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1. Functions of Bone and Skeletal System Structure of Bone Histology of Bone Tissue Blood and Nerve Supply of Bone Bone Formation Bone’s Role

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Functions of Bone and Skeletal System Structure of Bone Histology of Bone Tissue Blood and Nerve Supply of Bone Bone Formation Bone’s Role in Calcium Homeostasis Exercise and Bone Tissue Aging and Bone Tissue

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Support Protection Assistance in Movement Mineral Homeostasis Blood Cell Production Triglyceride Storage

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Support Structural framework of the body

Supports soft tissues Provides attachment points for tendons of skeletal

muscle

Protection Protects important internal organs For example…

Cranium protects brain Vertebrae protects spinal cord Ribs protect lungs and heart

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Assistance in Movement Skeletal muscle attaches to bone

Skeletal muscle contraction pulls on bone producing movement

Mineral Homeostasis Bone tissue stores several minerals

Acts to serve as a reservoir of critical minerals Calcium (99% of body’s content) Phosphorus

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Blood Cell Production Red bone marrow produces (Hemopoiesis)

Red blood cells White blood cells Platelets

Triglyceride Storage Yellow bone marrow

Triglycerides stored in adipose cells Serves as a potential chemical energy reserve

Diaphysis Epiphysis Metaphysis

Epiphyseal growth plate Articular cartilage

Perforating fibers Periosteum Medullary cavity Endosteum

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Long Bone Anatomy (Humerus)

Extracellular matrix surrounding widely separated cellsMatrix

25% water25% collagen fibers50% crystallized mineral salts

The most abundant mineral salt found in bone is calcium phosphate

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A process called calcification is initiated by bone-building cells called osteoblasts

Mineral salts are deposited and crystalize in the framework formed by the collagen fibers of the extracellular matrix

Bone’s flexibility depends on collagen fibers

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Four types of cells are present in bone tissue Osteogenic cells

Undergo cell division; the resulting cells develop into osteoblasts

Osteoblasts Bone-building cells Synthesize extracellular matrix of bone tissue

Osteocytes Mature bone cells Exchange nutrients and wastes with the blood

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Osteoclast Functions: Release enzymes that digest the mineral components of

bone matrix (resorption) Regulate blood calcium level

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Bone may be categorized as either: CompactSpongy

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Compact BoneResists the stresses produced by weight and

movementComponents of compact bone are arranged into

repeating structural units called osteons or Haversian Systems or Haversian Canals

Osteons consist of a central (Haversian) canal with concentrically arranged lamellae, lacunae, osteocytes, and canaliculi.

The result is a structure that resembles a tree trunk cut in half.

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OsteonCentral canals run longitudinally through boneAround the central canals are concentric

lamellae Rings of calcified matrix (like the rings of a tree

trunk)Between the lamellae are small spaces called

lacunae which contain osteocytesRadiating in all directions from the lacunae are

tiny canaliculi filled with extracellular fluid

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Osteon Canaliculi connect

lacunae, forming a system of interconnected canals Providing routes for

nutrients and oxygen to reach the osteocytes

The organization of osteons changes in response to the physical demands placed on the skeleton

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Spongy BoneLacks osteonsLamellae are arranged in a lattice of thin

columns called trabeculae Spaces between the trabeculae make bones lighter Trabeculae of spongy bone support and protect the

red bone marrow Hemopoiesis (blood cell production) occurs in

spongy bone

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Spongy BoneWithin each trabecula are lacunae that contain

osteocytesOsteocytes are nourished from the blood

circulating through the trabeculaeInterior bone tissue is made up primarily of

spongy boneThe trabeculae of spongy bone are oriented along

lines of stresshelps bones resist stresses without breaking

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Bone is richly supplied with blood Periosteal arteries accompanied by

nerves supply the periosteum and compact bone

Epiphyseal veins carry blood away from long bones

Nerves accompany the blood vessels that supply bones The periosteum is rich in sensory

nerves sensitive to tearing or tension

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The process by which bone forms is called ossification

Bone formation occurs in four situations:1) Formation of bone in an embryo2) Growth of bones until adulthood3) Remodeling of bone4) Repair of fractures

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Formation of Bone in an EmbryoCartilage formation and ossification occurs

during the sixth week of embryonic development

Bone formation follows one of two patterns Intramembranous ossification

Flat bones of the skull and mandible are formed in this way

“Soft spots” that help the fetal skull pass through the birth canal later become ossified forming the skull

