Chapter 6 Skeletal system
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What are the 5 functions of the skeletal system?
- 1) Support
- 2) Protection
- 3) Movement
- 4) Storage
- 5) Blood cell production
What are the 4 components of the skeletal system?
- 1) Bone
- 2) Cartilage
- 3) Tendons
- 4) Ligaments
What are the 3 types of cartilage?
- 1) Hyaline cartilage
- 2) Fibrocartilage
- 3) Elastic cartilage
What are chondroblasts and chondrocytes?
- Chondroblasts: cells that produce cartilage matrix
- Chondrocytes: chondroblast that has surrounded it self by matrix
What does space does a chondrocyte occupy?
Lacuna or lacunae
What is cartilage matrix composed of?
Contains collagen for strength, proteoglycans for resilience by trapping water.
What is the perichondrium?
Double layered sheath of connective tissue covering most cartilage.
What is the outer layer of the perichondrium made of?
Dense, irregular connective tissue with fibroblasts
What is the inner layer of the perichondrium made of?
More delicate than outer layer. Fever fibers and contains chondroblasts
How is cartilage innervated and vasularized?
Blood vessels and nerves only penetrate outer layer of perichondrium. They do not penetrate cartilage, so nutrients must diffuse through cartilage matrix to reach chondrocytes
What is articular cartilage?
Cartilage that covers the ends of bones at joints. It has no perichondrium, nerves, or blood vessels.
What are the two types of growth by which cartilage grows?
- 1) Appositional growth
- 2) Interstitial growth
What is appositional growth?
Addition of new cartilage matrix on surface of existing cartilage. Chondroblasts in inner perichondrium make new matrix on surface of existing cartilage. When chondroblasts make new cartilage that surrounds them, they are now chondrocytes in a new layer of cartilage.
What is interstitial growth?
Addition of new matrix within cartilage. Chondrocytes in matrix divide and make more matrix between cells thus increasing the thickness of cartilage.
What is bone matrix composed of?
35 % organic material (collagen and proteoglycans), 65% organic (hydroxyapatite)
Like reinforced concrete. Collagen= rebar, flexible strength. Mineral= concrete, weight bearing (compression) strength
What is osteogenesis imperfecta?
Genetic disorder that results in too little or poor quality of collagen. It results in brittle bone and fractures are much more likely.
What are osteoblasts?
Cells that make new bone matrix. Contain extensive ER, ribosomes, and Golgi apparatuses. Participate in appositional growth and communicate through gap junctions.
What do osteoblasts produce?
- 1) Collagen and proteoglycans packed by golgi into vesicles to be released by exocytosis
- 2) Release matrix vesicles when membrane pinched off to increase calcium and phosphate for hydroxyapatite crystals.
What is the purpose of the hydroxyapatite crystals made by osteoblasts?
Crystals act like "seeds" to stimulate hydroxyapatite formation and mineralization of matrix.
What are osteocytes?
Mature bone cells. They are osteoblasts that have been surrounded by bone matrix.
Where in the bone are osteocytes located?
They fill lacunae with their cell bodies. Cell processes fill canaliculi keeping cells in contact.
How do nutrients reach osteocytes in the bone?
No diffusion through bone matrix. Nutrients pass through gap junctions or fluid in canaliculi and lacunae.
What are osteoclasts?
Large, multinuclear cells that resorb bone.
How do osteoclasts resorb bone?
Release H+ to decalcify bone matrix and release enzymes to digest protein component. Some goes into cell through endocytosis.
What are mesenchymal cells and what develops from them?
Embryonic connective tissue. Adult connective tissue is developed from mesenchyme.
What are osteochondral pregenitor cells and what is developed from them?
Stem cells that become OSTEOBLASTS and CHONDROCYTES. Found in inner layer of perichondrium, periosteum and endosteum.
What are osteoclasts made from?
Stem cells in red bone marrow. Also stem cells from red marrow that make monocytes (WBC)
What are the two types of bone matrix?
- 1) Woven bone
- 2) Lamellar bone
What is woven bone?
Collagen fibers randomly oriented in many directions. Formed during fetal development and repair of fractures.
What happens to woven bone during bone remodeling?
Osteoclasts resorb woven bone and osteoblasts build new matrix of lamellar bone.
What is lamellar bone?
Mature bone, organized into thin layers (3-7 um) called lamellae.
Where are osteocytes located in lamellar bone?
In lacunae sandwiched between layers of lamellae.
What are the two types of bone?
- 1) Cancellous bone
- 2) Compact bone
What is cancellous bone?
Thin rods/plates of interconnecting bone called trabeculae. Less bone matrix and more space than compact bone. Bear weight and resist stretching/bending.
What are trabeculae?
Component of cancellous (spongy) bone. Each trabecula has several concentric lamellae with osteocytes in lacunae. Can realign if stress lines change in bone due to improper healing fracture.
Where do blood vessels fit into cancellous bone?
Blood vessels and bone marrow fit between trabeculae.
What is compact bone?
Dense bone with blood vessels that enter bone itself. Made of osteons with haversion canals for nerves and blood vessels.
What is an osteon?
