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The Skeletal System.
Function
Characteristics
- Function:
- 1. provide support and shape to the body
- 2. provide attachment sites for muscle (impt for movement)
- 3. protection
- 4. acts as a storage depot for essential minerals ( Calcium and phosphate)
- 5. blood-cell formation, fat storage (red/yellow marrow)
- [6. Energy metabolism: osteoblasts secrete hormones related to blood sugar)
- Characteristics:
- 1. composed of bones, cartilages, and joints
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Hyaline Cartilage
Characteristics
Location in skeletal system
- Characteristics:
- 1. Most abundant
- 2. Chondrocytes appear spherical
- 3. Made of collagen fibers
- 4. Resists compression well (bc of large amts of water)
- 5. Provides support through flexibility and resilience
- Location:
- 1. articular cartilage
- 2. cartilage attaching ribs to sternum
- 3. most cartilage in respiratory structures
- 4. forms embryonic skeleton
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Elastic Cartilage
Characteristics
Location
- Characteristics:
- 1. similar to hyaline, but matrix has elastic fibers as well as collagen
- 2. more elastic, more tolerant to repeated bending
- Location:
- 1. Epiglottis
- 2. External Ear
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Fibrocartilage:
Characteristics
Location
- Characteristics:
- 1. resists both strong compression and strong tension
- 2. consists of thick collagen fibers surrounding chondrocytes in lacunae
- Location:
- 1. anulus fibrosis portion of vertebral discs
- 2. articular discs of joints, ex. Knees
- 3. pubic symphysis
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Growth of Cartilage:
Appositional growth: "from outside"; chondroblasts in perichondrium actively secrete matrix
Interstitial growth: "from within"; chondrocytes within the cartilage divide and secrete new matrix
cartilage stops growing in late teens, and cells do not divide again
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Extracellular Bone Matrix Composition
- 35% organic components (collagen)
- - contribute flexibility and tensile strength
- - baking at high temperature destroys organic portion
- 65% inorganic hydroxyapatites (mineral salts: calcium phosphate)
- - provide exceptional hardness to resist compression, endurance
- - soaking in acid dissolves mineral salts
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Bone Cell Types
- 1. Osteogenic cells: stem cells that differentiate into bone-forming osteoblasts
- 2. OsteoBlasts: cells that actively produce and secrete organic components of bone matrix
- 3. Osteoid: bone matrix secreted by osteoblasts (crystallized calcium salts within)
- 4. Osteocytes: function to keep bone matrix healthy; no longer producing new osteoid
- 5. OsteoClasts: cells responsible for bone resorption (derived from white blood cells; secrete HCl and lysosomal enzymes)
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Bone Classification:
1. Long Bones
2. Short Bones
3. Flat Bones
4. Irregular Bones
- 1. Long bones: longer than they are wide; consists of a shaft and two ends
- most bones in limbs: humerus, femur, tibia, fibula, ulna, radius, phalanges, metacarpals
- 2. Short bones: typically cube-shaped; sesamoid bone: "sesame seed-shaped"
- occur in wrist and ankle: tarsals, carpals; coccyx, patella
- 3. Flat bones: thin, flattened, usually curved
- most cranial bones, ribs, sternum, scapula, clavicle
- 4. Irregular bones: anything that doesn't fit into the previous categories
- vertebrae, coxal, mandible, sacrum
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Gross Anatomy of Bones
- Diaphysis/ epiphyses
- Articular cartilage
- Periosteum: two sublayers: superficial layer of dense irregular connective tissue (resists tension), deep layer consisting of osteoblasts and osteoclasts (bone remodeling)
- Endosteum: covers internal bone surfaces (also osteogenic)
- Perforating (Sharpey's) fibers: bundles of collagen that connect periosteum to bone matrix
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Bone Markings:
Sites of Muscle and Ligament Attachment
Tuberosity, Crest, Trochanter, Line, Tubercle, Epicondyle, Spine, Process
Surfaces that form Joints
Head, Facet, Condyle
Depressions/ Openings
(for vessels and nerves)
Foramen, Groove, Fissure, Notch
(others)
Fossa, Meatus, Sinus
- Sites of Muscle and Ligament Attachment
- 1. Tuberosity - Large rounded projection; roughened
- 2. Crest - Narrow ridge of bone; usually prominent
- 3. Trochanter - Large, blunt, irregularly shaped process (only on femur)
- 4. Line - Narrow ridge of bone; less prominent than crest
- 5. Tubercle - Small, rounded projection or process
- 6. Epicondyle - Raised area on or above a condyle
- 7. Spine - Sharp, slender, often pointed projection
- 8. Process - Any bony prominence
