Bone as a Tissue and Organ Flashcards
What are the 2 types of bone?
cortical (compact)
trabecular (spongy, cancellous)
What are the 2 developmental pathways for bone?
intramembranous ossification
endochondral ossification
Describe the defined trajectory through out life.
what happens through puberty and into adulthood?
Why/how is this different in males and females?
What happens from the 3rd decade onward
the skeleton follows a defined developmental trajectory
- through puberty to young adulthood-mass increases, more in males than females
- males have larger body size
- hormonal milieu in males promotes greater modeling than in females
- 3rd decade onward net skeletal mass declines, more in females then males so they are more susceptible to low trauma fractures
How is intramembranous (flat) bone formed? Where?
- formation of osteoblasts from mesenchymal stem cells present within what will become the periosteum
- formed de novo
- skull and ribs
- also in bone collar region of a healing fracture and at the periosteal surface of long bones as they model to acheive greater diameters.
Where are endochrondral bones and how do Endochondral bones form?
- replace previously formed cartilage
- grow in length by proliferation of chondrocytes growth plate which is a specalized structure present
- linear growth stops when the plate fuses
- long bones of the limb
What is the epiphyseal growth plate?
- highly organized tissue in which chondrocytes are arrayed in columns, with different postiions within the column occupied by cells at a distinct point of maturation
- chondrocytes in proliferation zone divide to replenish the plate
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How does growth occur at the epiphyseal growth plate?
- chrondrocytes in the proliferative zone divide replenishing the groth plate
- they then hypertrophy, undergo apoptosis, and are mineralized
- blood vessles invade zone of calcified cartilage, which is resorbed by chondroclasts and the space if filled by osteoblasts and bone matrix
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Linear growth is dependent on what?
the relative speed with which cells in the hypertrophic zone undergo apoptosis and thos in the proliferative zone divide
when does the epiphyseal plate close?
late adolescene in response to estrogen signaling
different anatomical sites fuse at different ages
What is the function of cortical bone?
- 80-85% of total bone
- function is structural providing protection for organs and levers used for movement
How is cortical bone organized?
- as osteons (Haversian systems) which are cylindrical structures in which concentric layers of bone matrix form lamellae around a central canal called a Haversian canal
- the lamellar organization of osteons is an important contributor to bone’s ability to resit fracture
What is the function of trabecular bone?
- 15-20% of totl bone
- function is primarily metabolic
- is most of bone surface area and most of remodeling activity
- large surface area allows for mineral homeostasis
- organized in lamellar fashion like cortical bone but the individual trabeculae are too small to contain osteons
bone tissue is constanly being broken down and revuilt, but not all bone does this at the same rate. What type of bone is the most actively remodeled?
trabecular bone bc of its prominant job in maintainsing mineral homestasis
What are the 2 essential principles to understanding bone remodeling?
- bone resorption adn bone formation are coupled processes
- bone resorption is relatively rapid, requiring about 2 weeks, while bone formation is slow, requiring 4-6 months for full mineralization to take place
How does remodeling of trabecular bone provide a mechanism for extracellular fluid to buffer its calcium and phosphate content?
- If you need more Ca and PO4:
- PTH and 1, 25 di-OH vitamin D are potent activators of osteoclast activity and cause bone resorption which involves dissoltion of bone mineral providing free Ca and PO4 that can enter the extracellular fluid and blood
- If theres too much:
- Ca and PO4 can be deposited into the bone
**essential for moment to moment mineral homestasis**