Concentric lamallae, osteocytes and a central haversion canal. Osteocytes lay inside lacunae between concentric lamellar rings. Canaliculi connect lacunae with osteocytes inside.
What are circumfrential lamellae and interstitial lamellae?
Circumfrential: thins plates that extend around entire compact bone just underneath periosteum.
Interstitial: In between osteons, remnants of circumfrential and concentric lamellae remodeling.
What are Volkmann's canals (perforating canals)?
Canals that run perpendicular to long axis of bone. Connect periosteum, medullary canals and Haversion canals (osteons).
How are nutrients and wastes exchanged in compact bone?
Periosteum or medullary canal --> Volkmann's canals --> Haversion canals --> canaliculi --> osteocyte cytoplasms
Waste go opposite direction
What are the 4 different bone shapes?
- 1) Long bones (UE and LE)
- 2) Short bones (almost cuboidal, wrist, ankles)
- 3) Flat bones (thin, flat, curved, skull, ribs, sternum, scapulae)
- 4) Irregular bones (vertabrae, facial bones)
What are the two main parts of the long bone structure and what are they made from?
- 1) Diaphysis (shaft), made from mostly compact bone, contains medullary cavity
- 2) Epiphysis (ends), made from mostly cencellous bone, contains epiphyseal plate/lines
What is the medullary cavity?
Cavity in the center of the diaphysis of long bones made of cancellous bone. Filled with red and yellow bone marrow.
What is the difference between fetal and adult bone marrow?
Fetal: all red marrow, replaced by yellow marrow just before birth and continues through adulthood. Eventually all long bongs are yellow. Elsewhere in the body, mix of red and yellow.
Where is the only red marrow found in adult long bones?
Proximal ends of humerus and femur
Where is the most common place to take bone marrow for donation?
Ileum in the hip is about 50/50 red and yellow.
What is the epiphyseal plate (growth plate)?
Cartilage between diaphysis and epiphsis. Site of growth in bone length. When growth stops, epiphyseal plate replaced with epiphyseal lines.
What is the periosteum?
Connective tissue that covers the outside of the bone. Two layers: Outer and inner layers.
What is the structure of the outer periosteum?
Outer fibrous layer is dense irregular collagenous with blood vessels and nerves.
What is the structure of the inner periosteum?
Inner cellular layer is a single layer of bone cells including osteoclasts, osteoblasts and osteochondral pregenitor cells.
What is the endosteum?
Single layer of cells lining inner surface of all cavities within bones (medullary, cavities in cancellous bones and compact bone).
Composed of osteoblasts, osteoclasts, and osteochondral pregenitor cells.
What is the structure of flat bones?
Interior framework of cancellous bone with outer layer of compact bone.
What are the structures of short and irregular bones?
Cancellous bone center surrounded by compact bone. No diaphysis because not elongated.
What part of irregular bones possess epiphyseal plates?
Processes (projections) of irregular bones possess epiphyseal growth plates and small epiphyses.
What are sinuses?
Air-filled spaces lined with mucous membranes. Found in flat and irregular bones of skull.
What are the two types of bone growth?
- 1) Intramembranous ossification
- 2) Endochondral ossification
What is intramembranous ossification?
Formation of bone within connective tissue in utero. ~ 8 wks into development.
Describe the process of intramembranous ossification
- 1) Osteochondrial pregenitor cells specialize to become osteoblasts and lay down bone matrix and form center of ossification ontop of collagen fibers of fetal connective tissue membrane forming tiny trabeculae of woven bone
- 2) Trabeculae enlarge as more bone matrix layed down. Cancellous bone forms as trabeculae join together, red marrow develops and cells surrounding developing bone specialize to form periosteum.
- 3) Osteoblasts from periosteum lay down bone matrix to form outer compact bone covering; remodeling turns woven bone into lamellar bone.
What are centers of ossification?
Locations in membrane where ossification starts. They expand to form a bone by gradually ossifying the membrane.
What are fontanels?
Large, membrane covered spaces between developing skull bones. Bones eventually grow together and all fontanels usually closed by age 2.
What is endochondral ossification?
Formation of bone within cartilage. Most of skeletal system develops this way. Starts with hyaline cartilage model of bone
What are the 4 zones at the epiphyseal plate?
- 1) Zone of resting cartilage
- 2) Zone of proliferation
- 3) Zone of hypertrophy
- 4) Zone of calcification
What is the zone of resting cartilage?
Proximal to epiphysis and contains chondrocytes that don't divide rapidly
What is the zone of proliferation?
New cartilage made by chondrocytes using interstitial growth. Chondrocytes divide and form columns like stacked coins.
What is the zone of hypertrophy?
Chondrocytes from zone of proliferation mature and enlarge
Zone of Calcification
Very thin with hypertrophied chondrocytes and calcified matrix. Chondrocytes die and replaced by osteoblasts from endosteum. Osteoblasts deposit new bone matrix on calcified cartilage matrix (appositional growth)
Why doesn't the epiphyseal plate thickness change during bone growth?
Rate of cartilage made and bone replacement are equal.
When does the epiphyseal plate close?
