211 ch 6 bone
Bone Tissue
Chapter 6
Skeletal Cartilages
• All contain chondrocytes in lacunae and extracellular matrix
• Three types
– Hyaline cartilage
• Provides support, flexibility, and resilience
• Collagen fibers only; most abundant type
• Articular, costal, respiratory, nasal cartilage
– Elastic cartilage
• Similar to hyaline cartilage, but contains elastic fibers
• External ear and epiglottis
– Fibrocartilage
• Thick collagen fibers—has great tensile strength
• Menisci of knee; vertebral discs
Classification of Bones
• 206 named bones in skeleton
• Divided into two groups
– Axial skeleton
• Long axis of body
• Skull, vertebral column, rib cage
– Appendicular skeleton
• Bones of upper and lower limbs
• Girdles attaching limbs to axial skeleton
Classification of Bones by Shape
• Long bones
– Longer than they are wide
– Limb, wrist, ankle bones
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• Short bones
– Cube-shaped bones (in wrist and ankle)
– Sesamoid bones (within tendons, e.g., Patella)
– Vary in size and number in different individuals
• Flat bones
– Thin, flat, slightly curved
– Sternum, scapulae, ribs, most skull bones
• Irregular bones
– Complicated shapes
– Vertebrae, coxal bones
Functions of Bones
• Support
– For body and soft organs
• Protection
– For brain, spinal cord, and vital organs
• Movement
– Levers for muscle action
• Mineral and growth factor storage
– Calcium and phosphorus, and growth factors reservoir
• Blood cell formation (hematopoiesis) in red marrow cavities of
certain bones
• Triglyceride (fat) storage in bone cavities
– Energy source
• Hormone production
– Osteocalcin
• Regulates bone formation
• Protects against obesity, glucose intolerance, diabetes
mellitus
Bones
• Are organs
– Contain different types of tissues
• Bone (osseous) tissue, nervous tissue, cartilage, fibrous
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connective tissue, muscle and epithelial cells in its blood
vessels
• Three levels of structure
– Gross anatomy
– Microscopic
– Chemical
Gross Anatomy
• Bone textures
– Compact and spongy bone
• Compact
– Dense outer layer; smooth and solid
• Spongy (cancellous or trabecular)
– Honeycomb of flat pieces of bone deep to compact called
trabeculae
Structure of Short, Irregular, and Flat Bones
• Thin plates of spongy bone covered by compact bone
• Plates sandwiched between connective tissue membranes
– Periosteum (outer layer) and endosteum
• No shaft or epiphyses
• Bone marrow throughout spongy bone; no marrow cavity
• Hyaline cartilage covers articular surfaces
Structure of Typical Long Bone
• Diaphysis
– Tubular shaft forms long axis
– Compact bone surrounding medullary cavity
• Epiphyses
– Bone ends
– External compact bone; internal spongy bone
– Articular cartilage covers articular surfaces
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– Between is epiphyseal line
• Remnant of childhood bone growth at epiphyseal plate
Membranes: Periosteum
• White, double-layered membrane
• Covers external surfaces except joint surfaces
• Outer fibrous layer of dense irregular connective tissue
– Perforating/Sharpey's fibers secure to bone matrix
• Osteogenic layer abuts bone
– Contains primitive stem cells – osteogenic cells
• Many nerve fibers and blood vessels
• Anchoring points for tendons and ligaments
Membranes: Endosteum
• Delicate connective tissue membrane covering internal bone
surface
• Covers trabeculae of spongy bone
• Lines canals that pass through compact bone
• Contains osteogenic cells that can differentiate into other bone
cells
Hematopoietic Tissue in Bones
• Red marrow
– Found within trabecular cavities of spongy bone and diploë of
flat bones (e.g., Sternum)
– In medullary cavities and spongy bone of newborns
– Adult long bones have little red marrow
• Heads of femur and humerus only
– Red marrow in diploë and some irregular bones is most active
– Yellow marrow can convert to red, if necessary
Bone Markings
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• Sites of muscle, ligament, and tendon attachment on external
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surfaces
