Download Bone - Humble ISD

Chapter 6
Bones and Skeletal Tissues
Cartilage: Basic Structure, Types and Locations
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
Growth of Cartilage
•
Appositional growth
– Cells secrete matrix against external face of existing cartilage
•
Interstitial growth
– Chondrocytes divide and secrete new matrix, expanding cartilage from within
•
Calcification of cartilage
– Occurs during normal bone growth
• Youth and old age
– Hardens, but cacified cartilage is not bone
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
© 2013 Pearson Education, Inc.
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
Functions of Bones
•
Seven important functions
– Support
– Protection
– Movement
– Mineral and growth factor storage
– Blood cell formation
– Triglyceride (fat) storage
– Hormone production
Functions of Bones
•
Support
– For body and soft organs
•
Protection
– For brain, spinal cord, and vital organs
© 2013 Pearson Education, Inc.
•
Movement
– Levers for muscle action
Functions of Bones
•
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 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
© 2013 Pearson Education, Inc.
•
•
•
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)
© 2013 Pearson Education, Inc.
– 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
© 2013 Pearson Education, Inc.
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
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
© 2013 Pearson Education, Inc.
Microscopic Anatomy of Bone: Compact Bone
•
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
– Align along lines of stress to help resist it
– No osteons
© 2013 Pearson Education, Inc.
– 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
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
© 2013 Pearson Education, Inc.
–Forms flat bones, e.g. clavicles and cranial bones
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
–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 Deposit
•Evidence of new matrix deposit by osteoblasts
Bone Resorption
•Is function of osteoclasts
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
© 2013 Pearson Education, Inc.
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
Fracture Classification
Refer to Table6.2
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
© 2013 Pearson Education, Inc.
–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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.