Bone

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For other uses, see Bone (disambiguation) or Bones (disambiguation); note that this article uses anatomical terminology. Bone_sentence_0

Bone_table_infobox_0

BoneBone_header_cell_0_0_0
IdentifiersBone_header_cell_0_1_0
MeSHBone_header_cell_0_2_0 Bone_cell_0_2_1
TA98Bone_header_cell_0_3_0 Bone_cell_0_3_1
TA2Bone_header_cell_0_4_0 ,Bone_cell_0_4_1
THBone_header_cell_0_5_0 Bone_cell_0_5_1
FMABone_header_cell_0_6_0 Bone_cell_0_6_1

A bone is a rigid organ that constitutes part of the vertebrate skeleton in animals. Bone_sentence_1

Bones protect the various organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and enable mobility. Bone_sentence_2

Bones come in a variety of shapes and sizes and have a complex internal and external structure. Bone_sentence_3

They are lightweight yet strong and hard, and serve multiple functions. Bone_sentence_4

Bone tissue (osseous tissue) is a hard tissue, a type of dense connective tissue. Bone_sentence_5

It has a honeycomb-like matrix internally, which helps to give the bone rigidity. Bone_sentence_6

Bone tissue is made up of different types of bone cells. Bone_sentence_7

Osteoblasts and osteocytes are involved in the formation and mineralization of bone; osteoclasts are involved in the resorption of bone tissue. Bone_sentence_8

Modified (flattened) osteoblasts become the lining cells that form a protective layer on the bone surface. Bone_sentence_9

The mineralised matrix of bone tissue has an organic component of mainly collagen called ossein and an inorganic component of bone mineral made up of various salts. Bone_sentence_10

Bone tissue is a mineralized tissue of two types, cortical bone and cancellous bone. Bone_sentence_11

Other types of tissue found in bones include bone marrow, endosteum, periosteum, nerves, blood vessels and cartilage. Bone_sentence_12

In the human body at birth, there are approximately 270 bones present; many of these fuse together during development, leaving a total of 206 separate bones in the adult, not counting numerous small sesamoid bones. Bone_sentence_13

The largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear. Bone_sentence_14

The Greek word for bone is ὀστέον ("osteon"), hence the many terms that use it as a prefix—such as osteopathy. Bone_sentence_15

Structure Bone_section_0

Bone is not uniformly solid, but consists of a flexible matrix (about 30%) and bound minerals (about 70%) which are intricately woven and endlessly remodeled by a group of specialized bone cells. Bone_sentence_16

Their unique composition and design allows bones to be relatively hard and strong, while remaining lightweight. Bone_sentence_17

Bone matrix is 90 to 95% composed of elastic collagen fibers, also known as ossein, and the remainder is ground substance. Bone_sentence_18

The elasticity of collagen improves fracture resistance. Bone_sentence_19

The matrix is hardened by the binding of inorganic mineral salt, calcium phosphate, in a chemical arrangement known as calcium hydroxylapatite. Bone_sentence_20

It is the bone mineralization that give bones rigidity. Bone_sentence_21

Bone is actively constructed and remodeled throughout life by special bone cells known as osteoblasts and osteoclasts. Bone_sentence_22

Within any single bone, the tissue is woven into two main patterns, known as cortical and cancellous bone, and each with different appearance and characteristics. Bone_sentence_23

Cortical bone Bone_section_1

The hard outer layer of bones is composed of cortical bone, which is also called compact bone as it is much denser than cancellous bone. Bone_sentence_24

It forms the hard exterior (cortex) of bones. Bone_sentence_25

The cortical bone gives bone its smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult human skeleton. Bone_sentence_26

It facilitates bone's main functions—to support the whole body, to protect organs, to provide levers for movement, and to store and release chemical elements, mainly calcium. Bone_sentence_27

It consists of multiple microscopic columns, each called an osteon or Haversian system. Bone_sentence_28

Each column is multiple layers of osteoblasts and osteocytes around a central canal called the haversian canal. Bone_sentence_29

Volkmann's canals at right angles connect the osteons together. Bone_sentence_30

The columns are metabolically active, and as bone is reabsorbed and created the nature and location of the cells within the osteon will change. Bone_sentence_31

Cortical bone is covered by a periosteum on its outer surface, and an endosteum on its inner surface. Bone_sentence_32

The endosteum is the boundary between the cortical bone and the cancellous bone. Bone_sentence_33

The primary anatomical and functional unit of cortical bone is the osteon. Bone_sentence_34

Cancellous bone Bone_section_2

Cancellous bone, also called trabecular or spongy bone, is the internal tissue of the skeletal bone and is an open cell porous network. Bone_sentence_35