Joint surfaces
Conduits for blood vessels and nerves
Projections
Depressions
Openings
Microscopic Anatomy of Bone: Cells of Bone Tissue
• Five major cell types
• Each specialized form of same basic cell type
– Osteogenic cells
– Osteoblasts
– Osteocytes
– Bone lining cells
– Osteoclasts
Osteogenic Cells
• Also called osteoprogenitor cells
– Mitotically active stem cells in periosteum and endosteum
– When stimulated differentiate into osteoblasts or bone lining cells
• Some persist as osteogenic cells
Osteoblasts
• Bone-forming cells
• Secrete unmineralized bone matrix or osteoid
– Includes collagen and calcium-binding proteins
• Collagen = 90% of bone protein
• Actively mitotic
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Osteocytes
• Mature bone cells in lacunae
• Monitor and maintain bone matrix
• Act as stress or strain sensors
– Respond to and communicate mechanical stimuli to osteoblasts
and osteoclasts (cells that destroy bone) so bone remodeling
can occur
Bone Lining Cells
• Flat cells on bone surfaces believed to help maintain matrix
• On external bone surface called periosteal cells
• Lining internal surfaces called endosteal cells
Osteoclasts
• Derived from hematopoietic stem cells that become macrophages
• Giant, multinucleate cells for bone resorption
• When active rest in resorption bay and have ruffled border
– Ruffled border increases surface area for enzyme degradation of
bone and seals off area from surrounding matrix
Microscopic Anatomy of Bone:
Compact Bone
• Also called lamellar bone
• Osteon or haversian system
– Structural unit of compact bone
– Elongated cylinder parallel to long axis of bone
– Hollow tubes of bone matrix called lamellae
• Collagen fibers in adjacent rings run in different directions
– Withstands stress – resist twisting
Microscopic Anatomy of Bone: Compact Bone
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• Canals and canaliculi
– Central (haversian) canal runs through core of osteon
• Contains blood vessels and nerve fibers
• Perforating (volkmann's) canals
– Canals lined with endosteum at right angles to central canal
– Connect blood vessels and nerves of periosteum, medullary
cavity, and central canal
• Lacunae—small cavities that contain osteocytes
• Canaliculi—hairlike canals that connect lacunae to each other and
central canal
Canaliculi Formation
• Osteoblasts secreting bone matrix maintain contact with each other
and osteocytes via cell projections with gap junctions
• When matrix hardens and cells are trapped the canaliculi form
– Allow communication
– Permit nutrients and wastes to be relayed from one osteocyte to
another throughout osteon
Lamellae
• Interstitial lamellae
– Incomplete lamellae not part of complete osteon
– Fill gaps between forming osteons
– Remnants of osteons cut by bone remodeling
• Circumferential lamellae
– Just deep to periosteum
– Superficial to endosteum
– Extend around entire surface of diaphysis
– Resist twisting of long bone
Microscopic Anatomy of Bone:
Spongy Bone
• Appears poorly organized
• Trabeculae
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– Align along lines of stress to help resist it
– No osteons
– Contain irregularly arranged lamellae and osteocytes
interconnected by canaliculi
– Capillaries in endosteum supply nutrients
Chemical Composition of Bone: Organic Components
• Includes cells and osteoid
– Osteogenic cells, osteoblasts, osteocytes, bone- lining cells, and
osteoclasts
– Osteoid—1/3 of organic bone matrix secreted by osteoblasts
• Made of ground substance (proteoglycans and glycoproteins)
• Collagen fibers
• Contributes to structure; provides tensile strength and
flexibility
• Resilience of bone due to (sacrificial) bonds in or between
collagen molecules
– Stretch and break easily on impact to dissipate energy and
prevent fracture
– If no addition trauma, bonds re-form
Chemical Composition of Bone: Inorganic Components
• Hydroxyapatites (mineral salts)
– 65% of bone by mass
– Mainly of tiny calcium phosphate crystals in and around collagen
fibers
– Responsible for hardness and resistance to compression
