Bone

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Bone
Left femur of extinct elephant, Alaska, Ice Age Wellcome L0057714.jpg
A bone dating from the Pleistocene Ice Age of an extinct species of elephant.
Bertazzo S - SEM deproteined bone - wistar rat - x10k.tif
A scanning electronic micrograph of bone at 10,000× magnification.
Identifiers
MeSH D001842
TA A02.0.00.000
TH H3.01.00.0.00001
FMA 30317
Anatomical terminology

A bone is a rigid organ that constitutes part of the vertebrate skeleton in animals. 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. Bones come in a variety of shapes and sizes and have a complex internal and external structure. They are lightweight yet strong and hard, and serve multiple functions.

Contents

Bone tissue (osseous tissue) is a hard tissue, a type of dense connective tissue. It has a honeycomb-like matrix internally, which helps to give the bone rigidity. Bone tissue is made up of different types of bone cells. Osteoblasts and osteocytes are involved in the formation and mineralization of bone; osteoclasts are involved in the resorption of bone tissue. Modified (flattened) osteoblasts become the lining cells that form a protective layer on the bone surface. 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 tissue is a mineralized tissue of two types, cortical bone and cancellous bone. Other types of tissue found in bones include bone marrow, endosteum, periosteum, nerves, blood vessels and cartilage.

In the human body at birth, there are approximately 270 bones present; many of these fuse together during development, leaving a total of 212 separate bones in the adult, not counting numerous small sesamoid bones. [1] [2] The largest bone in the body is the femur or thigh-bone, and the smallest is the stapes in the middle ear.

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

Structure

Cross-section of bone Bone cross-section.svg
Cross-section of bone

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

Bone matrix is 90 to 95% composed of elastic collagen fibers, also known as ossein, [3] and the remainder is ground substance. [4] The elasticity of collagen improves fracture resistance. [5] The matrix is hardened by the binding of inorganic mineral salt, calcium phosphate, in a chemical arrangement known as calcium hydroxylapatite. It is the bone mineralization that give bones rigidity.

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

Cortical bone

Cross-section details of a long bone Illu compact spongy bone.jpg
Cross-section details of a long bone

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. It forms the hard exterior (cortex) of bones. 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.[ citation needed ] 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. It consists of multiple microscopic columns, each called an osteon. Each column is multiple layers of osteoblasts and osteocytes around a central canal called the haversian canal. Volkmann's canals at right angles connect the osteons together. The columns are metabolically active, and as bone is reabsorbed and created the nature and location of the cells within the osteon will change. Cortical bone is covered by a periosteum on its outer surface, and an endosteum on its inner surface. The endosteum is the boundary between the cortical bone and the cancellous bone. [6] The primary anatomical and functional unit of cortical bone is the osteon.

Cancellous bone

Micrograph of cancellous bone Spongy bone - trabecules.jpg
Micrograph of cancellous bone

Cancellous bone, also called trabecular or spongy bone, [6] is the internal tissue of the skeletal bone and is an open cell porous network. Cancellous bone has a higher surface-area-to-volume ratio than cortical bone and it is less dense. This makes it weaker and more flexible. The greater surface area also makes it suitable for metabolic activities such as the exchange of calcium ions. Cancellous bone is typically found at the ends of long bones, near joints and in the interior of vertebrae. Cancellous bone is highly vascular and often contains red bone marrow where hematopoiesis, the production of blood cells, occurs. The primary anatomical and functional unit of cancellous bone is the trabecula. The trabeculae are aligned towards the mechanical load distribution that a bone experiences within long bones such as the femur. As far as short bones are concerned, trabecular alignment has been studied in the vertebral pedicle. [7] Thin formations of osteoblasts covered in endosteum create an irregular network of spaces, [8] known as trabeculae. Within these spaces are bone marrow and hematopoietic stem cells that give rise to platelets, red blood cells and white blood cells. [8] 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. Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone. [9]

The words cancellous and trabecular refer to the tiny lattice-shaped units (trabeculae) that form the tissue. It was first illustrated accurately in the engravings of Crisóstomo Martinez. [10]