Cancellous bone has a higher surface-area-to-volume ratio than cortical bone and it is less dense. Bone_sentence_36

This makes it weaker and more flexible. Bone_sentence_37

The greater surface area also makes it suitable for metabolic activities such as the exchange of calcium ions. Bone_sentence_38

Cancellous bone is typically found at the ends of long bones, near joints and in the interior of vertebrae. Bone_sentence_39

Cancellous bone is highly vascular and often contains red bone marrow where hematopoiesis, the production of blood cells, occurs. Bone_sentence_40

The primary anatomical and functional unit of cancellous bone is the trabecula. Bone_sentence_41

The trabeculae are aligned towards the mechanical load distribution that a bone experiences within long bones such as the femur. Bone_sentence_42

As far as short bones are concerned, trabecular alignment has been studied in the vertebral pedicle. Bone_sentence_43

Thin formations of osteoblasts covered in endosteum create an irregular network of spaces, known as trabeculae. Bone_sentence_44

Within these spaces are bone marrow and hematopoietic stem cells that give rise to platelets, red blood cells and white blood cells. Bone_sentence_45

Trabecular marrow 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. Bone_sentence_46

Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone. Bone_sentence_47

The words cancellous and trabecular refer to the tiny lattice-shaped units (trabeculae) that form the tissue. Bone_sentence_48

It was first illustrated accurately in the engravings of Crisóstomo Martinez. Bone_sentence_49

Bone marrow Bone_section_3

Bone marrow, also known as myeloid tissue in red bone marrow, can be found in almost any bone that holds cancellous tissue. Bone_sentence_50

In newborns, all such bones are filled exclusively with red marrow or hematopoietic marrow, but as the child ages the hematopoietic fraction decreases in quantity and the fatty/ yellow fraction called marrow adipose tissue (MAT) increases in quantity. Bone_sentence_51

In adults, red marrow is mostly found in the bone marrow of the femur, the ribs, the vertebrae and pelvic bones. Bone_sentence_52

Bone cells Bone_section_4

Bone is a metabolically active tissue composed of several types of cells. Bone_sentence_53

These cells include osteoblasts, which are involved in the creation and mineralization of bone tissue, osteocytes, and osteoclasts, which are involved in the reabsorption of bone tissue. Bone_sentence_54

Osteoblasts and osteocytes are derived from osteoprogenitor cells, but osteoclasts are derived from the same cells that differentiate to form macrophages and monocytes. Bone_sentence_55

Within the marrow of the bone there are also hematopoietic stem cells. Bone_sentence_56

These cells give rise to other cells, including white blood cells, red blood cells, and platelets. Bone_sentence_57

Osteoblast Bone_section_5

Osteoblasts are mononucleate bone-forming cells. Bone_sentence_58

They are located on the surface of osteon seams and make a protein mixture known as osteoid, which mineralizes to become bone. Bone_sentence_59

The osteoid seam is a narrow region of newly formed organic matrix, not yet mineralized, located on the surface of a bone. Bone_sentence_60

Osteoid is primarily composed of Type I collagen. Bone_sentence_61

Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. Bone_sentence_62

The osteoblast creates and repairs new bone by actually building around itself. Bone_sentence_63

First, the osteoblast puts up collagen fibers. Bone_sentence_64

These collagen fibers are used as a framework for the osteoblasts' work. Bone_sentence_65

The osteoblast then deposits calcium phosphate which is hardened by hydroxide and bicarbonate ions. Bone_sentence_66

The brand-new bone created by the osteoblast is called osteoid. Bone_sentence_67

Once the osteoblast is finished working it is actually trapped inside the bone once it hardens. Bone_sentence_68

When the osteoblast becomes trapped, it becomes known as an osteocyte. Bone_sentence_69

Other osteoblasts remain on the top of the new bone and are used to protect the underlying bone, these become known as lining cells. Bone_sentence_70

Osteocyte Bone_section_6

Osteocytes are cells of mesenchymal origin and originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produced. Bone_sentence_71

The spaces the cell body of osteocytes occupy within the mineralized collagen type I matrix are known as lacunae, while the osteocyte cell processes occupy channels called canaliculi. Bone_sentence_72

The many processes of osteocytes reach out to meet osteoblasts, osteoclasts, bone lining cells, and other osteocytes probably for the purposes of communication. Bone_sentence_73

Osteocytes remain in contact with other osteocytes in the bone through gap junctions—coupled cell processes which pass through the canalicular channels. Bone_sentence_74

Osteoclast Bone_section_7

Osteoclasts are very large multinucleate cells that are responsible for the breakdown of bones by the process of bone resorption. Bone_sentence_75