Bone
• Half as strong as steel in resisting compression
• As strong as steel in resisting tension
• Last long after death because of mineral composition
– Reveal information about ancient people
– Can display growth arrest lines
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• Horizontal lines on bones
• Proof of illness - when bones stop growing so nutrients can
help fight disease
Bone Development
• Ossification (osteogenesis)
– Process of bone tissue formation
– Formation of bony skeleton
• Begins in 2nd month of development
– Postnatal bone growth
• Until early adulthood
– Bone remodeling and repair
• Lifelong
Two Types of Ossification
• Endochondral ossification
– Bone forms by replacing hyaline cartilage
– Bones called cartilage (endochondral) bones
– Forms most of skeleton
• Intramembranous ossification
– Bone develops from fibrous membrane
– Bones called membrane bones
– Forms flat bones, e.g. clavicles and cranial bones
Endochondral Ossification
• Forms most all bones inferior to base of skull
– Except clavicles
• Begins late in 2nd month of development
• Uses hyaline cartilage models
• Requires breakdown of hyaline cartilage prior to ossification
Endochondral Ossification
• Begins at primary ossification center in center of shaft
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– Blood vessel infiltration of perichondrium converts it to
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periosteum  underlying cells change to osteoblasts
Bone collar forms around diaphysis of cartilage model
Central cartilage in diaphysis calcifies, then develops cavities
Periosteal bud invades cavities  formation of spongy bone
Diaphysis elongates & medullary cavity forms
Epiphyses ossify
Intramembranous Ossification
• Forms frontal, parietal, occipital, temporal bones, and clavicles
• Begins within fibrous connective tissue membranes formed by
mesenchymal cells
• Ossification centers appear
• Osteoid is secreted
• Woven bone and periosteum form
• Lamellar bone replaces woven bone & red marrow appears
Postnatal Bone Growth
• Interstitial (longitudinal) growth
– Increase in length of long bones
• Appositional growth
– Increase in bone thickness
Interstitial Growth:
Growth in Length of Long Bones
• Requires presence of epiphyseal cartilage
• Epiphyseal plate maintains constant thickness
– Rate of cartilage growth on one side balanced by bone
replacement on other
• Concurrent remodeling of epiphyseal ends to maintain proportion
• Calcification zone
– Surrounding cartilage matrix calcifies, chondrocytes die and
deteriorate
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• Ossification zone
– Chondrocyte deterioration leaves long spicules of calcified
cartilage at epiphysis-diaphysis junction
– Spicules eroded by osteoclasts
– Covered with new bone by osteoblasts
– Ultimately replaced with spongy bone
Interstitial Growth:
Growth in Length of Long Bones
• Near end of adolescence chondroblasts divide less often
• Epiphyseal plate thins then is replaced by bone
• Epiphyseal plate closure
– Bone lengthening ceases
• Requires presence of cartilage
– Bone of epiphysis and diaphysis fuses
– Females – about 18 years
– Males – about 21 years
Appositional Growth: Growth in Width
• Allows lengthening bone to widen
• Occurs throughout life
• Osteoblasts beneath periosteum secrete bone matrix on external
bone
• Osteoclasts remove bone on endosteal surface
• Usually more building up than breaking down
–  Thicker, stronger bone but not too heavy
Hormonal Regulation of Bone Growth
• Growth hormone
– Most important in stimulating epiphyseal plate activity in infancy
and childhood
• Thyroid hormone
– Modulates activity of growth hormone
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Ensures proper proportions
• Testosterone (males) and estrogens (females) at puberty
– Promote adolescent growth spurts
– End growth by inducing epiphyseal plate closure
• Excesses or deficits of any cause abnormal skeletal growth
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Bone Homeostasis
• Recycle 5-7% of bone mass each week