Bone marrow

Bone marrow, also known as myeloid tissue in red bone marrow, can be found in almost any bone that holds cancellous tissue. 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. In adults, red marrow is mostly found in the bone marrow of the femur, the ribs, the vertebrae and pelvic bones. [11]

Bone cells

Bone cells 604 Bone cells.jpg
Bone cells

Bone is a metabolically active tissue composed of several types of cells. 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. Osteoblasts and osteocytes are derived from osteoprogenitor cells, but osteoclasts are derived from the same cells that differentiate to form macrophages and monocytes. [12] Within the marrow of the bone there are also hematopoietic stem cells. These cells give rise to other cells, including white blood cells, red blood cells, and platelets. [13]

Osteoblast

Light micrograph of decalcified cancellous bone displaying osteoblasts actively synthesizing osteoid, containing two osteocytes. Active osteoblasts.jpg
Light micrograph of decalcified cancellous bone displaying osteoblasts actively synthesizing osteoid, containing two osteocytes.

Osteoblasts are mononucleate bone-forming cells. They are located on the surface of osteon seams and make a protein mixture known as osteoid, which mineralizes to become bone. [14] 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 I collagen. Osteoblasts also manufacture hormones, such as prostaglandins, to act on the bone itself. The osteoblast creates and repairs new bone by actually building around itself. First, the osteoblast puts up collagen fibers. These collagen fibers are used as a framework for the osteoblasts' work. The osteoblast then deposits calcium phosphate which is hardened by hydroxide and bicarbonate ions. The brand new bone created by the osteoblast is called osteoid. [15] Once the osteoblast is finished working it is actually trapped inside the bone once it hardens. When the osteoblast becomes trapped, it becomes known as an osteocyte. [16] Other osteoblasts remain on the top of the new bone and are used to protect the underlying bone, these become known as lining cells.

Osteocyte

Osteocytes are mostly inactive osteoblasts. [12] Osteocytes originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produced. [6] 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. [17] Osteocytes remain in contact with other cells in the bone through gap junctions—coupled cell processes—which pass through small channels in the bone matrix called the canaliculi.

Osteoclast

Osteoclasts are very large multinucleate cells that are responsible for the breakdown of bones by the process of bone resorption. New bone is then formed by the osteoblasts. Bone is constantly remodelled by the resorption of osteoclasts and created by osteoblasts. [12] Osteoclasts are large cells with multiple nuclei located on bone surfaces in what are called Howship's lacunae (or resorption pits). These lacunae are the result of surrounding bone tissue that has been reabsorbed. [18] Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with phagocytic-like mechanisms similar to circulating macrophages. [12] Osteoclasts mature and/or migrate to discrete bone surfaces. Upon arrival, active enzymes, such as tartrate resistant acid phosphatase, are secreted against the mineral substrate.[ citation needed ] The reabsorption of bone by osteoclasts also plays a role in calcium homeostasis. [18]

Extracellular matrix

Bones consist of living cells embedded in a mineralized organic matrix. 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. Above 30% of the acellular part of bone consists of the organic components, and 70% of salts. [19] The collagen fibers give bone its tensile strength, and the interspersed crystals of hydroxyapatite give bone its compressive strength. These effects are synergistic. [19]

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). [19] 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. [19]

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, [19] as well as non-collagenous proteins such as osteocalcin, osteopontin or bone sialoprotein. Collagen consists of strands of repeating units, which give bone tensile strength, and are arranged in an overlapping fashion that prevents shear stress. The function of ground substance is not fully known. [19] Two types of bone can be identified microscopically according to the arrangement of collagen: woven and lamellar.

Transmission electron micrograph of decalcified woven bone matrix displaying characteristic irregular orientation of collagen fibers. Woven bone matrix.jpg
Transmission electron micrograph of decalcified woven bone matrix displaying characteristic irregular orientation of collagen fibers.