New bone is then formed by the osteoblasts. Bone_sentence_76

Bone is constantly remodeled by the resorption of osteoclasts and created by osteoblasts. Bone_sentence_77

Osteoclasts are large cells with multiple nuclei located on bone surfaces in what are called Howship's lacunae (or resorption pits). Bone_sentence_78

These lacunae are the result of surrounding bone tissue that has been reabsorbed. Bone_sentence_79

Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with phagocytic-like mechanisms similar to circulating macrophages. Bone_sentence_80

Osteoclasts mature and/or migrate to discrete bone surfaces. Bone_sentence_81

Upon arrival, active enzymes, such as tartrate-resistant acid phosphatase, are secreted against the mineral substrate. Bone_sentence_82

The reabsorption of bone by osteoclasts also plays a role in calcium homeostasis. Bone_sentence_83

Composition Bone_section_8

Main article: Extracellular matrix Bone_sentence_84

Bones consist of living cells embedded in a mineralized organic matrix. Bone_sentence_85

This matrix consists of organic components, mainly type I collagen—"organic" referring to materials produced as a result of the human body—and inorganic components, primarily hydroxyapatite and other salts of calcium and phosphate. Bone_sentence_86

Above 30% of the acellular part of bone consists of the organic components, and 70% of salts. Bone_sentence_87

The collagen fibers give bone its tensile strength, and the interspersed crystals of hydroxyapatite give bone its compressive strength. Bone_sentence_88

These effects are synergistic. Bone_sentence_89

The inorganic composition of bone (bone mineral) is primarily formed from salts of calcium and phosphate, the major salt being hydroxyapatite (Ca10(PO4)6(OH)2). Bone_sentence_90

The exact composition of the matrix may be subject to change over time due to nutrition and biomineralization, with the ratio of calcium to phosphate varying between 1.3 and 2.0 (per weight), and trace minerals such as magnesium, sodium, potassium and carbonate also being found. Bone_sentence_91

Type I collagen composes 90–95% of the organic matrix, with remainder of the matrix being a homogenous liquid called ground substance consisting of proteoglycans such as hyaluronic acid and chondroitin sulfate, as well as non-collagenous proteins such as osteocalcin, osteopontin or bone sialoprotein. Bone_sentence_92

Collagen consists of strands of repeating units, which give bone tensile strength, and are arranged in an overlapping fashion that prevents shear stress. Bone_sentence_93

The function of ground substance is not fully known. Bone_sentence_94

Two types of bone can be identified microscopically according to the arrangement of collagen: woven and lamellar. Bone_sentence_95

Bone_unordered_list_0

  • Woven bone (also known as fibrous bone), which is characterized by a haphazard organization of collagen fibers and is mechanically weak.Bone_item_0_0
  • Lamellar bone, which has a regular parallel alignment of collagen into sheets ("lamellae") and is mechanically strong.Bone_item_0_1

Woven bone is produced when osteoblasts produce osteoid rapidly, which occurs initially in all fetal bones, but is later replaced by more resilient lamellar bone. Bone_sentence_96

In adults woven bone is created after fractures or in Paget's disease. Bone_sentence_97

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. Bone_sentence_98

It is soon replaced by lamellar bone, which is highly organized in concentric sheets with a much lower proportion of osteocytes to surrounding tissue. Bone_sentence_99

Lamellar bone, which makes its first appearance in humans in the fetus during the third trimester, is stronger and filled with many collagen fibers parallel to other fibers in the same layer (these parallel columns are called osteons). Bone_sentence_100

In cross-section, the fibers run in opposite directions in alternating layers, much like in plywood, assisting in the bone's ability to resist torsion forces. Bone_sentence_101

After a fracture, woven bone forms initially and is gradually replaced by lamellar bone during a process known as "bony substitution." Bone_sentence_102

Compared to woven bone, lamellar bone formation takes place more slowly. Bone_sentence_103

The orderly deposition of collagen fibers restricts the formation of osteoid to about 1 to 2 µm per day. Bone_sentence_104

Lamellar bone also requires a relatively flat surface to lay the collagen fibers in parallel or concentric layers. Bone_sentence_105

Deposition Bone_section_9

The extracellular matrix of bone is laid down by osteoblasts, which secrete both collagen and ground substance. Bone_sentence_106

These synthesise collagen within the cell, and then secrete collagen fibrils. Bone_sentence_107

The collagen fibers rapidly polymerise to form collagen strands. Bone_sentence_108

At this stage they are not yet mineralised, and are called "osteoid". Bone_sentence_109

Around the strands calcium and phosphate precipitate on the surface of these strands, within days to weeks becoming crystals of hydroxyapatite. Bone_sentence_110

In order to mineralise the bone, the osteoblasts secrete vesicles containing alkaline phosphatase. Bone_sentence_111

