A Review

on

Bone as a living dynamic tissue

Submitted to: Dr. Manoj Kumar ShahDepartment of SurgeryRampur Campus, Rampur

Sumbitted by:Suraj SubediRoll no : 29B. V. Sc & A.H. 8th SemRampur Campus, Rampur

Dec, 2010

Introduction

Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue. Bones come in a variety of shapes and have a complex internal and external structure they are lightweight, yet strong and hard, in addition to fulfilling their many other functions. One of the types of tissue that makes up bone is the mineralized osseous tissue, also called bone tissue, that gives it rigidity and a honeycomb-like three-dimensional internal structure. Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage.

Bone is a dynamic, living tissue; not the hard, dry, lifeless frame. About 30% of bone is living tissue, cells, and blood vessels. The blood vessels go in and out of the bone carrying oxygen and nutrients, and taking away wastes. Bones contain marrow which produces red blood cells and white blood cells. Bones have nerves that can feel pressure and pain. About 45% of bone is mineral (primarily calcium and phosphorus), giving bone its hardness and rigidity and storing these minerals for future use. Bone releases some of this mineral when other body parts, such as nerves, may need them. Bone also contains the proteins, collagen and elastin. Finally, about 25% of bone is made up of water.

Bone tissue consists of compact bone (cortical or solid bone) and spongy bone (trabecular or cancellous bone). Compact bone is made up of structural units called Haversian systems. The system is composed of concentrically arranged layers of hard inorganic matrix surrounding a microscopic central Haversian canal. Blood vessels and nerves pass through the canal. Spongy bone is like a network of hardened bars with spaces between them filled with marrow.

Bone tissue is made and maintained by several types of cells: osteoblasts, osteocytes, and osteoclasts. Osteoblasts make new bone by hardening the protein, collagen, with minerals. Osteocyctes maintain bone, passing nutrients and wastes back and forth between the blood and bone tissues. Osteoclasts destroy bone, releasing minerals into the blood. All through life, bone is continually being reconstructed and reshaped. A young animal has very soft bones made up of cartilage. As it grows, the cartilage is replaced by calcium (ossification). When it reaches the maturity, the bones stop getting longer or bigger, but there is still a lot of growing going on. Old bone cells dissolve and are replaced by new bone cells. Because bone keeps growing, the body is able to repair any breaks that may occur.

Classification of Bone

On the basis of their naked eye appearance in regard to shape, size and structure, the bone has been generally classified into following types.

Long Bones: these are the long hollow cylindrical types of bones found in the limbs. These are weight bearing bones and act as levers. A long bone has a shaft and two expanded ends. The shaft contains a large medullary cavity. These bones ossify in cartilage. e.g. Femur, Humerus

Modified long bones – small long bones like clavicle in dogs, rabbit and fowl do not possess medullary cavity and therefore considered as modified long bones. Miniature long– These are long bones but small in size. E.g. metacarpels of dogs

Short bones: these are small pieces of partially smooth surfaced bones mainly found in the joints and help in their mobility. Most of these bones present six surfaces. These are mainly composed of spongy substance with a layer of cortical compact bone. e.g. carpal bones

Flat bones: These are flat irregular boney plates designed for enclosing cavities which contain important organs of the body. Flat bones are composed of two plates of compact bone with intervening spongy bone. e.g. scapula, some cranial bones of skull.

Irregular bones: These are also small bones with rough irregular surfaces generally found in the mid-line of the skeleton. Their so many projections help in the attachment of the various muscles. They are composed of spongy substance with the thin covering of compact substances. e.g. vertebra

Pneumatic bones: Some long bones of birds have cavities inside their bodies which accommodate air sacs through pneumatic foramen in living conditions. e.g. humerus of fowl. Bones situated closely to the nasal cavity which contain air-filled cavities are also considered as pneumatic bones. e.g. frontal bone, maxilla and ethmoid.

Sesamoid bones: These are small seasame (seed) like bones develop within the tendon. They work as pully to avoid friction. e.g Patella.

Visceral bones: These are fouund in the ciscera of some animals and birds. e.g. os penis in dog, os cordis in ruminants, os opticus in some fowl etc.

