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Lecture 10: BONE
Everything is connected to everything else
The Skeletal System: Bone Tissue
Functions of Bone and Skeletal System
Structure of Bone
Histology of Bone Tissue
Blood and Nerve Supply of Bone
Bone Formation
Bones Role in Calcium Homeostasis
Exercise and Bone TissueAging and Bone Tissue
Overview
Bone tissue forms most of the SKELETON,
which allows us to move, provides support,
and protection.
The study of bone structure and bone
disorders is known as OSTEOLOGY.
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Overview
Functions of Bone:
1. SUPPORT - provides a point of attachment for
SKELETAL MUSCLE.
2. PROTECTION - Protects internal organs frominjury. Cranial bones protect the brain, the ribcage protects the thoracic cavity, etc
3. MOVEMENT - Assists in movement (muscleattaches to bone).
Overview
Functions of Bone (continued)
4. MINERAL HOMEOSTASIS
Bone tissue stores several minerals, especially
CALCIUM and PHOSPHORUS.
5. BLOOD CELL PRODUCTION
Red bone marrow is the site of production of red
and white blood cells and platelets. The
process of blood cell formation -
HEMOPOIESIS.
6. ENERGY STORAGE
Lipids are stored in YELLOW bone marrow.
Overview
The Skeletal System consists of bone tissue,
cartilage, red and yellow bone marrow,
periosteum and endosteum.
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Bone Anatomy
Basic Structure of typical LONG bone:
1. DIAPHYSIS - shaft or main LONG portion.
2. EPIPHYSIS - distal and proximal ENDS or
extremities.
Bone Anatomy
Basic Structure of typical LONG bone:
3. METAPHYSIS - region where the Diaphysis
joins the Epiphysis.
In a growing bone, each metaphysis includes an
EPIPHYSEAL PLATE which is a layer of hyaline
cartilage that allows the diaphysis of the bone to
grow in length.
4. ARTICULAR CARTILAGE - a thin layer ofhyaline cartilage covering the epiphysis wherebone forms an articulation or joint.
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Bone Anatomy
5. PERIOSTEUM - a membrane around the
surface of bone not covered by articular
cartilage.
6. MEDULLARY (MARROW) CAVITY
space within the Diaphysis that contains
YELLOW or fatty bone marrow.
7. ENDOSTEUM - lines the medullary cavity.
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Histology of Bone Tissue
Extracellular matrix surrounding widely
separated cells
Matrix
25% water
25% collagen fibers
50% crystallized mineral salts
The most abundant mineral salt is
calcium phosphate
Histology of Bone Tissue
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
Bones flexibilitydepends on collagenfibers
Bone Histology
OSSEOUS TISSUE contains an abundant
matrix with widely separated cells.
4 types of cells in bone tissue:
1. OSTEOPROGENITOR CELLS - these candivide, are unspecialized cells, derived fromMESENCHYME (remember, we said allconnective tissue is derived from this).
Found in the periosteum and endosteum.
Can divide and become OSTEOBLASTS.
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Bone Histology
2. OSTEOBLASTS
CANT DIVIDE.
Osteoblasts secrete MATRIX.
Form bone, but have lost their ability to divide bymitosis.
Secrete components that build bone tissue.
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Bone Histology
3. OSTEOCYTES
CANT DIVIDE.
DO NOT secrete matrix.
Osteocytes are mature bone cells that are derivedfrom OSTEOBLASTS.
NO mitotic potential.
Osteoblasts initially form bone tissue, osteocytes
no longer secrete bone tissue materials, butmaintain the daily activities of bone tissue.
Bone Histology
4. OSTEOCLASTS
clast - break
Function in bone RESORPTION (destruction ofthe matrix), which is important in repairing andmaintaining bone.
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Bone
Bone is not completely solid, but has many
small spaces within it.
Depending on the size and distribution of these
spaces, regions of bone may be classified as:
1. COMPACT (DENSE) BONE
2. CANCELLOUS (SPONGY) BONE
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Compact Bone
The main difference between compact and
cancellous bone is that in compact bone,
there are concentric ring structures of
OSTEONS (HAVERSIAN SYSTEMS) in
compact bone and an irregular arrangement
of osteons in SPONGY BONE.
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Compact Bone
Central canals run longitudinally through
bone.
Around these canals are CONCENTRIC
LAMELLAE - rings of hard, calcified matrix.
Between the lamellae are spaces called
LACUNAE that contain OSTEOCYTES.
Remember, osteocytes are mature bone cells that
no longer secrete matrix.