This cleaves the phosphate groups and acts as the foci for calcium and phosphate deposition. Bone_sentence_112

The vesicles then rupture and act as a centre for crystals to grow on. Bone_sentence_113

More particularly, bone mineral is formed from globular and plate structures. Bone_sentence_114

Types Bone_section_10

There are five types of bones in the human body: long, short, flat, irregular, and sesamoid. Bone_sentence_115

Bone_unordered_list_1

  • Long bones are characterized by a shaft, the diaphysis, that is much longer than its width; and by an epiphysis, a rounded head at each end of the shaft. They are made up mostly of compact bone, with lesser amounts of marrow, located within the medullary cavity, and areas of spongy, cancellous bone at the ends of the bones. Most bones of the limbs, including those of the fingers and toes, are long bones. The exceptions are the eight carpal bones of the wrist, the seven articulating tarsal bones of the ankle and the sesamoid bone of the kneecap. Long bones such as the clavicle, that have a differently shaped shaft or ends are also called modified long bones.Bone_item_1_2
  • Short bones are roughly cube-shaped, and have only a thin layer of compact bone surrounding a spongy interior. The bones of the wrist and ankle are short bones.Bone_item_1_3
  • Flat bones are thin and generally curved, with two parallel layers of compact bones sandwiching a layer of spongy bone. Most of the bones of the skull are flat bones, as is the sternum.Bone_item_1_4
  • Sesamoid bones are 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 the patella and the pisiform.Bone_item_1_5
  • Irregular bones do 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 the ethmoid and sphenoid bones.Bone_item_1_6

Terminology Bone_section_11

Main article: Anatomical terms of bone Bone_sentence_116

In the study of anatomy, anatomists use a number of anatomical terms to describe the appearance, shape and function of bones. Bone_sentence_117

Other anatomical terms are also used to describe the location of bones. Bone_sentence_118

Like other anatomical terms, many of these derive from Latin and Greek. Bone_sentence_119

Some anatomists still use Latin to refer to bones. Bone_sentence_120

The term "osseous", and the prefix "osteo-", referring to things related to bone, are still used commonly today. Bone_sentence_121

Some examples of terms used to describe bones include the term "foramen" to describe a hole through which something passes, and a "canal" or "meatus" to describe a tunnel-like structure. Bone_sentence_122

A protrusion from a bone can be called a number of terms, including a "condyle", "crest", "spine", "eminence", "tubercle" or "tuberosity", depending on the protrusion's shape and location. Bone_sentence_123

In general, long bones are said to have a "head", "neck", and "body". Bone_sentence_124

When two bones join together, they are said to "articulate". Bone_sentence_125

If the two bones have a fibrous connection and are relatively immobile, then the joint is called a "suture". Bone_sentence_126

Development Bone_section_12

The formation of bone is called ossification. Bone_sentence_127

During the fetal stage of development this occurs by two processes: intramembranous ossification and endochondral ossification. Bone_sentence_128

Intramembranous ossification involves the formation of bone from connective tissue whereas endochondral ossification involves the formation of bone from cartilage. Bone_sentence_129

Intramembranous ossification mainly occurs during formation of the flat bones of the skull but also the mandible, maxilla, and clavicles; the bone is formed from connective tissue such as mesenchyme tissue rather than from cartilage. Bone_sentence_130

The process includes: the development of the ossification center, calcification, trabeculae formation and the development of the periosteum. Bone_sentence_131

Endochondral ossification occurs in long bones and most other bones in the body; it involves the development of bone from cartilage. Bone_sentence_132

This process includes the development of a cartilage model, its growth and development, development of the primary and secondary ossification centers, and the formation of articular cartilage and the epiphyseal plates. Bone_sentence_133

Endochondral ossification begins with points in the cartilage called "primary ossification centers." Bone_sentence_134

They mostly appear during fetal development, though a few short bones begin their primary ossification after birth. Bone_sentence_135

They are responsible for the formation of the diaphyses of long bones, short bones and certain parts of irregular bones. Bone_sentence_136

Secondary ossification occurs after birth, and forms the epiphyses of long bones and the extremities of irregular and flat bones. Bone_sentence_137

The diaphysis and both epiphyses of a long bone are separated by a growing zone of cartilage (the epiphyseal plate). Bone_sentence_138

At 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). Bone_sentence_139

In the upper limbs, only the diaphyses of the long bones and scapula are ossified. Bone_sentence_140

The epiphyses, carpal bones, coracoid process, medial border of the scapula, and acromion are still cartilaginous. Bone_sentence_141

The following steps are followed in the conversion of cartilage to bone: Bone_sentence_142

