Chapter 6: Skeletal System Flashcards
What does the skeletal system include?
Bones, joints, and associated supporting tissues
Bones
- Main organs of the system
- Composed of more than osseous tissue
- Also has dense regular, irregular collagenous connective tissue, and bone marrow
Functions of the skeletal system
- Protection
- Mineral storage
- Acid-base homeostasis
- Blood cell formation
- Fat storage
- Movement
- Support
Protection
The skeleton protects vital organs such as the brain
Mineral storage and acid-base homeostasis
Bone stores minerals such as Ca2+ and PO4(3-), which are necessary for electrolyte and acid-base balance
Blood cell formation
Red bone marrow is the site of blood cell formation
Fat storage
Yellow bone marrow stores triglycerides
Movement
Muscles produce body movement via their attachment to bones
Support
The skeleton supports the weight of the body
Bone shapes
- Long bone
- Flat bone
- Short bone
- Irregular bone
- Sesamoid bone
- Sutural (Worminan) bone
Long bone
- Elongated shape
- Most limb bones
Flat bone
- Thin, flat
- Cranium, ribs, sternum
Short bone
- Cube-shaped
- Carpals, tarsals
Irregular bone
- Various shapes
- Vertebra, hip, coccyx
Sesamoid bone
- Small, flat, like a sesame seed
- Forms within tendons
- Patella
Sutural (Worminan) bone
- Small, flat, irregular shape
- Forms between bones of the skull at sutures
Parts of a long bone
- Periosteum
- Endosteum
- Diaphysis
- Metaphysis
- Epiphysis
- Epiphyseal plate
- Epiphyseal line
- Medullary cavity
Periosteum
- Outer covering
- Consists of dense irregular connective tissue
- Inside is a single layer of bone cells which contain osteoblasts, nerves, blood vessels, and osteochondral progenitor cells
- Fibers of tendon become continuous with fibers of periosteum
- Sharpey’s fibers
Sharpey’s Fibers
- Some periosteal fibers penetrate through the periosteum and into the bone
- This strengthens the attachment of tendon to bone
Endosteum
- Lines medullary cavity
- Has a layer of osteoprogenitor cells
Osteoprogenitor cells
Stem cells that produce osteoblasts
Diaphysis
Shaft of bone made out of compact bone
Metaphysis
Narrow zone where diaphysis connects to epiphysis
Epiphysis
- End of bone
- Cancellous bone (spongy bone)
Epiphyseal plate
- Growth plate
- Its presence means the bone is still growing
- Hyaline cartilage
- Present until growth stops
Epiphyseal line
Present when bone stops growing in length
Medullary cavity
- Red marrow in children
- Gradually changes to yellow in limb bones and skull
- Not in long bones
- Rest of skeleton is red
Diploe
- Flat bones do not have a marrow cavity
- Instead they have a diploe
Types of bone structure
- Compact bone
- Cancellous (spongy) bone
Compact Bone
- Hard, dense outer region
- Allows bone to resist linear compression, twisting, and other stresses
Cancellous Bone
- Inside cortical (compact) bone
- Honeycomb-like framework of bony struts
- Allows bones to resist forces from many directions
- Provides cavity for bone marrow
- Made of trabeculae
Types of bone marrow
- Red bone marrow
- Yellow bone marrow
Red Bone Marrow
- Loose connective tissue
- Supports islands of blood-forming hematopoietic cells
- Decreases with age
- Only present in the pelvis with adults
Why do children have more bone marrow?
They need more of it to assist with growth and development
Yellow Bone Marrow
Composed of triglycerides, blood vessels, and adipocytes
Bone Matrix
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Bone cells
- Osteoblasts
- Osteocytes
- Osteoclasts
- Stem cells/osteochondral progenitor cells
Osteoblasts
Make bone
Osteocytes
Mature bone cells
Osteoclasts
- Break down bone
Stem cells/osteochondral progenitor cells
Become chondroblasts or osteoblasts
Woven Bone
Collagen fibers are randomly oriented
Lamellar Bone
Mature bone in sheets
Ruffled border
Where cell membrane borders bone and resorption is taking place
How to osteoclasts reabsorb bone?
- H+ ions are pumped across the membrane
- Acid forms, eating away bone
- Enzymes are released that digest the bone
Osteon
- The unit of compact bone structure
- Also called a Haversian system
- Consists of concentric rings
Osteon Structure
- Concentric lamellae
- Interstitial lamellae
- Circumferential lamellae
- Perforating (Volkmann’s) canals
Concentric Lamellae
- Thin layers of bone that make up the rings found in an osteon
- 4-20 in each osteon
Interstitial lamellae
- Fills spaces between circular osteons
- Represent remnants of old osteons
Circumferential lamellae
- Outer and inner layers of lamellae just inside periosteium
- Found at the boundary with spongy bone
- Adds strength
Perforating (Volkmann’s) canals
- Originate from blood vessels in periosteum
- Perpendicular to central canals of neighboring osteons
- Connects osteons
Trabeculae
- Struts or ribs of bone
- Covered with endosteum
- Usually not arranged into osteons
- Communicate through calaiculi
What are trabeculae made of?
Concentric lamellae with osteocytes in lacunae
How do trabeculae communicate?
Through canaliculi
How does spongy bone get blood?
