocorticoid administration (Ch. 32).

Thyroxine stimulates osteoclast action, reducing bone density and liberating Ca2+. Osteoporosis occurs in

association with thyrotoxicosis, and care must be taken not to use excessive thyroxine dosage for treating

hypothyroidism (seeCh. 33).

Lumbar vertebrae

In human anatomy, the lumbar vertebraeare the five vertebrae between therib cageand thepelvis.They are the largest

segments of thevertebral columnand are characterized by the absence of theforamen transversariumwithin the transverse

process (as it is only found in the cervical region), and by the absence of facets on the sides of the body. They are designated

L1 to L5, starting at the top. The lumbar vertebrae help support the weight of the body, and permit movement.

Human anatomy[edit]General characteristics[edit]

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