Bone_ordered_list_2

  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.Bone_item_2_7
  2. Zone of cell proliferation. A little closer to the marrow cavity, chondrocytes multiply and arrange themselves into longitudinal columns of flattened lacunae.Bone_item_2_8
  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.Bone_item_2_9
  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.Bone_item_2_10
  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.Bone_item_2_11

Function Bone_section_13

Bone_table_general_1

Functions of BoneBone_header_cell_1_0_0
MechanicalBone_cell_1_1_0
SyntheticBone_cell_1_2_0
MetabolicBone_cell_1_3_0

Bones have a variety of functions: Bone_sentence_143

Mechanical Bone_section_14

See also: Skeleton, Human skeleton, and List of bones of the human skeleton Bone_sentence_144

Bones serve a variety of mechanical functions. Bone_sentence_145

Together the bones in the body form the skeleton. Bone_sentence_146

They provide a frame to keep the body supported, and an attachment point for skeletal muscles, tendons, ligaments and joints, which function 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). Bone_sentence_147

Bones protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs. Bone_sentence_148

Because of the way that bone is formed, bone has a high compressive strength of about 170 MPa (1,700 kgf/cm), poor tensile strength of 104–121 MPa, and a very low shear stress strength (51.6 MPa). Bone_sentence_149

This means that bone resists pushing (compressional) stress well, resist pulling (tensional) stress less well, but only poorly resists shear stress (such as due to torsional loads). Bone_sentence_150

While bone is essentially brittle, bone does have a significant degree of elasticity, contributed chiefly by collagen. Bone_sentence_151

Mechanically, bones also have a special role in hearing. Bone_sentence_152

The ossicles are three small bones in the middle ear which are involved in sound transduction. Bone_sentence_153

Synthetic Bone_section_15

The cancellous part of bones contain bone marrow. Bone_sentence_154

Bone marrow produces blood cells in a process called hematopoiesis. Bone_sentence_155

Blood cells that are created in bone marrow include red blood cells, platelets and white blood cells. Bone_sentence_156

Progenitor cells such as the hematopoietic stem cell divide in a process called mitosis to produce precursor cells. Bone_sentence_157

These include precursors which eventually give rise to white blood cells, and erythroblasts which give rise to red blood cells. Bone_sentence_158

Unlike red and white blood cells, created by mitosis, platelets are shed from very large cells called megakaryocytes. Bone_sentence_159

This process of progressive differentiation occurs within the bone marrow. Bone_sentence_160

After the cells are matured, they enter the circulation. Bone_sentence_161

Every day, over 2.5 billion red blood cells and platelets, and 50–100 billion granulocytes are produced in this way. Bone_sentence_162

As well as creating cells, bone marrow is also one of the major sites where defective or aged red blood cells are destroyed. Bone_sentence_163

Metabolic Bone_section_16

Bone_unordered_list_3

  • Mineral storage – bones act as reserves of minerals important for the body, most notably calcium and phosphorus.Bone_item_3_12

Determined by the species, age, and the type of bone, bone cells make up to 15 percent of the bone. Bone_sentence_164

Growth factor storage—mineralized bone matrix stores important growth factors such as insulin-like growth factors, transforming growth factor, bone morphogenetic proteins and others. Bone_sentence_165

Bone_unordered_list_4

  • Fat storage – marrow adipose tissue (MAT) acts as a storage reserve of fatty acids.Bone_item_4_13
  • Acid-base balance – bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts.Bone_item_4_14
  • Detoxification – bone tissues can also store heavy metals and other foreign elements, removing them from the blood and reducing their effects on other tissues. These can later be gradually released for excretion.Bone_item_4_15
  • Endocrine organ – bone controls phosphate metabolism by releasing fibroblast growth factor 23 (FGF-23), which acts on kidneys to reduce phosphate reabsorption. Bone cells also release a hormone called osteocalcin, which contributes to the regulation of blood sugar (glucose) and fat deposition. Osteocalcin increases both the insulin secretion and sensitivity, in addition to boosting the number of insulin-producing cells and reducing stores of fat.Bone_item_4_16
  • Calcium balance – 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 fixes circulating calcium in its mineral form, removing it from the bloodstream, resorption actively unfixes it thereby increasing circulating calcium levels. These processes occur in tandem at site-specific locations.Bone_item_4_17

Remodeling Bone_section_17

Main article: Bone remodeling Bone_sentence_166

Bone is constantly being created and replaced in a process known as remodeling. Bone_sentence_167

This ongoing turnover of bone is a process of resorption followed by replacement of bone with little change in shape. Bone_sentence_168

This is accomplished through osteoblasts and osteoclasts. Bone_sentence_169

Cells are stimulated by a variety of signals, and together referred to as a remodeling unit. Bone_sentence_170