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. In adults woven bone is created after fractures or in Paget'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 in concentric sheets with a much lower proportion of osteocytes to surrounding tissue. Lamellar bone, which makes its first appearance in humans in the fetus during the third trimester, [21] is stronger and filled with many collagen fibers parallel to other fibers in the same layer (these parallel columns are called osteons). 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. 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 2  µm per day. Lamellar bone also requires a relatively flat surface to lay the collagen fibers in parallel or concentric layers.[ citation needed ]

Deposition

The extracellular matrix of bone is laid down by osteoblasts, which secrete both collagen and ground substance. These synthesise collagen within the cell, and then secrete collagen fibrils. The collagen fibers rapidly polymerise to form collagen strands. At this stage they are not yet mineralised, and are called "osteoid". Around the strands calcium and phosphate precipitate on the surface of these strands, within days to weeks becoming crystals of hydroxyapatite. [19]

In order to mineralise the bone, the osteoblasts secrete vesicles containing alkaline 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. [22] [23]

Types

Structure of a long bone 603 Anatomy of Long Bone.jpg
Structure of a long bone
One way to classify bones is by their shape or appearance. Blausen 0229 ClassificationofBones.png
One way to classify bones is by their shape or appearance.

There are five types of bones in the human body: long, short, flat, irregular, and sesamoid. [24]

Terminology

In the study of anatomy, anatomists use a number of anatomical terms to describe the appearance, shape and function of bones. Other anatomical terms are also used to describe the location of bones. Like other anatomical terms, many of these derive from Latin and Greek. Some anatomists still use Latin to refer to bones. The term "osseous", and the prefix "osteo-", referring to things related to bone, are still used commonly today.

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

When two bones join together, they are said to "articulate". If the two bones have a fibrous connection and are relatively immobile, then the joint is called a "suture".

Development

Endochondral ossification Bone growth.png
Endochondral ossification
Section through a juvenile knee joint (rat) showing the cartilagineous growth plates Bone (1).jpg
Section through a juvenile knee joint (rat) showing the cartilagineous growth plates

The formation of bone is called ossification. During the fetal stage of development this occurs by two processes: intramembranous ossification and endochondral ossification. [29] Intramembranous ossification involves the formation of bone from connective tissue whereas endochondral ossification involves the formation of bone from cartilage.

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. The process includes: the development of the ossification center, calcification, trabeculae formation and the development of the periosteum.[ citation needed ]

Endochondral ossification occurs in long bones and most other bones in the body; it involves the development of bone from cartilage. 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. [30]

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 after birth. 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 the epiphyses of 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 (the epiphyseal plate). 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).[ 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. [31]

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. [32]
  2. Zone of cell proliferation. A little closer to the marrow cavity, chondrocytes multiply and arrange themselves into longitudinal columns of flattened lacunae. [32]
  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. [32]
  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. [32]
  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. [32]

Function

Functions of Bone
Mechanical
  • Protection
  • Gives structure
  • Facilitates movement
  • Facilitates hearing
Synthetic
Metabolic

Bones have a variety of functions:

Mechanical

Bones serve a variety of mechanical functions. Together the bones in the body form the skeleton. 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).

Bones protect internal organs, such as the skull protecting the brain or the ribs protecting the heart and lungs. Because of the way that bone is formed, bone has a high compressive strength of about 170 MPa (1800 kgf/cm²), [5] poor tensile strength of 104–121 MPa, and a very low shear stress strength (51.6 MPa). [33] [34] 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). While bone is essentially brittle, bone does have a significant degree of elasticity, contributed chiefly by collagen.

Mechanically, bones also have a special role in hearing. The ossicles are three small bones in the middle ear which are involved in sound transduction.

Synthetic

The cancellous part of bones contain bone marrow. Bone marrow produces blood cells in a process called hematopoiesis. [35] Blood cells that are created in bone marrow include red blood cells, platelets and white blood cells. [36] Progenitor cells such as the hematopoietic stem cell divide in a process called mitosis to produce precursor cells. These include precursors which eventually give rise to white blood cells, and erythroblasts which give rise to red blood cells. [37] Unlike red and white blood cells, created by mitosis, platelets are shed from very large cells called megakaryocytes. [38] This process of progressive differentiation occurs within the bone marrow. After the cells are matured, they enter the circulation. [39] Every day, over 2.5 billion red blood cells and platelets, and 50–100 billion granulocytes are produced in this way. [13]