Endochondral ossificationThe replacement of cartilage by boneMost bones of the body are formed in this way including

long bones

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1

Blood capillary

Ossification center

Mesenchymal cell

Osteoblast

Collagen fiber

Development of ossification center

Mandible

Flat boneof skull

1

Blood capillary

Ossification center

Mesenchymal cell

Osteoblast

Osteocyte in lacuna

Canaliculus

Osteoblast

Newly calcified bonematrix

Development of ossification center

Calcification

Mandible

Flat boneof skull

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Collagen fiber

1

Blood capillary

Ossification center

Mesenchymal cell

Osteoblast

Development of ossification center

Calcification

Mandible

Flat boneof skull

2

Collagen fiber

Osteocyte in lacuna

Canaliculus

Osteoblast

Newly calcified bonematrix

Mesenchymecondenses

Blood vessel

Spongy bonetrabeculae

Osteoblast

Formation of trabeculae3

1

Blood capillary

Ossification center

Mesenchymal cell

Osteoblast

Mesenchymecondenses

Blood vessel

Spongy bonetrabeculae

Osteoblast

Periosteum

Spongy bone tissue

Compact bone tissue

Development of ossification center

Calcification Formation of trabeculae

Development of the periosteum

Mandible

Flat boneof skull

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4

2

Collagen fiber

Osteocyte in lacuna

Canaliculus

Osteoblast

Newly calcified bonematrix

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Growth in Length The growth in length of

long bones involves two major events:1) Growth of cartilage

on the epiphyseal plate2) Replacement of

cartilage by bone tissue in the epiphyseal plate

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Osteoclasts dissolve the calcified cartilage, and osteoblasts invade the area laying down bone matrix

The activity of the epiphyseal plate is the way bone can increase in length

At adulthood, the epiphyseal plates close and bone replaces all the cartilage leaving a bony structure called the epiphyseal line

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Growth in Thickness Bones grow in thickness at the outer surface

Remodeling of Bone Bone forms before birth and continually renews itself The ongoing replacement of old bone tissue by new bone

tissue Old bone is continually destroyed and new bone is

formed in its place throughout an individual’s life

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A balance must exist between the actions of osteoclasts and osteoblasts

If too much new tissue is formed, the bones become abnormally thick and heavy

Excessive loss of calcium weakens the bones, as occurs in osteoporosis

Or they may become too flexible, as in rickets and osteomalacia

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1 Development ofcartilage model

Hyalinecartilage

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

1 Development ofcartilage model

Growth ofcartilage model

2

Hyalinecartilage

Uncalcifiedmatrix

Calcifiedmatrix

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

1 Development ofcartilage model

Development ofprimary ossificationcenter

Growth ofcartilage model

2 3

Hyalinecartilage

Uncalcifiedmatrix

Calcifiedmatrix

Nutrientartery

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

Periosteum

Primaryossificationcenter

Spongybone

1

Hyalinecartilage

Calcifiedmatrix

Periosteum(covering compact bone)

Uncalcifiedmatrix

Calcifiedmatrix

Medullarycavity

Nutrientartery and vein

Nutrientartery

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

Development ofcartilage model

Development ofprimary ossificationcenter

Development ofthe medullarycavity

Growth ofcartilage model

Periosteum

Primaryossificationcenter

2 3 4

Spongybone

Uncalcifiedmatrix

1 Development ofcartilage model

Development ofprimary ossificationcenter

Development ofthe medullarycavity

Growth ofcartilage model

2 3 4

Hyalinecartilage

Calcifiedmatrix

Periosteum(covering compact bone)

Uncalcifiedmatrix

Calcifiedmatrix

Medullarycavity

Nutrientartery and vein

Nutrientartery

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

Periosteum

Primaryossificationcenter

Secondaryossificationcenter

Nutrientartery and vein

Uncalcifiedmatrix

Epiphysealartery andvein

Development of secondaryossification center

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Spongybone

Uncalcifiedmatrix

1

Articular cartilage

Spongy bone

Epiphyseal plate

Secondaryossificationcenter

Nutrientartery and vein

Uncalcifiedmatrix

Epiphysealartery andvein

Formation of articular cartilageand epiphyseal plate

Development of secondaryossification center

Development ofcartilage model

Development ofprimary ossificationcenter

Development ofthe medullarycavity

Growth ofcartilage model

2 3 4

5 6

Hyalinecartilage

Uncalcifiedmatrix

Calcifiedmatrix

Periosteum(covering compact bone)

Uncalcifiedmatrix

Calcifiedmatrix

Medullarycavity

Nutrientartery and vein

Nutrientartery

Perichondrium

Proximalepiphysis

Distalepiphysis

Diaphysis

Periosteum

Primaryossificationcenter

Spongybone

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Normal bone metabolism depends on several factors Minerals

Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling

VitaminsVitamin A stimulates activity of osteoblastsVitamin C is needed for synthesis of collagenVitamin D helps build bone by increasing the absorption

of calcium from foods in the gastrointestinal tract into the blood

Vitamins K and B12 are also needed for synthesis of bone proteins

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Hormones

During childhood, the hormones most important to bone growth are growth factors (IGFs), produced by the liver IGFs stimulate osteoblasts, which in turn promote cell division

at the epiphyseal plate, and enhance protein synthesis

Thyroid hormones also promote bone growth by stimulating osteoblasts

Insulin promotes bone growth by increasing the synthesis of bone proteins

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HormonesEstrogen and testosterone cause a dramatic

effect on bone growth Cause of the sudden “growth spurt” that occurs

during the teenage year Promote changes in females, such as widening of

the pelvis Shut down growth at epiphyseal plates

Parathyroid hormone, calcitriol, and calcitonin are other hormones that can affect bone remodeling

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Fracture Types Open (compound) fracture

The broken ends of the bone protrude through the skin Closed (simple) fracture

Does not break the skin Comminuted fracture

The bone is splintered, crushed, or broken into pieces Greenstick fracture

A partial fracture in which one side of the bone is broken and the other side bends Impacted fracture

One end of the fractured bone is forcefully driven into another Pott’s fracture

Fracture of the fibula, with injury of the tibial articulation Colles’ fracture

A fracture of the radius in which the distal fragment is displaced Stress fracture

A series of microscopic fissures in bone

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Calcium and phosphorus needed to strengthen and harden new bone after a fracture are deposited only gradually and may take several months

The repair of a bone fracture involves the following steps 1) Formation of fracture hematoma

Blood leaks from the torn ends of blood vessels, a clotted mass of blood forms around the site of the fracture

2) Fibrocartilaginous callus formation Fibroblasts invade the fracture site and produce collagen fibers

bridging the broken ends of the bone 3) Bony callus formation

Osteoblasts begin to produce spongy bone trabeculae joining portions of the original bone fragments

4) Bone remodeling Compact bone replaces spongy bone

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Compact boneSpongy bone

Periosteum

Fracture hematoma

Fracturehematoma

BonefragmentOsteocyte

Red bloodcell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginouscallus

Collagen fiber

Chondroblast

Cartilage

Fibrocartilaginous callus formation2

Compact boneSpongy bone

Periosteum

Fracture hematoma

Fracturehematoma

BonefragmentOsteocyte

Red bloodcell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Bony callus

Spongy bone

Osteoblast

Bony callus formation

Osteocyte

3

Compact boneSpongy bone

Periosteum

Fracture hematoma

Fracturehematoma

BonefragmentOsteocyte

Red bloodcell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginouscallus

Collagen fiber

Chondroblast

Cartilage

Fibrocartilaginous callus formation2

Spongy bone

OsteoblastOsteoclast

New compactbone

Bony callus formation Bone remodeling

Osteocyte

3 4

Compact boneSpongy bone

Periosteum

Fracture hematoma

Fracturehematoma

BonefragmentOsteocyte

Red bloodcell

Blood vessel

Formation of fracture hematoma

Phagocyte

Osteon

1

Phagocyte

Osteoblast

Fibroblast

Fibrocartilaginouscallus

Collagen fiber

Chondroblast

Cartilage

Fibrocartilaginous callus formation2

Bony callus

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Bone is the body’s major calcium reservoir Levels of calcium in the blood are maintained

by controlling the rates of calcium resorption from bone into blood and of calcium deposition from blood into boneBoth nerve and muscle cells depend on calcium

ions (Ca2+) to function properlyBlood clotting also requires Ca2+

Many enzymes require Ca2+ as a cofactor

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To review the different types of fractures, their descriptions and accompanying radiographs and/or drawings, visit the following sites:

http://www.rush.edu/rumc/page-P07392.html http://www.nlm.nih.gov/medlineplus/ency/imagep

ages/1096.htm http://orthoinfo.aaos.org/fact/thr_report.cfm?

thread_id=125&topcategory=about%20orthopaedics

Actions that help elevate blood Ca2+ levelParathyroid hormone (PTH) regulates

Ca2+ exchange between blood and bone tissuePTH increases the number and activity of

osteoclastsPTH acts on the kidneys to decrease loss of

Ca2+ in the urinePTH stimulates formation of calcitriol a

hormone that promotes absorption of calcium from foods in the gastrointestinal tract