Approximately 10% of the skeletal mass of an adult is remodelled each year. Bone_sentence_171

The purpose of remodeling is to regulate calcium homeostasis, repair microdamaged bones from everyday stress, and to shape the skeleton during growth. Bone_sentence_172

Repeated stress, such as weight-bearing exercise or bone healing, results in the bone thickening at the points of maximum stress (Wolff's law). Bone_sentence_173

It has been hypothesized that this is a result of bone's piezoelectric properties, which cause bone to generate small electrical potentials under stress. Bone_sentence_174

The action of osteoblasts and osteoclasts are controlled by a number of chemical enzymes that either promote or inhibit the activity of the bone remodeling cells, controlling the rate at which bone is made, destroyed, or changed in shape. Bone_sentence_175

The cells also use paracrine signalling to control the activity of each other. Bone_sentence_176

For example, the rate at which osteoclasts resorb bone is inhibited by calcitonin and osteoprotegerin. Bone_sentence_177

Calcitonin is produced by parafollicular cells in the thyroid gland, and can bind to receptors on osteoclasts to directly inhibit osteoclast activity. Bone_sentence_178

Osteoprotegerin is secreted by osteoblasts and is able to bind RANK-L, inhibiting osteoclast stimulation. Bone_sentence_179

Osteoblasts can also be stimulated to increase bone mass through increased secretion of osteoid and by inhibiting the ability of osteoclasts to break down osseous tissue. Bone_sentence_180

Increased secretion of osteoid is stimulated by the secretion of growth hormone by the pituitary, thyroid hormone and the sex hormones (estrogens and androgens). Bone_sentence_181

These hormones also promote increased secretion of osteoprotegerin. Bone_sentence_182

Osteoblasts can also be induced to secrete a number of cytokines that promote reabsorption of bone by stimulating osteoclast activity and differentiation from progenitor cells. Bone_sentence_183

Vitamin D, parathyroid hormone and stimulation from osteocytes induce osteoblasts to increase secretion of RANK-ligand and interleukin 6, which cytokines then stimulate increased reabsorption of bone by osteoclasts. Bone_sentence_184

These same compounds also increase secretion of macrophage colony-stimulating factor by osteoblasts, which promotes the differentiation of progenitor cells into osteoclasts, and decrease secretion of osteoprotegerin. Bone_sentence_185

Bone volume Bone_section_18

Bone volume is determined by the rates of bone formation and bone resorption. Bone_sentence_186

Recent research has suggested that certain growth factors may work to locally alter bone formation by increasing osteoblast activity. Bone_sentence_187

Numerous bone-derived growth factors have been isolated and classified via bone cultures. Bone_sentence_188

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. Bone_sentence_189

Evidence suggests that bone cells produce growth factors for extracellular storage in the bone matrix. Bone_sentence_190

The release of these growth factors from the bone matrix could cause the proliferation of osteoblast precursors. Bone_sentence_191

Essentially, bone growth factors may act as potential determinants of local bone formation. Bone_sentence_192

Research has suggested that cancellous bone volume in postmenopausal osteoporosis may be determined by the relationship between the total bone forming surface and the percent of surface resorption. Bone_sentence_193

Clinical significance Bone_section_19

See also: Bone disease Bone_sentence_194

A number of diseases can affect bone, including arthritis, fractures, infections, osteoporosis and tumours. Bone_sentence_195

Conditions relating to bone can be managed by a variety of doctors, including rheumatologists for joints, and orthopedic surgeons, who may conduct surgery to fix broken bones. Bone_sentence_196

Other doctors, such as rehabilitation specialists may be involved in recovery, radiologists in interpreting the findings on imaging, and pathologists in investigating the cause of the disease, and family doctors may play a role in preventing complications of bone disease such as osteoporosis. Bone_sentence_197

When a doctor sees a patient, a history and exam will be taken. Bone_sentence_198

Bones are then often imaged, called radiography. Bone_sentence_199

This might include ultrasound X-ray, CT scan, MRI scan and other imaging such as a Bone scan, which may be used to investigate cancer. Bone_sentence_200

Other tests such as a blood test for autoimmune markers may be taken, or a synovial fluid aspirate may be taken. Bone_sentence_201

Fractures Bone_section_20

Main article: Bone fracture Bone_sentence_202

In normal bone, fractures occur when there is significant force applied, or repetitive trauma over a long time. Bone_sentence_203

Fractures can also occur when a bone is weakened, such as with osteoporosis, or when there is a structural problem, such as when the bone remodels excessively (such as Paget's disease) or is the site of the growth of cancer. Bone_sentence_204