As well as creating cells, bone marrow is also one of the major sites where defective or aged red blood cells are destroyed. [13]

Metabolic

°Determined by the species, age, and the type of bone, bone cells make up to 15 percent of the bone. Growth factor storage – mineralized bone matrix stores important growth factors such as insulin-like growth factors, transforming growth factor, bone morphogenetic proteins and others.[ citation needed ]

Remodeling

Bone is constantly being created and replaced in a process known as remodeling. This ongoing turnover of bone is a process of resorption followed by replacement of bone with little change in shape. This is accomplished through osteoblasts and osteoclasts. Cells are stimulated by a variety of signals, and together referred to as a remodeling unit. Approximately 10% of the skeletal mass of an adult is remodelled each year. [44] The purpose of remodeling is to regulate calcium homeostasis, repair microdamaged bones from everyday stress, and to shape the skeleton during growth.[ citation needed ] Repeated stress, such as weight-bearing exercise or 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 piezoelectric properties, which cause bone to generate small electrical potentials under stress. [45]

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. The cells also use paracrine signalling to control the activity of each other.[ citation needed ] For example, the rate at which osteoclasts resorb bone is inhibited by calcitonin and osteoprotegerin. Calcitonin is produced by parafollicular cells in the thyroid 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. [46]

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.[ citation needed ] Increased secretion of osteoid is stimulated by the secretion of growth hormone by the pituitary, thyroid hormone and the sex hormones (estrogens and androgens). These hormones also promote increased secretion of osteoprotegerin. [46] Osteoblasts can also be induced to secrete a number of cytokines that promote reabsorbtion of bone by stimulating osteoclast activity and differentiation from progenitor cells. 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. 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.[ citation needed ]

Bone volume

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. [47] 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. [47] Research has suggested that cancellous bone volume in postemenopausal osteoporosis may be determined by the relationship between the total bone forming surface and the percent of surface resorption. [48]

Clinical significance

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

When a doctor sees a patient, a history and exam will be taken. Bones are then often imaged, called radiography. 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. [49] Other tests such as a blood test for autoimmune markers may be taken, or a synovial fluid aspirate may be taken. [49]

Fractures

Radiography used to identify possible bone fractures after a knee injury. Xraymachine.JPG
Radiography used to identify possible bone fractures after a knee injury.

In normal bone, fractures occur when there is significant force applied, or repetitive trauma over a long time. 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. [50] 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. Not all fractures are painful. [50] When serious, depending on the fractures type and location, complications may include flail chest, compartment syndromes or fat embolism. Compound fractures involve the bone's penetration through the skin. Some complex fractures can be treated by the use of bone grafting procedures that replace missing bone portions.

Fractures and their underlying causes can be investigated by X-rays, CT scans and MRIs. [50] Fractures are described by their location and shape, and several classification systems exist, depending on the location of the fracture. A common long bone fracture in children is a Salter–Harris fracture. [51] When fractures are managed, pain relief is often given, and the fractured area is often immobilised. This is to promote bone healing. In addition, surgical measures such as internal fixation may be used. Because of the immobilisation, people with fractures are often advised to undergo rehabilitation. [50]

Tumours

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. [52]

Cancer

Cancer can arise in bone tissue, and bones are also a common site for other cancers to spread (metastasise) to. [53] Cancers that arise in bone are called "primary" cancers, although such cancers are rare. [53] Metastases within bone are "secondary" cancers, with the most common being breast cancer, lung cancer, prostate cancer, thyroid cancer, and kidney cancer. [53] Secondary cancers that affect bone can either destroy bone (called a "lytic" cancer) or create bone (a "sclerotic" cancer). Cancers of the bone marrow inside the bone can also affect bone tissue, examples including leukemia and multiple myeloma. Bone may also be affected by cancers in other parts of the body. Cancers in other parts of the body may release parathyroid hormone or parathyroid hormone-related peptide. This increases bone reabsorption, and can lead to bone fractures.

Bone tissue that is destroyed or altered as a result of cancers is distorted, weakened, and more prone to fracture. 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. If the cancer is metastatic, then there might be other symptoms depending on the site of the original cancer. Some bone cancers can also be felt.