Functions of bone

Mechanical• Protection : Bones can serve to protect internal organs, such as the skull protecting the brain or

the ribs protecting the heart and lungs. • Shape : Bones provide a frame to keep the body supported. • Movement : Bones, skeletal muscles, tendons, ligaments and joints 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.

• Sound transduction : Bones are important in the mechanical aspect of overshadowed hearing.

Synthetic

• Blood production : The marrow, located within the medullary cavity of long bones and interstices of cancellous bone, produces blood cells in a process called haematopoiesis.

Metabolic

• Mineral storage : Bones act as reserves of minerals important for the body, most notably calcium and phosphorus.

• Growth factor storage : Mineralized bone matrix stores important growth factors such as insulin-like growth factors, transforming growth factor, bone morphogenetic proteins and others.

• Fat Storage : The yellow bone marrow acts as a storage reserve of fatty acids. • Acid-base balance : Bone buffers the blood against excessive pH changes by absorbing or

releasing alkaline salts. • 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.

• 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 Formation in the Body

The process of bone formation is known as ossification. Calcification is an event in the process of ossification. There are two types of ossification.

a) Intramembranous or mesenchymal ossification

b) Intracartilagenous of endochondral ossification

A) 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 steps in intramembranous ossification are:

i. Development of ossification center: At a point (center of ossification) osteoblasts are differentiated from the mesenchymal cells. A mesh-work of collagen fibers produced by the osteoblasts appear between the cells. It become vascularized by capillary network. The osteoblasts produce other organic intracellular substances like mucoprotein, glycoprotein, mucopolysaccharide etc. The organic non-calcified matrix is called osteoid.

ii. Calcification: The martix is calcified by osteoblasts.

iii.Formation of trabeculae : Few osteoblasts become entrapped by the surrounding matrix and are transformed in radiating manner from the center. Thus trabeculae are formed between the cells. The trabeculae join each other to form cancellous bone. The osteoblasts surrounding the bony spicules deposit more bones to the free ends and sides and thus calcification is spread and the bone becomes compact.

iv. Development of periosteum : The periosteum is developed from the condensation of mesenchyme.

B) Endochondral Ossification occurs where a cartilage precursor of the bone is formed during embryonic development and then, starting from the beginning of the fetal period ossification begins in cartilage.

It is carried out by replacing hyaline cartilage. Steps of endochondral ossification are:

• Development of cartilage model • Growth of cartilage model • Development of the primary ossification center • Development of the secondary ossification center • Formation of articular cartilage and epiphyseal plate

The process occurs in various stages as:

Stage I

A cartilaginous model is formed by the condensation of mesenchymal tissue. A perichondrium appears around the cartilage. The cartilage cells (chondroblasts) at the mid-section of the model proliferate by mitosis and are arranged in the rows towards the ends. They mature and get hypertrophied. The hypertrophied cells produce alkaline phosphatase and precipitate calcium salt at the matrix. The surrounding calcification cause death of the cartilage cells and thereby forms spaces – the primary areolae. This zone is known as primary ossification center. At the same time osteoblasts appear at the inner layer of perichondrium and form subperiosteal collar bone around the primary ossification center. The perichondrium is then called periosteum.

Stage II

At this stage, the collar bone is eroded by the increased activity of the subpeiosteal osteoclasts and the periosteal buds containing osteoblasts, osteoclasts and blood vessels enter into the primrary ossification center. The osteoclasts absorb the irregular clacified mass and form secondary large areolae. These secondary areolae lead to the formation of marrow cavity which subsequently becomes filled up by bone marrow.

Stage III

It is the true stage of bone formation. In this stage, the osteoblasts appear and lay down lamellated bone. Subsequently a number of longitudional groove appear on the outer surface of bony lamella. The ridges of each groove proliferate and enclose a small bood vessels and groove develops into tunel. The lining osteoblasts of the tunnel convert the tunnel into haversian system by the proliferation and differentiation into osteocytes. The whole process is repeated again and again and the ossification extends longitudionally. Secondary ossification centres called the epiphysis appear at the

ends of the cartilagenous model and present at the time of birth. At the ends of long bones, a layer of cartilage doesnot ossify and remain as articular cartilage throughout life. In the growth phase, a portion of of the cartilagenous model remains as epiphyseal cartilage between the epiphysis and diaphysis. This epiphyseal cartilage helps longitudional growth of the bone and is replaced totally by the bone when the growth is complete. The width of bone increases by the deposition of subperiosoteal membrane bone.

