ch. 6 bones tissues Flashcards
Cartilage
• Skeletal cartilage – Water lends resiliency – Contains no blood vessels or nerves – Perichondrium surrounds • Dense connective tissue girdle – Contains blood vessels for nutrient delivery – Resists outward expansion • All contain chondrocytes in lacunae and extracellular matrix
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
Appositional growth
– Cells secrete matrix against external face of existing cartilage
Interstitial growth
– Chondrocytes divide and secrete new matrix, expanding cartilage from within
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 • Short bones • Flat bones • Irregular bones Classification of Bones by Shape
Long bones
– Longer than they are wide
– Limb, wrist, ankle bones
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
Seven important functions of bone
– Support – Protection – Movement – Mineral and growth factor storage – Blood cell formation – Triglyceride (fat) storage – Hormone production
Bones
• Are organs – Contain different types of tissues • Bone (osseous) tissue, nervous tissue, cartilage, fibrous 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
– 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
– 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
- Sites of muscle, ligament, and tendon attachment on external surfaces
- Joint surfaces
- Conduits for blood vessels and nerves
- Projections
- Depressions
- Openings
Bone Markings
• Projections
– Most indicate stresses created by muscle pull or joint modifications
• Depressions and openings
• Usually allow nerves and blood vessels to pass
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
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
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
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
Spongy Bone
• Appears poorly organized
• Trabeculae
– 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
• 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
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
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
• Result of five zones within cartilage
– Resting (quiescent) zone
– Proliferation (growth) zone
– Hypertrophic zone
– Calcification zone
– Ossification (osteogenic) zone
• Resting (quiescent) zone
– Cartilage on epiphyseal side of epiphyseal plate
– Relatively inactive
• Proliferation (growth) zone
• Calcification zone
– Surrounding cartilage matrix calcifies, chondrocytes die and deteriorate
• 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
– Cartilage on diaphysis side of epiphyseal plate • 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
– Rapidly divide pushing epiphysis away from diaphysis lengthening
• Hypertrophic zone
– Older chondrocytes closer to diaphysis and their lacunae enlarge and erode interconnecting spaces
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
– 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
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 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
– Osteoid seam
• Unmineralized band of bone matrix
– Calcification front
• Abrupt transition zone between osteoid seam and older mineralized bone
• Trigger not confirmed
– Mechanical signals involved
– Endosteal cavity concentrations of calcium and phosphate ions for hydroxyapatite formation
– Matrix proteins bind and concentrate calcium
– Enzyme alkaline phosphatase for mineralization
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
• 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
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
Calcium Homeostasis
• Even minute changes in blood calcium dangerous
– Severe neuromuscular problems
• Hyperexcitability (levels too low)
• Nonresponsiveness (levels too high)
– Hypercalcemia
• Sustained high blood calcium levels
• Deposits of calcium salts in blood vessels, kidneys can interfere with function
Other Hormones Affecting Bone Density
• Leptin – Hormone released by adipose tissue – Role in bone density regulation • Inhibits osteoblasts in animals • Serotonin – Neurotransmitter regulating mood and sleep – Most made in gut – Secreted into blood after eating • Interferes with osteoblast activity • Serotonin reuptake inhibitors
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
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
Results of Hormonal and Mechanical Influences
• Hormonal controls determine whether and when remodeling occurs to changing blood calcium levels
• Mechanical stress determines where remodeling occurs
Bone Repair
• Fractures – Breaks – Youth • Most result from trauma – Old age • Most result of weakness from bone thinning
Fracture Classification
• Three “either/or” fracture classifications
– Position of bone ends after fracture
• Nondisplaced—ends retain normal position
• Displaced—ends out of normal alignment
– Completeness of break
• Complete—broken all the way through
• Incomplete—not broken all the way through