Cancers of the bone are managed according to their type, their stage, prognosis, and what symptoms they cause. Many primary cancers of bone are treated with radiotherapy. Cancers of bone marrow may be treated with chemotherapy, and other forms of targeted therapy such as immunotherapy may be used. [54] 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.

Painful conditions

Osteoporosis

Reduced bone mineral density in Osteoporosis (R), increasing the likelihood of fractures. Osteoporosis in Bones.jpg
Reduced bone mineral density in Osteoporosis (R), increasing the likelihood of fractures.

Osteoporosis is a disease of bone where there is reduced bone mineral density, increasing the likelihood of fractures. [55] 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. This density is measured using dual energy X-ray absorptiometry (DEXA), with the term "established osteoporosis" including the presence of a fragility fracture. [56] 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. [55] Osteoporosis usually has no symptoms until a fracture occurs. [55] 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. [55]

Osteoporosis treatment includes advice to stop smoking, decrease alcohol consumption, exercise regularly, and have a healthy diet. Calcium and trace mineral supplements may also be advised, as may Vitamin D. When medication is used, it may include bisphosphonates, Strontium ranelate, and hormone replacement therapy. [57]

Osteopathic medicine

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. As of 2012, over 77,000 physicians in the United States are trained in osteopathic medical schools. [58]

Osteology

Human femurs and humerus from Roman period, with evidence of healed fractures Paleopathology; Human femurs from Roman period, Tell Fara Wellcome L0008764.jpg
Human femurs and humerus from Roman period, with evidence of healed fractures

The study of bones and teeth is referred to as osteology. It is frequently used in anthropology, archeology and forensic science for a variety of tasks. This can include determining the nutritional, health, age or injury status of the individual the bones were taken from. Preparing fleshed bones for these types of studies can involve the process of maceration.

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

Other animals

Skeletal fluorosis in a cow's leg, due to industrial contamination Fluworoze egzostozes1-800h.jpg
Skeletal fluorosis in a cow's leg, due to industrial contamination
Leg and pelvic girdle bones of bird Bird leg and pelvic girdle skeleton EN.gif
Leg and pelvic girdle bones of bird

Bird skeletons are very lightweight. Their bones are smaller and thinner, to aid flight. Among mammals, bats come closest to birds in terms of bone density, suggesting that small dense bones are a flight adaptation. Many bird bones have little marrow due to their being hollow. [59]

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

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. [60]

The extinct predatory fish Dunkleosteus had sharp edges of hard exposed bone along its jaws. [61] [62]

Many animals possess an exoskeleton that is not made of bone. These include insects and crustaceans.

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, [63] among others. [64]

Many animals, particularly herbivores, practice osteophagy – the eating of bones. This is presumably carried out in order to replenish lacking phosphate.

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

Society and culture

Bones of slaughtered cattle on a farm in Namibia Bones of cattle on a farm in Namibia.jpg
Bones of slaughtered cattle on a farm in Namibia

Bones from slaughtered animals have a number of uses. In prehistoric times, they have been used for making bone tools. [65] 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.[ citation needed ]

Bone glue can be made by prolonged boiling of ground or cracked bones, followed by filtering and evaporation to thicken the resulting fluid. Historically once important, bone glue and other animal glues today have only a few specialized uses, such as in antiques restoration. Essentially the same process, with further refinement, thickening and drying, is used to make gelatin.

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

Ground bones are used as an organic phosphorus-nitrogen fertilizer and as additive in animal feed. Bones, in particular after calcination to bone ash, are used as source of calcium phosphate for the production of bone china and previously also phosphorus chemicals.[ citation needed ]

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

Oracle bone script was a writing system used in Ancient China based on inscriptions in bones. Its name originates from oracle bones, which were mainly ox clavicle. 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.

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

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.

Various cultures throughout history have adopted the custom of shaping an infant's head by the practice of artificial cranial deformation. A widely practised custom in China was that of foot binding to limit the normal growth of the foot.

Additional images

See also

Related Research Articles

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Bone healing, or fracture healing, is a proliferative physiological process in which the body facilitates the repair of a bone fracture.