Cartilage is converted to bone at the growth plate in 5 stages.

1. Hypertrophy – the cartilage grows rapidly and the chondrocytes line up in columns that point in the direction of growth.

2. Calcification of cartilage matrix

3. Death of chondrocytes.

4. Replacement of calcified cartilage by bone.

5. Remodelling of the new bone

Macrostructure of Bones

• Consist of cortical bone, trabecular bone, and marrow • Distribution of each depends on anatomical location • Long bones

• Tubular in shape • Length of bone greater than breadth

• Some long bones may actually be short • Consist of a shaft and two enlarged, curved ends • Shaft of bone consists of cortical bone surrounding medullary cavity (filled with

marrow) - diaphysis • Ends of bone consist of cortical shell of bone surrounding trabecular bone • Examples: Femur, Tibia and fibula, Humerus, Radius and ulna• After birth, longitudinal growth of long bones occurs in two regions

• Epiphysis - • Highly trabecular region at most proximal and distal ends of long bone

• Metaphysis - • Highly trabecular region at proximal and distal end of diaphysis

• The epiphysis and methaphysis are separated by epiphyseal cartilage plate (growth plate) that ossifies when growth has stopped

• Throughout life, radial growth can occur at two surfaces • Periosteum -

• Outer surface of bone, through which blood supply reaches the bone • Endosteum -

• Inner surface of bone, in contact with medullary canal • Bone can be deposited or resorbed at these two surfaces • Resorbtion can occur in any region of the bone, typically followed by deposition

of bone in the same region -- termed remodeling

• Short bones • Cuboidal in shape • Consist of cortical shell with inner trabecular core • Exist only in the wrist and foot

• Carpal and tarsal bones • Flat bones

• Consist of two plates of cortical bone with trabecular tissue in between • Generally curved rather than flat • Examples:

• Calvaria (top of skull) • Sternum (breast bone) • Scapula (shoulder blade) • Ribs

• Irregular bones • Various shapes • Composition depends on bone • Examples:

• Facial bones • Vertebrae (bones of spine)

• Consist of thin cortical shell surrounding trabecular core

a) Epiphysis (end) and Epiphyseal line: The epiphysis is the end of the long bone. Externally it has a thin layer of compact bone while inner it is cancellous. Epiphysis is caped with articular cartilage.

b) Diaphysis (shaft): Has compact bone with a central cavity. It resists bending forces.

c) Articular Cartilage: is found in the end of long bones. It is smooth slippery and bloodless.

d) Periosteum: It is a fibrous, vascular, sensitive life support covering or sheath of the bone. It provides nutrient rich blood for bone cells and also a source of bone-developing cells during growth or after a fracture.

e) Cancellous or spongy bone and marrow: It appears as a tiny beam of bones arranged like a lattice. Red marrow packs the space between beams of some epiphysis as well as elsewhere.

f) Compact Bone: A dense bone found in diaphysis. It is arranged in concentric layers.

g) Medullary cavity: Medullary cavity of diaphysis serves to tighten bone and provides space for its marrow.

h) Nutrient Artery: Each long bone contains an oblique tunnel in its shaft for the passage of a nutrient artery which supplies the shaft.

Compact (cortical) bone

The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces. Its porosity is 5-30%. This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult skeleton. Compact bone may also be referred to as dense bone.

Trabecular bone/ Spongy bone

Filling the interior of the bone is the trabecular bone tissue (an open cell porous network also called cancellous or spongy bone), which is composed of a network of rod- and plate-like elements that make the overall organ lighter and allow room for blood vessels and marrow. Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone. Its porosity is 30-90%.If for any reason there is an alteration in the strain to which the cancellous is subjected, there is a rearrangement of the trabeculae.