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Compact Bone
Radiating in all directions form lacunae are
CANALICULI, which connect lacunae with
one another and with central canals.
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Spongy Bone
Compared to compact bone, spongy bone
has no formal osteons.
Instead, lamellae are arranged in irregular
configurations of thin plates called TRABECULAE.
Spongy bone makes up most of the bone
tissue in short, flat, and irregular shaped
bones and the epiphyses of long bones.
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Spongy Bone
Spongy bone in the hips, ribs, sternum,
vertebrae, skull, and ends of long bones is
the only site of HEMOPOIESIS in adults. Where blood cells are PRODUCED.
Compact Bone
How do nutrients actually get into bone?
From the PERIOSTEUM - blood vessels, nerves,etc, penetrate the compact bone tissue
through PERFORATING (VOLKMANNS)CANALS.
These connect with the MEDULLARY CAVITY,
PERIOSTEUM, and CENTRAL (HAVERSIAN)CANALS.
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Bone Formation: Ossification
Bones in infants are not hard or rigid.
There is a constant process of bone being
broken down and rebuilt.
This process by which bone forms is calledOSSIFICATION.
Ossification begins around the 6th or 7th
week of embryonic life and continues
throughout adulthood.
Bone Formation: Ossification
Bone formation follows one of two patterns:
1. INTRAMEMBRANOUS OSSIFICATION
2. ENDOCHONDRAL OSSIFICATION
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Bone Formation: Ossification
Intramembranous ossification:
Refers to bone formation DIRECTLY within loose
fibrous connective tissue membranes.
Forms DIRECTLY from MESENCHYME withoutgoing through a CARTILAGE stage first.
Fontanels in an infant skull (soft spots) are
composed of loose fibrous connective tissue and
are eventually replaced by bone through
Intramembranous Ossification.
Bone Formation: Ossification
Endochondral Ossification:
Refers to formation of bone IN hyaline cartilage.
Mesenchyme is transformed into
CHONDROBLASTS - cartilage precursors, thatproduce a hyaline cartilage matrix that is graduallyREPLACED BY BONE.
Bone Formation: Ossification
These two kinds of ossification DO NOT lead
to differences in STRUCTURE of mature
bones.
They simply indicate different methods in
bone FORMATION.
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Bone Formation: Ossification
The first stage in development of bone is
migration of MESENCHYMAL cells into an
area where bone formation is about to begin. These cells increase in number and size.
In some skeletal structures, they becomechondroblasts and produce cartilage; in othersthey become osteoblasts and will form bonetissue.
Bone Formation: Ossification
Intramembranous Ossification Steps:
1. At the site where bone will develop,MESENCHYME cells cluster together and
differentiate into OSTEOPROGENITOR CELLSand then into OSTEOBLASTS.
The place where this cluster occurs is called the
CENTER OF OSSIFICATION
Osteoblasts secrete matrix and become surrounded
by it.
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Bone Formation: Ossification
Intramembranous Ossification Steps:
2. Matrix secretion stops and those cells become
OSTEOCYTES and lie within lacunae. Calcium and other minerals start to be deposited
and the MATRIX starts to calcify.
Bone Formation: Ossification
Intramembranous Ossification Steps:
3. As bone matrix starts to form, it develops into
TRABECULAE that fuse with one another to form
precursor bone with the appearance of SPONGYbone.
Mesenchyme cells start to condense on the outer
surface of the developing bone.
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Bone Formation: Ossification
Intramembranous Ossification Steps:
4. The condensed mesenchyme develops intoPERIOSTEUM.
Most surface layers of the spongy bone are
replaced by compact bone, but spongy bone
remains in the center of the bone.
Eventually, newly formed bone is remodeled and
reshaped until reaching its final adult shape and
size.
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Bone Formation: Ossification
Endochondral Ossification steps:
Replacement of CARTILAGE by bone.
Most bones in the body are formed this way.
1. Development of a cartilage precursor. At the site where bone will form, mesenchyme cells gather
in the rough shape of the future bone.
These cells differentiate into CHONDROBLASTS that
produce hyaline cartilage.
A PERICHONDRIUM develops around the cartilage
structure.
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Bone Formation: Ossification
Endochondral Ossification Steps:
2. GROWTH of CARTILAGE
Grows by continual cell division.
A nutrient artery penetrates the Perichondrium.
This stimulates OSTEOPROGENITOR CELLS in
the perichondrium to form OSTEOBLASTS.
Once the perichondrium starts to form bone, it is
now called PERIOSTEUM.