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Actions that work to decrease blood Ca2+ level

The thyroid gland secretes calcitonin (CT) which inhibits activity of osteoclasts

The result is that CT promotes bone formation and decreases blood Ca2+ level

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Bone tissue alters its strength in response to changes in mechanical stressUnder stress, bone tissue becomes stronger through

deposition of mineral salts and production of collagen fibers by osteoblasts

Unstressed bones diminishes because of the loss of bone minerals and decreased numbers of collagen fibers

The main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of gravity

Weight-bearing activities help build and retain bone mass

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The level of sex hormones diminishes during middle age, especially in women after menopauseA decrease in bone mass occursBone resorption by osteoclasts outpaces bone

deposition by osteoblasts Female bones generally are smaller and less

massive than malesLoss of bone mass in old age has a greater

adverse effect in females

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There are two principal effects of aging on bone tissue: 1) Loss of bone mass

Results from the loss of calcium from bone matrix The loss of calcium from bones is one of the symptoms in osteoporosis

2) Brittleness Results from a decreased rate of protein synthesis Collagen fibers gives bone its tensile strength The loss of tensile strength causes the bones to become very brittle and

susceptible to fracture

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Osteoporosis: A decrease in the amount and strength of bone tissue owing to decreases in hormone output. In osteoporosis, bone resorption outpaces bone formation. This cause bones to become fragile and more likely to break. If not prevented or if left untreated, osteoporosis can progress painlessly until a bone breaks. These fractures occur typically in the hip, spine, and wrist.

Any bone can be affected, but of special concern are fractures of the hip and spine which almost always requires hospitalization and major surgery. It can impair a person's ability to walk unassisted and may cause prolonged or permanent disability or even death. Spinal or vertebral fractures can also cause loss of height, severe back pain, and deformity

Paget’s disease causes a malfunction in the normal process of bone remodeling. When an area of bone is destroyed in a person with Paget’s disease, the bone that replaces it is soft and porous. Soft bone can be weak and easily bend, leading to shortening of the affected part of the body. 

The bone replacement also takes place very quickly and excess bone may be formed. This can cause the bone to get larger, be painful and break easily.

Bone affected by Paget’s disease tends to have more blood vessels than normal.  This causes an increase in the blood supply to the area, and as a result the area may feel warmer than usual.

The disease can affect any bone but more commonly affects the spine, pelvis, skull, femur and the tibia.

Paget’s disease can lead to other medical conditions including osteoarthritis, kidney stones and heart disease.

Paget’s disease is also called osteitis deformans. It is named after Sir James Paget, an English doctor who first described the disease in 1876.

How common is Paget’s disease?  

The exact number of people with Paget’s disease is not known. Men are more likely to be affected by Paget’s disease than women. It usually affects people over age 40.

Paget’s disease is estimated to affect 3% of people over 40.  However the exact number is not known because many people who have it do not know it.  It occurs all over the world but is more prevalent in some areas, such as in Europe and Australia.

It tends to affect men more than women, and usually those over the age of 40.

What are the warning signs of Paget’s disease?  

Because Paget’s disease comes on slowly many people do not know they have it. If you have Paget’s disease the first warning sign may be pain in or over a bone. The area may feel extra warm. You may feel also feel tired. If Paget’s disease affects the bones in a leg, the shape of the bone may change and your legs

may bend or bow out. If it affects the skull, your head may get bigger.  It can also cause you to have trouble hearing. Paget’s disease can cause your bones to break more easily. Often Paget’s affects only one or two bones.

In many cases, Paget's disease takes a very mild course and a person with it may not have any symptoms. Those who do have symptoms may be affected in various ways. 

If you have Paget’s disease your bones may break easily because they are weakened. Your bones may also bend, and if your leg bones are affected you may notice that your legs bow, or a leg may appear to shrink. If your spine is involved, you may feel pain in your back. If the bones in your spine bend or grow larger than usual this can put pressure on your nerves, and you may feel pain or numbness in other areas of your body also.

If Paget’s disease affects your skull, your head may increase in size from front to back.  Hearing loss may result if there is involvement of some of the small bones in the middle ear or pressure is placed on the nerves related to hearing.

In late stages of the disease, your hip joint may become damaged if the bones of your pelvis have been involved.

Usually only one or a few bones are affected.  However, the disease can be widespread and affect all bones. Because of the increased number of blood vessels in bones affected by Paget’s disease your heart has to work harder to pump blood to them.  If many bones are affected this can cause strain on your heart and lead to other problems.

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