Compact bone is composed of cylindrical structures called osteons or Haversian systems. An osteon consists of concentric lamellae of bone matrix, mainly collagen fibres, surrounding a central canal called the Haversian canal, which contains small blood vessels and nerves. The long axis of osteons is usually parallel to the long axis of the bone. The collagen fibres within any one lamella are generally parallel with one another, but the collagen fibres in the different lamellae of an osteon are oriented at different angles. This increases the strength of the osteon. Canals called Volkmann’s canals link the Haversian canals of different osteons with one another and with the marrow cavity. They provide the major route for blood vessels from the marrow cavity to the Haversian canals of osteons.

Between the lamellae of an osteon are lacunae containing bone cells called osteocytes. Canaliculi connect the lacunae with one another and with the Haversian canal. The canaliculi contain the processes of the osteocytes, which communicate with one another via gap junctions. Thus nutrients and other substances, such as hormones, can pass from blood vessels in the Haversian canal to distant osteocytes via a sort of bucket brigade. Osteocytes can be involved in both bone deposition and bone resorption.

In addition to the lamellae of osteons, lamellae not belonging to any osteon can be seen in compact bone. These are called interstitial lamellae and are the remnants of previous osteons. They reflect the fact that bone is not static but is constantly being remodelled. In addition, the inner and outer surfaces of long bone have lamellae that run the length of the shaft. They are called, respectively, the inner and outer circumferential lamellae.

Spongy bone is composed of bone spicules, also called trabeculae, of varying shapes and sizes. The spaces between the spicules are filled with marrow. The composition of spongy bone (cells and matrix) is the same as that of compact bone. In spongy bone, however, the lamellae of collagen are not arranged concentrically around a central canal, but run parallel to one another. Osteocytes sit in lacunae between lamellae.

The microscopic difference between compact and cancellous bone is that compact bone consists of haversian sites and osteons, while cancellous bones do not. Also, bone surrounds blood in the compact bone, while blood surrounds bone in the cancellous bone.

Cellular structure

There are several types of cells constituting the bone. These are:

• Osteoprogenitor cells (periosteal and endosteal): Just as cartilage is surrounded by a perichondrium, bone is surrounded by a periosteum of dense connective tissue. As in the perichondrium, the periosteum has two layers: an outer fibrous layer with typical fibroblasts, and an inner cellular layer, which contains osteoprogenitor cells. The osteoprogenitor cells in this location are called periosteal cells. They are capable of giving rise to osteoblasts, which secrete the extracellular matrix of bone.

The marrow surface of compact bone, and the spicules of spongy bone, are lined by an (often single) layer of cells called the endosteum (endosteal cells). Like the periosteal cells, these endosteal cells are also osteoprogenitor cells, capable of becoming osteoblasts. (The two names periosteal cells and endosteal cells refer to their different locations, both function as osteoprogenitor cells).

• Osteoblasts are mononucleate bone forming cells that descend from osteoprogenitor cells. They are located on the surface of osteoid seams and make a protein mixture known as osteoid, which mineralizes to become bone. The osteiod 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. They robustly produce alkaline phosphatase, an enzyme that has a role in the mineralisation of bone, as well as many matrix proteins. Osteoblasts are the immature bone cells.

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

• Osteocytes originate from osteoblasts that have migrated into and become trapped and surrounded by bone matrix that they themselves produce. The spaces they occupy are known as lacunae. Osteocytes have many processes that reach out to meet osteoblasts and other osteocytes probably for the purposes of communication. Their functions include to varying degrees: formation of bone, matrix maintenance and calcium homeostasis. Osteocytes can both secrete and resorb matrix. They have also been shown to act as mechano-sensory receptors — regulating the bone's response to stress and mechanical load. They are mature bone cells.

• Osteoclasts are the cells responsible for bone resorption (remodeling of bone to reduce its volume). Osteoclasts are large, multinucleated cells located on bone surfaces in what are called Howship's lacunae or resorption pits. These lacunae, or resorption pits, are left behind after the breakdown of the bone surface. Because the osteoclasts are derived from a monocyte stem-cell lineage, they are equipped with phagocytic-like mechanisms similar to circulating macrophages. 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.