Bone Formation: Ossification
Endochondral Ossification Steps:
3. Development of the PRIMARY ossification
center:
Capillaries grow into disintegrating cartilage and
form the Primary ossification center.
This is where bone tissue will replace most of the
cartilage.
OSTEOBLASTS begin to deposit bone matrix over
the remnants of calcified cartilage.
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Bone Formation: Ossification
Endochondral Ossification Steps:
3. Development of the PRIMARY ossification
center: The ossification center enlarges towards the ends
of the bone.
Osteoclasts break down former spongy bone areas
leaving a new medullary cavity.
The cavity fills with red bone marrow.
Bone Formation: Ossification
Endochondral Ossification Steps:
4. Development of Diaphysis and Epiphysis:
The shaft (diaphysis), which was spongy bone, is
now replaced by compact bone with a core of red
bone marrow-filled MEDULLARY CAVITY.
When blood vessels enter the epiphyses (ends),
SECONDARY OSSIFICATION CENTERS develop,
usually around the time of birth.
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Bone Formation: Ossification
Endochondral Ossification Steps:
5. SECONDARY ossification centers:
Bone formation is similar to that in Primary areas.
However, SPONGY bone remains in the interior of
the epiphyses (no medullary cavity).
Hyaline cartilage remains covering the epiphyses
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Bone Growth During Infancy, Childhood
and Adolescence Growth in Length
The growth in length oflong bones involves two
major events: 1) Growth of cartilage
on the epiphysealplate
2) Replacement ofcartilage by bonetissue in theepiphyseal plate
Bone Growth During Infancy, Childhood
and Adolescence
Osteoclasts dissolve the
calcified cartilage, andosteoblasts invade the arealaying down bone matrix
The activity of the epiphysealplate is the way bone canincrease in length
At adulthood, the epiphysealplates close and bone replacesall the cartilage leaving a bonystructure called the epiphysealline
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Bone Growth During Infancy, Childhood
and Adolescence 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 isformed in its place throughout an individuals life
Bone Homeostasis
Just like skin, bone forms before birth, but
there is a constant RENEWAL of bone tissue
after birth.
REMODELING is the ongoing replacement of oldbone tissue by new bone tissue.
Ex. The distal end of the femur is replaced aboutevery 4 months.
But some areas in the femur may never be replaced
during an individuals lifetime.
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Remodeling
OSTEOCLASTS are responsible for BONE
RESPORTION (destruction of the matrix).
The body maintains a delicate balance betweenbuilding too much new bone too fast by
osteoblasts and osteoclasts removing mineralsand collagen.
Remodeling
OSTEOPOROSIS
Condition where the basic problem is that boneresorption outpaces bone formation and the
bones weaken.
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Remodeling
PAGETS DISEASE
When there is abnormal ACCELERATION of theremodeling process.
Osteoblastic bone formation is extensive andthere is irregular THICKENING of bone.
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Remodeling
OSTEOMYELITIS:
Inflammation of bone, especially bone marrow,
caused by a pathogenic organism, oftenStaphylococcusaureus.
Remodeling
In the basic process of resorption,
osteoclasts secrete protein-digesting
LYSOSOMAL enzymes that digest matrix
materials.
Factors Affecting Bone Growth and Bone
Remodeling Normal bone metabolism depends on several factors
Minerals Large amounts of calcium and phosphorus and
smaller amounts of magnesium, fluoride, andmanganese are required for bone growth andremodeling
Vitamins Vitamin A stimulates activity of osteoblasts
Vitamin C is needed for synthesis of collagen
Vitamin D helps build bone by increasing theabsorption of calcium from foods in the gastrointestinaltract into the blood
Vitamins K and B12 are also needed for synthesis ofbone proteins
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Factors Affecting Bone Growth and Bone
Remodeling Hormones
During childhood, the hormones most important to
bone growth are growth factors (IGFs), produced bythe liver IGFs stimulate osteoblasts, promote cell division at the
epiphyseal plate, and enhance protein synthesis
Thyroid hormones also promote bone growth bystimulating osteoblasts
Insulin promotes bone growth by increasing thesynthesis of bone proteins
Factors Affecting Bone Growth and Bone
Remodeling Hormones
Estrogen and testosterone cause a dramaticeffect on bone growth Cause of the sudden growth spurt that occurs
during the teenage year
Promote changes in females, such as widening ofthe pelvis
Shut down growth at epiphyseal plates
Parathyroid hormone, calcitriol, and calcitoninare other hormones that can affect bone
remodeling Weight Bearing Exercise
Fracture and Repair of Bone
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
Potts 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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Fracture and Repair of Bone
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Fracture and Repair of Bone
Calcium and phosphorus needed to strengthen andharden new bone after a fracture are deposited onlygradually and may take several months
The repair of a bone fracture involves the followingsteps 1) Formation of fracture hematoma
Blood leaks from the torn ends of blood vessels, a clottedmass of blood forms around the site of the fracture
2) Fibrocartilaginous callus formation
Fibroblasts invade the fracture site and produce collagenfibers 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
Compact boneSpongy bone
Periosteum
Fracture hematoma
Fracturehematoma
Bonefragment
Osteocyte
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
Bonefragment
Osteocyte
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
Bonefragment
Osteocyte
Red bloodcell
Blood vessel
Formation of fracture hematoma
Phagocyte
Osteon
1
Phagocyte
Osteoblast
Fibroblast
Fibrocartilaginouscallus
Collagen fiber
Chondroblast
Cartilage
Fibrocartilaginous callus formation2
Spongy bone
Osteoblast
Osteoclast
New compactbone
Bony callus formation Bone remodeling
Osteocyte
3 4
Compact boneSpongy bone
Periosteum
Fracture hematoma
Fracturehematoma
Bonefragment
Osteocyte
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 Fractures
1. Formation of Fracture Hematoma Blood vessels crossing the bone are broken as
a result of the fracture.