– Spongy bone replaced ~ every 3-4 years
– Compact bone replaced ~ every 10 years
• Older bone becomes more brittle
– Calcium salts crystallize
– Fractures more easily
• Consists of bone remodeling and bone repair
Bone Homeostasis: Bone Remodeling
• Consists of both bone deposit and bone resorption
• Occurs at surfaces of both periosteum and endosteum
• Remodeling units
– Adjacent osteoblasts and osteoclasts
Bone Deposit
• Evidence of new matrix deposit by osteoblasts
• Trigger not confirmed
Bone Resorption
• Is function of osteoclasts
– Dig depressions or grooves as break down matrix
– Secrete lysosomal enzymes that digest matrix and protons (H+)
– Acidity converts calcium salts to soluble forms
• Osteoclasts also
– Phagocytize demineralized matrix and dead osteocytes
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Transcytosis allow release into interstitial fluid and then into
blood
– Once resorption complete, osteoclasts undergo apoptosis
• Osteoclast activation involves PTH and T cell-secreted proteins
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Control of Remodeling
• Occurs continuously but regulated by genetic factors and two
control loops
– Negative feedback hormonal loop for Ca2+ homeostasis
• Controls blood Ca2+ levels; Not bone integrity
– Responses to mechanical and gravitational forces
Importance of Calcium
• Functions in
– Nerve impulse transmission
– Muscle contraction
– Blood coagulation
– Secretion by glands and nerve cells
– Cell division
• 1200 – 1400 grams of calcium in body
– 99% as bone minerals
– Amount in blood tightly regulated (9-11 mg/dl)
– Intestinal absorption requires Vitamin D metabolites
– Dietary intake required
Hormonal Control of Blood Ca2+
• Parathyroid hormone (PTH)
– Produced by parathyroid glands
– Removes calcium from bone regardless of bone integrity
• Calcitonin may be involved
– Produced by parafollicular cells of thyroid gland
– In high doses lowers blood calcium levels temporarily
Negative Feedback Hormonal Loop for blood Ca2+ Homeostasis
Controlled by parathyroid hormone (PTH)
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 Blood Ca2+ levels
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PTH release
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PTH stimulates osteoclasts to degrade bone matrix, releasing Ca2+
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Blood Ca2+ levels
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PTH release ends
Bone Homeostasis: Response to
Mechanical Stress
• Bones reflect stresses they encounter
– Long bones thickest midway along diaphysis where bending
stresses greatest
• Bones stressed when weight bears on them or muscles pull on
them
– Usually off center so tends to bend bones
– Bending compresses on one side; stretches on other
Results of Mechanical Stressors:
Wolff's Law
• Bones grow or remodel in response to demands placed
on it
• Explains
– Handedness (right or left handed) results in thicker and stronger
bone of that upper limb
– Curved bones thickest where most likely to buckle
– Trabeculae form trusses along lines of stress
– Large, bony projections occur where heavy, active muscles
attach
– Bones of fetus and bedridden featureless
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How Mechanical Stress Causes Remodeling
• Electrical signals produced by deforming bone may cause
remodeling
– Compressed and stretched regions oppositely charged
• Fluid flows within canaliculi appear to provide remodeling stimulus
Homeostatic Imbalances
• Osteomalacia
– Calcium salts not adequate
• Rickets (osteomalacia of children)
– Cause: Vitamin D deficiency or insufficient dietary calcium
–Osteoporosis
– Sex hormones maintain normal bone health and density
• As secretion wanes with age osteoporosis can develop
• Diet poor in calcium and protein
• Hormone-related conditions
– Hyperthyroidism
– Low blood levels of thyroid-stimulating hormone
– Diabetes mellitus
• Males with prostate cancer taking androgen-suppressing drugs
– Electrical stimulation; Daily ultrasound treatments hasten repair
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