- Surfaces that form Joints
- 1. Head - bony expansion carried on a narrow neck
- 2. Facet - smooth, nearly flat articular surface
- 3. Condyle - rounded articular projection, often articulating with a fossa
- Depressions/ Openings
- (for vessels and nerves)
- 1. Foramen - round/oval opening through a bone
- 2. Groove - Furrow
- 3. Fissure - Narrow, slit-like opening
- 4. Notch - Indentation at the edge of a structure
- (others)
- 5. Fossa - Shallow, basin-like depression, often an articular surface
- 6. Meatus - Canal-like passageway
- 7. Sinus - Cavity within a bone, filled with air and lined with mucous membrane
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Microscopic Structure of Compact Bone
- Central (Haversian) Canal: carries blood vessels, nerves, lymphatic vessels
- Osteocyte: mature bone cells in Lacunae: chambers
- Concentric Lamellae: circular tubes surrounding a central canal like rings in a tree trunk
- Interstitial Lamellae: incomplete remnants of Circumferential Lamellae: at internal and external layer of compact bone
- Osteon: central canal and all concentric lamellae surrounding it
- Canaliculi: tiny canals going outward from central canal to lacunae; provides connecting network for nutrient supply
- Perforating (Volkmann's) Canals: run perpendicular to central canal, from periosteum to bone interior
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Bone Development and Growth
Intramembranous Ossification
- Membrane bones form directly from mesenchyme without cartilaginous model
- - All bones in skull except base, and clavicles
- Week 8 of embryonic development: mesenchymal cells cluster and become osteoblasts that secrete organic bone matrix that become mineralized --> become osteocytes
- New bone tissue forms btwn blood vessels that result in woven bone tissue, mesenchyme condenses externally and becomes periosteum.
- Trabeculae at periphery thicken and form compact bone, sandwiching an internal spongy bone.
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Bone Development and Growth
Endochondral Ossification
- All bones from the base of the skull down, except clavicles
- - first modeled in hyaline cartilage, gradually replaced by bone
- - begins late in second month of development, not complete until early adulthood
- - increases in length and width
- 1. Bone collar forms around diaphysis
- 2. Cartilage calcifies in center of diaphysis
- 3. Periosteal bud (blood vessel) invades internal cavity, forming spongy bone (primary ossification center)
- 4. Diphysis elongates, medullary cavity forms; secondary ossification centers appear in epiphyses
- 5. Epiphyses ossify; cartilaginous epiphyseal plates form (hylaine cartilage only in epiphyseal plate and articular cartilages)
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Anatomy of Epiphyseal Plate
- Resting Zone: cartilage cells nearest epiphysis; small and inactive
- Proliferation Zone: cartilage cells forming tall stacks, top cells divide quickly, pushing epiphyseal plate further away, lengthening bone
- Hypertrophic Zone: older chondrocytes enlarge and signal matrix to calcify
- Calcification Zone: matrix becomes calcified, chondrocytes die, leaving trabeculae
- Ossification Zone: trabeculae covered with bone tissue by osteoblasts
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Repair of Bone Fractures
- Simple fracture: bone breaks cleanly, does not penetrate the skin
- Compound fracture: bone breaks in multiple pieces; penetrates skin
- Closed reduction: bone ends realigned by physician (no surgery required)
- Open reduction: bones realigned by pins/wires (through surgery)
- Healing requires 6-8wks for a simple fracture in healthy young adults
- 1. Hematoma formation (inflammation occurs)
- 2. Fibrocartilaginous callus formation: new blood vessels form, proliferation of osteoblasts, filling of cavity with soft callus which becomes dense CT containing fibro- and hyaline cartilage.
- 3. Bony callus formation: Within a week, bone trabeculae form via endochondral ossification and unite the two fragments
- 4. Bone remodeling occurs: months after repair, bone remodels due to mechanical stresses
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Bone Diseases
Osteoporosis
Osteomalacia and Rickets
- Osteoporosis: "bone porous condition"
- low bone mass and deterioration of microscoopic architecture
- - bone resorption outpaces bone deposition (osteoclast activity > osteoblast activity)
- - treated by supplemental calcium and vitamin D
- Osteomalacia and Rickets: "soft bones"
- bones are inadequately mineralized (lack of vitamin D and calcium phosphate)
- - calcification does not occur, bone softens and weakens
- - epiphyses in children become thickened
- - cured by drinking vitamin D-fortified milk, or sunlight exposure
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