- There are no central canals to supply blood
- Obtain blood from vessels in bone marrow
Types of bone development
- Intramembranous ossification
- Encochondral ossification
- Both methods produce woven bone that is then remodeled
- After remodeling, bone is indistinguishable
Intramembranous ossification
Take place in connective tissue membrane
Encochondral ossification
Take place in cartilage
Ossification
The process of bone formation
When does ossification begin?
- In the embryonic period
- Continues through childhood, with most bones completing formation by age 7
Mechanisms for ossification
- First bone formed is immature primary (woven) bone
- Usually primary bone is broken down by osteoclasts and replaced with mature secondary bone
Immature Primary Bone
Irregularly arranged collagen bundles, osteocytes, and sparse inorganic matrix
Mature Secondary bone
- Lamellar bone
- More inorganic matrix and increased strength
What bones does intramembranous ossification make?
Many flat bones (skull and clavicles) during fetal development
Fetal primary bone
- Formed within mesenchymal membrane
- Composed of embryonic connective tissue
- Rich in blood
Flat bone structure
2 outer layers of compact bone with layer of spongy bone in the middle
Order of ossification for flat bones
- Spongy middle layer first
- Begins from primary ossification center
Progression of mesenchymal cells
They differentiate into osteogenic cells, then osteoblasts, at the primary ossification site
How many primary ossification centers do larger bones have?
- More than one
- These lead to pieces of bone that fuse together over time
Which bones undergo endochondral ossification?
All bones below the head except clavicles
When does endochondral ossification begin?
- In the fetal stage for most bones
- Some bones (wrist and ankle) ossify much later
- Many bones complete ossification by age 7
Where does endochondral ossification begin?
From within a model of hyaline cartilage
Hyaline cartilage model
Consists of chondrocytes, collagen, and ECM surrounded by connective tissue membrane (perichondrium) and immature cartilage cells (chondroblasts)
What do long bones have in the epiphysis?
Secondary ossification centers
Steps of endochondral ossification
- Differentiate into osteogenic cells, then osteoblasts, forming periosteum
- Bone begins to form where osteoblasts have built bone collar on external surface of bone
- Internal cartilage begins to calcify at the same time
- Osteoclasts etch oping for blood vessel and bone cell entry
- Osteoblasts replace calcified cartilage with early spongy bone
- Cavities enlarge and combine, developing the medullary cavity
- Secondary ossification centers develop in epiphysis
- Remaining ossified cartilage replaced by bone
- Osteoclasts enlarge medullary cavity, fills with bone marrow
- Epiphyses finish ossifying
Osteoporosis
Bone disease in which bones become weak and brittle due to inadequate inorganic matrix
What causes osteoporosis?
- Dietary: calcium and/or vitamin D deficiency
- Female gender
- Advanced age
- Lack of exercises
- Hormones (lack of estrogen in postmenopausal women)
- Genetic factors
Appositional Growth
- Bone growth in width
- Does not result in immediate formation of osteons
- Forms new circumferential lamellae
What happens to old circumferential lamellae?
They are removed or restructured into osteons
Bone remodeling
Continuous process of bone formation and loss after growth in length is finished
Reasons for bone remodeling cycle
- Maintenance of calcium ion homeostasis
- Replacement of primary bone with secondary bone
- Bone repair
- Replacement of old brittle bone with newer bone
- Adaptation to tension and stress
Bone remodeling in response to compression
Bone deposition
Bone remodeling in response to tension
Bone deposition
Bone remodeling in response to continuous pressure
Bone resorption
How do intestines affect concentration of calcium ions in body fluids?
When calcium concentration drops below normal rate of intestinal absorption increases
How do the kidneys affect concentration of calcium ions in body fluids?
When calcium concentration drops below normal osteoclasts are stimulated to increase release of stored calcium ions
How do bones affect concentration of calcium ions in body fluids?
When calcium concentration drops below normal kidneys retain calcium ions
What happens when calcium is low in the body?
- Parathyroid gland will secret PTH (parathyroid Hormone)
- Kidneys and the small intestine absorb calcium and phosphate
- PTH will stimulate the kidneys to release calcitriol which will also effect the kidneys and the small intestine to absorb calcium and phosphate
- PTH will go to the bones and directly increase activity of osteoclast to break down bone and release into the blood calcium and phosphate
What happens when calcium is high in the body?
- Thyroid gland will secret calcitonin
- Calcitonin effect the kidneys and the small intestine to not absorb calcium and phosphate
- Calcitonin will cause the kidneys and the small intestine to not absorb calcium and phosphate
- It will go to the bones and indirectly inhibit PTH from activating osteoclast and as a result osteoblast with deposit calcium and phosphate back into the bone
Types of bone fractures
- Open (compound)
- Closed (simple)
- Incomplete
- Complete
- Greenstick
- Hairline
- Comminuted
Open (compound) fracture
- Bone break with open wound
- Bone may be sticking out of wound
Closed (simple) fracture
Non perforated skin
Incomplete fracture
Doesn’t extend across the bone
Complete fracture
Extends across the bone
Greenstick fracture
Incomplete fracture that occurs on the convex side of the curve of a bone
Hairline fracture
- Incomplete where two sections of bone do not separate
- Common in skull fractures
Comminuted fracture
Complete with break into more than 2 pieces
Bone Repair
- Hematoma (blood clot) fills in gap between bone fragments
- Fibroblasts and chondroblasts form soft callus
- Osteoblasts build hard (bone) callus
- Bone callus is remodeled and primary bone is replaced with secondary bond