Molecular structure

• Mineral phase (69 wt%): • Majority is hydroxyapatite [HA] (calcium phosphate) • Also citrate, carbonate, fluoride, and hydroxyl ions

• Organic phase (22 wt%) • Collagen (90-96 wt%) • Cellular components (osteoclasts, osteoblasts, osteocytes)

• Water (9 wt%)

1) Matrix

The majority of bone is made of the bone matrix. It has inorganic and organic parts. Bone is formed by the hardening of this matrix entrapping the cells. When these cells become entrapped from osteoblasts they become osteocytes.

Inorganic

The inorganic composition of bone (bone mineral) is formed from carbonated hydroxyapatite (Ca10(PO4)6OH2) with lower crystallinity. The matrix is initially laid down as unmineralised osteoid (manufactured by osteoblasts). Mineralisation involves osteoblasts secreting 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, distributed among the collagen fibrils of bone and forming yet larger structure. Hydroxyapatite crystals form slender needles in the collagen fiber matrix. The resulting mineral containing fibrils form lamellar sheets.

Organic

The organic part of matrix is mainly composed of Type I collagen. This is synthesised intracellularly as tropocollagen and then exported, forming fibrils. The organic part is also composed of various growth factors, the functions of which are not fully known. Factors present include glycosaminoglycans, osteocalcin, osteonectin, bone sialo protein, osteopontin and Cell Attachment Factor. One of the main things that distinguish the matrix of a bone from that of another cell is that the matrix in bone is hard.

2) Woven or lamellar bone (Microstructure of cortical bone)

Two types of bone can be identified microscopically according to the pattern of collagen forming the osteoid (collagenous support tissue of type I collagen embedded in glycosaminoglycan gel):

1) Woven bone characterised by haphazard organisation of collagen fibers and is mechanically weak, and

2) Lamellar bone which has a regular parallel alignment of collagen into sheets (lamellae) and is mechanically strong.

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 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). 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 2µm per day. Lamellar bone also requires a relatively flat surface to lay the collagen fibers in parallel or concentric layers.

Vascular supply and circulation

In a typical long bone, blood is supplied by three separate systems: a nutrient artery, periosteal vessels, and epiphyseal vessels. The diaphysis and metaphysis are nourished primarily by the nutrient artery. One or two nutrient artery enter the shaft of the long bones in oblique direction through nutrient foramina, which passes through the cortex into the medullary cavity and then ramifies outward through haversian and Volkmann canals to supply the cortex. Extensive vessels in the periosteum, the membrane surrounding the bone, supply the superficial layers of the cortex and connect with the nutrient-artery system. In the event of obstruction of the nutrient artery, periosteal vessels are capable of meeting the needs of both systems. The epiphyses are supplied by a separate system that consists of a ring of arteries entering the bone along a circular band between the growth plate and the joint capsule. In the adult these vessels become connected to the other two systems at the metaphyseal-epiphyseal junction, but while the growth plate is open there is no such connection, and the epiphyseal vessels are the sole source of nutrition for the growing cartilage; therefore they are essential for skeletal growth.

Drainage of blood is by a system of veins that runs parallel with the arterial supply and by veins leaving the cortical periosteum through muscle insertions. Muscle contraction milks blood outward, giving rise to a centrifugal pattern of flow from the axial nutrient artery through the cortex and out through muscle attachments.

Bone Remodeling and Wolff’s Law

Wolff's law is a theory developed by the German Anatomist/Surgeon Julius Wolff (1836–1902) in the 19th century that states that bone in a healthy person or animal will adapt to the loads it is placed under. His theory proposed bone as a living tissue which is constantly undergoing modeling and remodeling.

Wolff proposed that changes in the form and function of bones, or changes in function alone, are followed by changes in the internal structure and shape of the bone in accordance with mathematical laws. If loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading. The internal architecture of the trabeculae undergoes adaptive changes, followed by secondary changes to the external cortical portion of the bone, perhaps becoming thicker as a result. The converse is true as well: if the loading on a bone decreases, the bone will become weaker due to turnover, it is less metabolically costly to maintain and there is no stimulus for continued remodeling that is required to maintain bone mass.