Common fractures include wrist fractures and hip fractures, associated with osteoporosis, vertebral fractures associated with high-energy trauma and cancer, and fractures of long-bones. Bone_sentence_205

Not all fractures are painful. Bone_sentence_206

When serious, depending on the fractures type and location, complications may include flail chest, compartment syndromes or fat embolism. Bone_sentence_207

Compound fractures involve the bone's penetration through the skin. Bone_sentence_208

Some complex fractures can be treated by the use of bone grafting procedures that replace missing bone portions. Bone_sentence_209

Fractures and their underlying causes can be investigated by X-rays, CT scans and MRIs. Bone_sentence_210

Fractures are described by their location and shape, and several classification systems exist, depending on the location of the fracture. Bone_sentence_211

A common long bone fracture in children is a Salter–Harris fracture. Bone_sentence_212

When fractures are managed, pain relief is often given, and the fractured area is often immobilised. Bone_sentence_213

This is to promote bone healing. Bone_sentence_214

In addition, surgical measures such as internal fixation may be used. Bone_sentence_215

Because of the immobilisation, people with fractures are often advised to undergo rehabilitation. Bone_sentence_216

Tumours Bone_section_21

Main article: Bone tumour Bone_sentence_217

There are several types of tumour that can affect bone; examples of benign bone tumours include osteoma, osteoid osteoma, osteochondroma, osteoblastoma, enchondroma, giant cell tumour of bone, and aneurysmal bone cyst. Bone_sentence_218

Cancer Bone_section_22

Main article: Bone metastases Bone_sentence_219

Cancer can arise in bone tissue, and bones are also a common site for other cancers to spread (metastasise) to. Bone_sentence_220

Cancers that arise in bone are called "primary" cancers, although such cancers are rare. Bone_sentence_221

Metastases within bone are "secondary" cancers, with the most common being breast cancer, lung cancer, prostate cancer, thyroid cancer, and kidney cancer. Bone_sentence_222

Secondary cancers that affect bone can either destroy bone (called a "lytic" cancer) or create bone (a "sclerotic" cancer). Bone_sentence_223

Cancers of the bone marrow inside the bone can also affect bone tissue, examples including leukemia and multiple myeloma. Bone_sentence_224

Bone may also be affected by cancers in other parts of the body. Bone_sentence_225

Cancers in other parts of the body may release parathyroid hormone or parathyroid hormone-related peptide. Bone_sentence_226

This increases bone reabsorption, and can lead to bone fractures. Bone_sentence_227

Bone tissue that is destroyed or altered as a result of cancers is distorted, weakened, and more prone to fracture. Bone_sentence_228

This may lead to compression of the spinal cord, destruction of the marrow resulting in bruising, bleeding and immunosuppression, and is one cause of bone pain. Bone_sentence_229

If the cancer is metastatic, then there might be other symptoms depending on the site of the original cancer. Bone_sentence_230

Some bone cancers can also be felt. Bone_sentence_231

Cancers of the bone are managed according to their type, their stage, prognosis, and what symptoms they cause. Bone_sentence_232

Many primary cancers of bone are treated with radiotherapy. Bone_sentence_233

Cancers of bone marrow may be treated with chemotherapy, and other forms of targeted therapy such as immunotherapy may be used. Bone_sentence_234

Palliative care, which focuses on maximising a person's quality of life, may play a role in management, particularly if the likelihood of survival within five years is poor. Bone_sentence_235

Painful conditions Bone_section_23

Bone_unordered_list_5

Osteoporosis Bone_section_24

Main article: Osteoporosis Bone_sentence_236

Osteoporosis is a disease of bone where there is reduced bone mineral density, increasing the likelihood of fractures. Bone_sentence_237

Osteoporosis is defined in women by the World Health Organization as a bone mineral density of 2.5 standard deviations below peak bone mass, relative to the age and sex-matched average. Bone_sentence_238

This density is measured using dual energy X-ray absorptiometry (DEXA), with the term "established osteoporosis" including the presence of a fragility fracture. Bone_sentence_239

Osteoporosis is most common in women after menopause, when it is called "postmenopausal osteoporosis", but may develop in men and premenopausal women in the presence of particular hormonal disorders and other chronic diseases or as a result of smoking and medications, specifically glucocorticoids. Bone_sentence_240

Osteoporosis usually has no symptoms until a fracture occurs. Bone_sentence_241

For this reason, DEXA scans are often done in people with one or more risk factors, who have developed osteoporosis and are at risk of fracture. Bone_sentence_242

Osteoporosis treatment includes advice to stop smoking, decrease alcohol consumption, exercise regularly, and have a healthy diet. Bone_sentence_243

Calcium and trace mineral supplements may also be advised, as may Vitamin D. Bone_sentence_244