Usually between 12 and 25 yrs old
Bone growth at articular cartilage
Similar to epiphyseal plate, but articular cartilage lasts for life, doesn't ossify.
What are the factors affecting bone growth?
Nutrition, Hormones, Sex hormones
How does nutrition affect bone growth?
Vitamin D required for Ca absorption. Insufficient fat absorption = Vit D insufficiency because Vit D is fat soluble.
What is rickets?
Vitamin D insufficiency in children. Decreased bone mineralization, bowed bones, inflammed joints.
What is "Adult rickets"?
Osteomalacia. Insufficient fat absorption
Which hormones affect bone growth?
Growth hormone, testosterone, and estrogen
Excessive bone/cartilage formation at epiphyseal plate. Most common is pituitary gigantism
What is acromegaly?
Growth of connective tissue after epiphyseal plate ossifies and increases diameter of bones especially face and hands.
What is pituitary dwarfism?
Low levels of growth hormone. Smaller than average but normal proportions.
How do sex hormones affect bone growth?
- Testosterone and estrogen initially stimulates bone growth at puberty but also stimulate ossification of epihpyseal plate.
- Estrogen causes epiphyseal plate ossification faster
What is achondroplasia?
- Most common type of dwarfism.
- Normal sized trunk and head. Shorter than normal limbs.
- Usually caused by spontaneous mutation (during sperm oocyte production) of gene regulating bone growth. Increased inhibition of chondrocyte division at epihpyseal plate.
Basic Multicellular Unit (BMU) containing osteoblasts and osteoclasts resorb and lay new bone. Avg lifespan of BMU is 6 months, renews entire skeleton every 10 years.
Open (compound) fracture
Fragment of bone protrudes out of skin
Closed (simple) fracture
Skin not perforated
Tissues around closed fracture damaged
Doesn't completely extended across bone
Broken into at least 2 fragments
Green stick fracture
Incomplete fracture on convex side of bone
Incomplete fracture, 2 sections don't separate. Common in skull fractures.
Complete fracture, more than 2 pieces, usually 2 major and 1 smaller fragments
One fragment driven into cancellous portion of other
Along long axis of bone
Perpendicular to long axis of bone
Helical course around bone
Obliquely in relation to long axis.
Rough, toothed, broken ends
Breakage lines radiating from 1 central point
Rate of bone resorbtion greater than bone formation. More common in women. Strong genetic component.
At what age does bone mass begin decreasing?
35 yrs of age. Women can lose 50% of cancellous bone. Men can lose 25%.
How do genetics affect bone mass?
- 60% of peak bone mass is genetically determined
- 40% attributed to environmental factors
How does estrogen affect bone loss?
Decreased levels of estrogen increases bone loss. Estrogen inhibits parathyroid hormone (PTH) which is involved in stimulating osteoclasts. After menopause decreased cancellous bone in vertebrae and forearm.
How does testosterone affect bone loss?
Slowly decreases bone mass after 65. Men have denser bones.
What are some treatments for osteoporosis?
- Supplemental Ca and Vitamin D
- Weight-bearing exercises
- Hormone replacement therapy (now discouraged)
- Selective estrogen receptor modulators
- Calcitonin nasal spray
Why is hormone replacement therapy discouraged as a treatment for osteoporosis?
- Increased risk of breast and uterine cancer, MI, CVAs, blood clots
- Decrease PTH= decrease osteoclast activity which slows down bone loss, but does not increase bone mass.
How is Selective estrogen receptor modulators treatment different from hormone replacement therapy for osteoporosis?
Binds to estrogen receptors in bone but inhibit binding in breast and uterine tissue.
How are statins used to treat osteoporosis?
- Inhibit cholesterol synthesis
- Increase osteoblast activity
What are the 4 steps in bone repair?
- 1) Hematoma formation
- 2) Callus formation
- 3) Callus ossification
- 4) Bone remodeling
Mass of blood released from blood vessels that is confined in an organ or space
Hematoma formation during bone repair
1st step of bone repair. Clot stops bleeding at fracture site disrupting blood vessels in bone causing ischemia for osteocytes and bone tissue dies.
Callus Formation during bone repair
- 2nd step. Mass of tissue forms at fracture site and connects 2 bone fragments. Callus forms as clot desolves and removed by macrophages.
- Internal and external calluses (cartilage and woven bone)
Callus Ossification during bone repair
- 3rd step. Calluses ossified to form woven, canellous bone.
- Similar to fetal bone development.
- Complete in 4-6 weeks.
- Immobilization is critical up to then because refracture of delicate bone matrix is easy.
Bone remodeling during bone repair
- Woven bone of internal callus and dead bone adjacent to fracture is replaced by compact bone.
- Osteons from both sides of fracture extend across fracture line and "peg" 2 fragments together.
- Takes a long time.
Usually equal in/out Ca in bone. Parathyroid hormone (PTH) and calcitonin
- Responds to low serum Ca
- Increases osteoclasts activity, Vit D production in kidneys, and calcium absorption in intestines.
- Thyroid gland
- Responds to increased serum Ca
- Decreases osteoclast activity, more Ca stored in bone.
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