– Whether skin is penetrated
• Open (compound) - skin is penetrated
• Closed (simple) – skin is not penetrated
• Also described by location of fracture
• External appearance
• Nature of break
Fracture Treatment and Repair
• Treatment
– Reduction
• Realignment of broken bone ends
• Closed reduction – physician manipulates to correct position
• Open reduction – surgical pins or wires secure ends
– Immobilization by cast or traction for healing
• Depends on break severity, bone broken, and age of patient
Stages of Bone Repair: HEMATOMA Forms
- Torn blood vessels hemorrhage
- Clot (hematoma) forms
- Site swollen, painful, and inflamed
Stages of Bone Repair:
Fibrocartilaginous Callus Forms
• Capillaries grow into hematoma
• Phagocytic cells clear debris
• Fibroblasts secrete collagen fibers to span break and connect broken ends
• Fibroblasts, cartilage, and osteogenic cells begin reconstruction of bone
– Create cartilage matrix of repair tissue
– Osteoblasts form spongy bone within matrix• Mass of repair tissue called fibrocartilaginous callus
Stages of Bone Repair:
Bony Callus Forms
- Within one week new trabeculae appear in fibrocartilaginous callus
- Callus converted to bony (hard) callus of spongy bone
- ~2 months later firm union forms
Stages of Bone Repair:
Bone Remodeling Occurs
- Begins during body callus formation
- Continues for several months
- Excess material on diaphysis exterior and within medullary cavity removed
- Compact bone laid down to reconstruct shaft walls
- Final structure resembles original because responds to same mechanical stressors
Homeostatic Imbalances
• Osteomalacia – Bones poorly mineralized – Calcium salts not adequate – Soft, weak bones – Pain upon bearing weight • Rickets (osteomalacia of children) – Bowed legs and other bone deformities – Bones ends enlarged and abnormally long – Cause: Vitamin D deficiency or insufficient dietary calcium
Osteoporosis
– Group of diseases
– Bone resorption outpaces deposit
– Spongy bone of spine and neck of femur most susceptible
• Vertebral and hip fractures common
Risk Factors for Osteoporosis
• Risk factors
– Most often aged, postmenopausal women
• 30% 60 – 70 years of age; 70% by age 80
• 30% caucasian women will fracture bone because of it
– Men to lesser degree
– Sex hormones maintain normal bone health and density
• As secretion wanes with age osteoporosis can develop
Additional Risk Factors for Osteoporosis
• Petite body form
• Insufficient exercise to stress bones
• Diet poor in calcium and protein
• Smoking
• Hormone-related conditions
– Hyperthyroidism
– Low blood levels of thyroid-stimulating hormone
– Diabetes mellitus
• Immobility
• Males with prostate cancer taking androgen-suppressing drugs
Treating Osteoporosis
• Traditional treatments
– Calcium
– Vitamin D supplements
– Weight-bearing exercise
– Hormone replacement therapy
• Slows bone loss but does not reverse it
• Controversial due to increased risk of heart attack, stroke, and breast cancer
• Some take estrogenic compounds in soy as substitute
New Drugs for Osteoporosis Treatment
• Bisphosphonates
– Decrease osteoclast activity and number
– Partially reverse in spine
• Selective estrogen receptor modulators
– Mimic estrogen without targeting breast and uterus
• Statins
– Though for lowering cholesterol also increase bone mineral density
• Denosumab
– Monoclonal antibody
– Reduces fractures in men with prostate cancer
– Improves bone density in elderly
Preventing Osteoporosis
• Plenty of calcium in diet in early adulthood
• Reduce carbonated beverage and alcohol consumption
– Leaches minerals from bone so decreases bone density
• Plenty of weight-bearing exercise
– Increases bone mass above normal for buffer against age-related bone loss
Paget’s Disease
• Excessive and haphazard bone deposit and resorption
– Bone made fast and poorly – called pagetic bone
• Very high ratio of spongy to compact bone and reduced mineralization
– Usually in spine, pelvis, femur, and skull
• Rarely occurs before age 40
• Cause unknown - possibly viral
• Treatment includes calcitonin and biphosphonates
Developmental Aspects of Bones
- Embryonic skeleton ossifies predictably so fetal age easily determined from X rays or sonograms
- Most long bones begin ossifying by 8 weeks
- Primary ossification centers by 12 weeks
- At birth, most long bones well ossified (except epiphyses)
- At age 25 ~ all bones completely ossified and skeletal growth ceases
Age-related Changes in Bone
• Children and adolescents
– Bone formation exceeds resorption
• Young adults
– Both in balance; males greater mass
• Bone density changes over lifetime largely determined by genetics
– Gene for Vitamin D’s cellular docking determines mass early in life and osteoporosis risk as age
• Bone mass, mineralization, and healing ability decrease with age beginning in 4th decade
– Except bones of skull
– Bone loss greater in whites and in females
– Electrical stimulation; Daily ultrasound treatments hasten repair