When medication is used, it may include bisphosphonates, Strontium ranelate, and hormone replacement therapy. Bone_sentence_245

Osteopathic medicine Bone_section_25

Main article: Osteopathic medicine in the United States Bone_sentence_246

Osteopathic medicine is a school of medical thought originally developed based on the idea of the link between the musculoskeletal system and overall health, but now very similar to mainstream medicine. Bone_sentence_247

As of 2012, over 77,000 physicians in the United States are trained in osteopathic medical schools. Bone_sentence_248

Osteology Bone_section_26

The study of bones and teeth is referred to as osteology. Bone_sentence_249

It is frequently used in anthropology, archeology and forensic science for a variety of tasks. Bone_sentence_250

This can include determining the nutritional, health, age or injury status of the individual the bones were taken from. Bone_sentence_251

Preparing fleshed bones for these types of studies can involve the process of maceration. Bone_sentence_252

Typically anthropologists and archeologists study bone tools made by Homo sapiens and Homo neanderthalensis. Bone_sentence_253

Bones can serve a number of uses such as projectile points or artistic pigments, and can also be made from external bones such as antlers. Bone_sentence_254

Other animals Bone_section_27

Main articles: Bird anatomy and Exoskeleton Bone_sentence_255

Bird skeletons are very lightweight. Bone_sentence_256

Their bones are smaller and thinner, to aid flight. Bone_sentence_257

Among mammals, bats come closest to birds in terms of bone density, suggesting that small dense bones are a flight adaptation. Bone_sentence_258

Many bird bones have little marrow due to their being hollow. Bone_sentence_259

A bird's beak is primarily made of bone as projections of the mandibles which are covered in keratin. Bone_sentence_260

A deer's antlers are composed of bone which is an unusual example of bone being outside the skin of the animal once the velvet is shed. Bone_sentence_261

The extinct predatory fish Dunkleosteus had sharp edges of hard exposed bone along its jaws. Bone_sentence_262

Many animals possess an exoskeleton that is not made of bone. Bone_sentence_263

These include insects and crustaceans. Bone_sentence_264

The proportion of cortical bone that is 80% in the human skeleton may be much lower in other animals, especially in marine mammals and marine turtles, or in various Mesozoic marine reptiles, such as ichthyosaurs, among others. Bone_sentence_265

Many animals, particularly herbivores, practice osteophagy—the eating of bones. Bone_sentence_266

This is presumably carried out in order to replenish lacking phosphate. Bone_sentence_267

Many bone diseases that affect humans also affect other vertebrates—an example of one disorder is skeletal fluorosis. Bone_sentence_268

Society and culture Bone_section_28

Bones from slaughtered animals have a number of uses. Bone_sentence_269

In prehistoric times, they have been used for making bone tools. Bone_sentence_270

They have further been used in bone carving, already important in prehistoric art, and also in modern time as crafting materials for buttons, beads, handles, bobbins, calculation aids, head nuts, dice, poker chips, pick-up sticks, ornaments, etc. A special genre is scrimshaw. Bone_sentence_271

Bone glue can be made by prolonged boiling of ground or cracked bones, followed by filtering and evaporation to thicken the resulting fluid. Bone_sentence_272

Historically once important, bone glue and other animal glues today have only a few specialized uses, such as in antiques restoration. Bone_sentence_273

Essentially the same process, with further refinement, thickening and drying, is used to make gelatin. Bone_sentence_274

Broth is made by simmering several ingredients for a long time, traditionally including bones. Bone_sentence_275

Bone char, a porous, black, granular material primarily used for filtration and also as a black pigment, is produced by charring mammal bones. Bone_sentence_276

Oracle bone script was a writing system used in Ancient China based on inscriptions in bones. Bone_sentence_277

Its name originates from oracle bones, which were mainly ox clavicle. Bone_sentence_278

The Ancient Chinese (mainly in the Shang dynasty), would write their questions on the oracle bone, and burn the bone, and where the bone cracked would be the answer for the questions. Bone_sentence_279

To at someone is considered bad luck in some cultures, such as Australian aborigines, such as by the Kurdaitcha. Bone_sentence_280

The wishbones of fowl have been used for divination, and are still customarily used in a tradition to determine which one of two people pulling on either prong of the bone may make a wish. Bone_sentence_281

Various cultures throughout history have adopted the custom of shaping an infant's head by the practice of artificial cranial deformation. Bone_sentence_282

A widely practised custom in China was that of foot binding to limit the normal growth of the foot. Bone_sentence_283

Additional images Bone_section_29

Bone_unordered_list_6

  • Bone_item_6_24
  • Bone_item_6_25
  • Bone_item_6_26

See also Bone_section_30

Bone_unordered_list_7


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Bone.