Blood leaks into the area and forms a clotcalled a FRACTURE HEMATOMA.
Bone cells at the fracture site die due to lack ofblood flow.
The hematoma serves as a focus of cellularinvasion to help heal the fracture.
White blood cells start to flow into the area andbegin to clean up debris.
6-8 hours after fracture.
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Bone Fractures
2. Fibrocartilaginous Callus FOrmation Capillaries begin to grow into the hematoma
and organized tissue called a PROCALLUSforms.
A CALLUS is a mass of repair tissue thatbridges the ends of broken bones.
Fibroblasts begin to secrete collagen whichhelps join the ends of the fracture.
Precursor cells develop into chondroblasts(cartilage cells) and the procallus is now calleda FIBROCARTILAGINOUS (SOFT) CALLUS.
Lasts about 3 weeks.
Bone Fractures
3. Bony Callus Formation
Osteoprogenitor cells begin to develop into
osteoblasts which begin to produce new spongy
bone that bridges the ends of the broken bone.
The Callus is then referred to as a BONY (HARD)CALLUS.
3-4 months.
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Bone Fractures
4. Bone Remodeling
The final phase is remodeling of the callus.
Dead portions of the original fracture are gradually
resorbed by osteoclasts.
Compact bone replaces spongy bone.
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Calcium Homeostasis
Bone is a major storage area for calcium.
It stores more than 99% of the total calcium in the
body.
The role of bone in calcium homeostasis is tobuffer the level of calcium in the blood.
If there is too much calcium in the blood - bonewill take back calcium. If there is too little - bonewill release calcium into blood.
Hormones regulate these exchanges.
Bones Role in Calcium HomeostasisActions that help elevate blood Ca2+ level Parathyroid hormone (PTH) regulates
Ca2+ exchange between blood and bonetissue PTH increases the number and activity of
osteoclasts
PTH acts on the kidneys to decrease loss ofCa2+ in the urine
PTH stimulates formation ofcalcitriol ahormone that promotes absorption of
calcium from foods in the gastrointestinaltract
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Bones Role in Calcium HomeostasisActions that work to decrease blood Ca2+
level
The thyroid gland secretes calcitonin (CT)whichinhibits activity of osteoclasts
The result is that CT promotes boneformation and decreases blood Ca2+ level
Exercise and Bone Tissue Bone tissue alters its strength in response to
changes in mechanical stress Under stress, bone tissue becomes stronger through
deposition of mineral salts and production of collagenfibers by osteoblasts
Unstressed bones diminishes because of the loss ofbone minerals and decreased numbers of collagenfibers
The main mechanical stresses on bone arethose that result from the pull of skeletal musclesand the pull of gravity
Weight-bearing activities help build and retainbone mass
Aging and Bone Tissue
Principal Effects of Aging:
1. LOSS of calcium and other minerals.
Usually begins after 30 in females and accelerates
around age 40-45.
Does not begin until about age 60 in males.
Loss of calcium is one of the problems in
OSTEOPOROSIS.
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Aging and Bone Tissue
Principal Effects of Aging:
2. Second effect is a decrease in the rate of
protein synthesis. This causes the bones to lose tensile strength or
become more brittle and more susceptible to
fracture.