Osteoblast bone-forming cells which secrete an extracellular matrix

Osteoblasts are cells with a single nucleus that synthesize bone. However, in the process of bone formation, osteoblasts function in groups of connected cells. Individual cells cannot make bone. A group of organized osteoblasts together with the bone made by a unit of cells is usually called the osteon.

Osteoid

In histology, osteoid is the unmineralized, organic portion of the bone matrix that forms prior to the maturation of bone tissue. Osteoblasts begin the process of forming bone tissue by secreting the osteoid as several specific proteins. When the osteoid becomes mineralized, it and the adjacent bone cells have developed into new bone tissue.

Osteopetrosis an extremely rare inherited disorder whereby the bones harden, becoming denser, sometimes causing bones to dissolve and break, caused by malfunctioning osteoclasts’ inability to resorb bone

Osteopetrosis, literally "stone bone", also known as marble bone disease or Albers-Schönberg disease, is an extremely rare inherited disorder whereby the bones harden, becoming denser, in contrast to more prevalent conditions like osteoporosis, in which the bones become less dense and more brittle, or osteomalacia, in which the bones soften. Osteopetrosis can cause bones to dissolve and break.

Osteoclast type of bone cell that breaks down bone tissue

An osteoclast is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and remodelling of bones of the vertebral skeleton. The osteoclast disassembles and digests the composite of hydrated protein and mineral at a molecular level by secreting acid and a collagenase, a process known as bone resorption. This process also helps regulate the level of blood calcium.

Osteocyte bone cell with single core

An osteocyte, a star-shaped type of bone cell, is the most commonly found cell in mature bone tissue, and can live as long as the organism itself. The adult human body has about 42 billion of them. Osteocytes do not divide and have an average half life of 25 years. They are derived from osteoprogenitor cells, some of which differentiate into active osteoblasts. Osteoblasts/osteocytes develop in mesenchyme.

Endochondral ossification Cartilaginous bone development that forms the long bones

Endochondral ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Unlike intramembranous ossification, which is the other process by which bone tissue is created, cartilage is present during endochondral ossification. Endochondral ossification is also an essential process during the rudimentary formation of long bones, the growth of the length of long bones, and the natural healing of bone fractures.

Intramembranous ossification Mesenchymal bone development that forms the non-long bones

Intramembranous ossification is one of the two essential processes during fetal development of the gnathostome skeletal system by which rudimentary bone tissue is created. Intramembranous ossification is also an essential process during the natural healing of bone fractures and the rudimentary formation of bones of the head.

Ossification formation of bone or of a bony substance, or the conversion of fibrous tissue or of cartilage into bone or a bony substance

Ossification in bone remodeling is the process of laying down new bone material by cells called osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in the formation of normal, healthy bone tissue: Intramembranous ossification is the direct laying down of bone into the primitive connective tissue (mesenchyme), while endochondral ossification involves cartilage as a precursor. In fracture healing, endochondral osteogenesis is the most commonly occurring process, for example in fractures of long bones treated by plaster of Paris, whereas fractures treated by open reduction and internal fixation with metal plates, screws, pins, rods and nails may heal by intramembranous osteogenesis.

Alveolar process thickened ridge of bone that contains the tooth sockets (dental alveoli) on bones that hold teeth

The alveolar process is the thickened ridge of bone that contains the tooth sockets on the jaw bones that hold teeth. In humans, the tooth-bearing bones are the maxilla and the mandible. The curved part of each alveolar process on the jaw is called the alveolar arch.

Bone resorption The process in which specialized cells known as osteoclasts degrade the organic and inorganic portions of bone, and endocytose and transport the degradation products.

Bone resorption is resorption of bone tissue, that is, the process by which osteoclasts break down the tissue in bones and release the minerals, resulting in a transfer of calcium from bone tissue to the blood.

Flat bone type of bone, whose principal function is either extensive protection or the provision of broad surfaces for muscular attachment

Flat bones are bones whose principal function is either extensive protection or the provision of broad surfaces for muscular attachment. These bones are expanded into broad, flat plates, as in the cranium (skull), the ilium (pelvis), sternum and the rib cage. The flat bones are: the occipital, parietal, frontal, nasal, lacrimal, vomer, hip bone, sternum, ribs, and scapulae. In the cranial bones, the layers of compact tissue are familiarly known as the tables of the skull; the outer one is thick and tough; the inner is thin, dense, and brittle, and hence is termed the vitreous (glass-like) table.