The theory is supported by the observation that bones atrophy when they are not mechanically stressed and hypertrophy when they are stressed. Although Wolff's proposal relates specifically to bone, the law has also been applied to other connective tissues such as ligaments and tendons.

Bone Remodeling

Normal bone is always undergoing remodeling. Healthy bone remodeling occurs at many simultaneous sites throughout the body where bone is experiencing growth, mechanical stress, micro-fractures, or breaks. This remodeling removes old bone tissue and replaces it with new bone tissue. About 20% of all bone tissue is replaced annually by the remodeling process. The remodeling cycle, removing and building tissue, continues throughout life and is typically “in balance” to maintain healthy bone.

There are five phases in the bone remodeling process:

ACTIVATION, RESORPTION, REVERSAL, FORMATION, and QUIESCENCE

The total process takes about 4 to 8 months, and occurs continually throughout our lives. This remodeling cycle involves bone “resorption” by the osteoclasts. The osteoclasts remove the old stressed or worn-out mineralized bone. This recreates a “resorption pit”. The “resorption” process causes osteoblasts to become attracted to the resorption “pit”. Osteoblasts rebuild new bone tissue by laying down an unmineralized matrix, called osteoid, which will eventually form new mineralized bone. When this rebuilding is complete, the area of bone remodeling rests until the next remodeling cycle begins.

B one R emode l ing Cyc le

Phase Phase Events

Activation1. Pre-osteoclasts are attracted to the remodeling sites.

2. Pre-osteoclasts fuse to form multinucleated osteoclasts.

Resorption

3. Osteoclasts dig out a cavity, called a resorption pit, in spongy bone or burrow a tunnel in compact bone.

4. Calcium can be released into the blood for use in various body functions.

5. Osteoclasts disappear.

Reversal

6. Mesenchymal stem cells, pre-cursors to osteoblasts, appear along the burrow or pit where they...

7. proliferate (increase in numbers) and differentiate (change) into pre-osteoblasts, then ...

Formation

8. mature into osteoblasts at the surface of the burrow or pit and ...

9. release osteoid at the site, forming a new soft nonmineralized matrix.

10. The new matrix is mineralized with calcium and phosphorous.

Quiescence 11. Site, with resting lining cells, remains dormant until the next cycle.

Conclusion:

The bone is a dynamic, living tissue; not the hard, dry, lifeless frame. Although it is hard, it has

a definite cellular structure and molecular structure. Bone is made up of living and non-living

components. There are living cells (collagen, and various cells like osteoblast, osteoclast, osteocytes

etc. etc.) and non-living; minerals (calcium, Phosphorus, carbonates, water etc). It has a blood supply

and nerve innervations.

Like most other tissues, broken bone can repair itself. The healing of the bone tissues starts im-

mediately after an injury. The initial is an acute inflammatory phase to stop the bleeding by formation

of clots. Then a highly vascularized fibrous tissue is deposited by the blood around the broken ends.

This material begins the formation of a kind of protective, lumpy sleeve, called a callus, around the

broken ends, mainly cartilage. The callus hardens into spongy bone, normally within a month or two.

Then, the spongy bone begins to be reduced in size by bone-dissolving cells produced in the marrow,

while at the same time the spongy bone in the area of the break is beginning to be replaced by hard

bone.

The bone over the life period is undergoing constant changes of modeling and remodeling. Re-

modeling occurs at many simultaneous sites throughout the body where bone is experiencing growth,

mechanical stress, micro-fractures, or breaks. This remodeling removes old bone tissue and replaces it

with new bone tissue. Also there is removal of old and damaged cells with new bone cells. The bone is

dynamic in terms if shape as well. The bone not only maintains its cellular structure but also adapts it-

self to its new surroundings. i.e. increase in load to bone adapts itself accordingly by remodeling. This

may be increase in thickness of bone, increase in bone density etc. that it adapts to bear the increase or

decrease in work load. Also there are consequent changes in the molecular and cellular structure to

make bone thicker. Thus bone is a living dynamic tissue.

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