RANKL mammalian protein found in Homo sapiens

Receptor activator of nuclear factor kappa-Β ligand (RANKL), also known as tumor necrosis factor ligand superfamily member 11 (TNFSF11), TNF-related activation-induced cytokine (TRANCE), osteoprotegerin ligand (OPGL), and osteoclast differentiation factor (ODF), is a protein that in humans is encoded by the TNFSF11 gene.

Osteonecrosis of the jaw area of necrotic bone in the mandible or maxilla

Osteonecrosis of the jaw (ONJ) is a severe bone disease (osteonecrosis) that affects the jaws. Various forms of ONJ have been described over the last 160 years, and a number of causes have been suggested in the literature.

Bone remodeling The continuous turnover of bone matrix and mineral that involves first, an increase in resorption (osteoclastic activity) and later, reactive bone formation (osteoblastic activity). The process of bone remodeling takes place in the adult skeleton at

Bone remodeling is a lifelong process where mature bone tissue is removed from the skeleton and new bone tissue is formed. These processes also control the reshaping or replacement of bone following injuries like fractures but also micro-damage, which occurs during normal activity. Remodeling responds also to functional demands of the mechanical loading.

Sp7 transcription factor protein-coding gene in the species Homo sapiens

Transcription factor Sp7, also called Osterix (Osx), is a protein that in humans is encoded by the SP7 gene. It is a member of the Sp family of zinc-finger transcription factors It is highly conserved among bone-forming verterbrate species It plays a major role, along with Runx2 and Dlx5 in driving the differentiation of mesenchymal precursor cells into osteoblasts and eventually osteocytes. Sp7 also plays a regulatory role by inhibiting chondrocyte differentiation maintaining the balance between differentiation of mesenchymal precursor cells into ossified bone or cartilage. Mutations of this gene have been associated with multiple dysfunctional bone phenotypes in vertebrates. During development, a mouse embryo model with Sp7 expression knocked out had no formation of bone tissue. Through the use of GWAS studies, the Sp7 locus in humans has been strongly associated with bone mass density. In addition there is significant genetic evidence for its role in diseases such as Osteogenesis imperfecta (OI).

Eldecalcitol chemical compound

Eldecalcitol is a drug used in Japan for the treatment of osteoporosis. It is an analog of vitamin D. Osteoporosis is a common bone disease among the older generation, with an estimated prevalence of over 200 million people. This condition often results in bone fractures due to abnormally low bone mass density, and is a leading cause of disability, especially among developed countries with longer average life spans. Osteoporosis is more common in women than with men.

The human skeletal system is a complex organ in constant equilibrium with the rest of the body. In addition to support and structure of the body, bone is the major reservoir for many minerals and compounds essential for maintaining a healthy pH balance. The deterioration of the body with age renders the elderly particularly susceptible to and affected by poor bone health. Illnesses like osteoporosis, characterized by weakening of the bone's structural matrix, increases the risk of hip-fractures and other life-changing secondary symptoms. In 2010, over 258,000 people aged 65 and older were admitted to the hospital for hip fractures. Incidence of hip fractures is expected to rise by 12% in America, with a projected 289,000 admissions in the year 2030. Other sources estimate up to 1.5 million Americans will have an osteoporotic-related fracture each year. The cost of treating these people is also enormous, in 1991 Medicare spent an estimated $2.9 billion for treatment and out-patient care of hip fractures, this number can only be expected to rise.

Hard tissue is tissue which is mineralized and has a firm intercellular matrix. The hard tissues of humans are bone, tooth enamel, dentin, and cementum. The term is in contrast to soft tissue.

A bone biopsy is a procedure in which a small bone sample is removed from the outer layers of bone for examination, unlike a bone marrow biopsy, which involves the innermost part of the bone. The bone biopsy sample retains the architecture of bone when seen using histopathological examination slide.

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Footnotes