Week 1 - Bone Structure, Formation, & Modelling/Remodelling Flashcards
What are the main functions of bone?
- Support & protects vital organs
- Muscle attachment
- Hematopoiesis
- Mineral Reservoir - Ca & PO4
Histologically, what differentiates bone from other tissue?
Mineralized ECM
Briefly describe the composition of bone matrix
- 35% organic materials “Osteoid” –> Type I Collagen, glycoproteins, growth factors, cytokines
- 65% inorganic materials –> hydroxyapatite, other elements
- specialized bone cells –> osteoprogenitor, osteoblasts, osteocytes, osteoclasts
What is the outer surface of bone called?
- Describe its structure
Periosteum - two layers:
- Outer layer: connective tissue (fibroblasts, collagen, blood vessels)
- Inner layer: cellular layer (osteoprogenitor & osteoblast cells) –> has “bone-making potential”
- anchored to the bone by collagen (Sharpey) fibres
What is the inner lining of the bone called?
Endosteum
- one cell layer thick (progenitor, matrix-secreting, and bone-lining cells)
What are endosteal cells?
osteoprogenitor cells + bone-lining cells
What is found in the red marrow of bone?
Blood cells/vessels & supporting reticular meshwork
What is found in the yellow marrow of bone?
Mainly fat cells (especially in adults)
What are the functions of osteocytes?
- Maintain bone tissue
- Communication (with osteocytes & osteoblasts)
- Regulation of Ca2+
Osteoprogenitor cells have the potential to develop into what?
- osteoblasts
- chondroblasts
- fibroblasts
What are trabeculae?
Developing bone spicules
What is the function of osteoblasts?
Build bone:
- secrete Type I collagen & bone matrix protein
- calcify/mineralize the bone
What is the process of osteoblasts becoming osteocytes?
Osteoblasts secrete matrix, then get surrounded in that matrix, then become osteocytes “sentinel cells” which exist in lacuna
How/from what does an osteoclast develop?
Common myeloid progenitor --> monocyte progenitor --> osteoclast precursor (must be activated by RANKL - from stromal cell or soluble secreted RANKL from activated T-lymphocytes ) --> inactive osteoclast --> resorbing osteoclast
What are the stages of resorption by osteoclasts?
- Migration
- Attachment & cell proliferation
- Degradation of cellular matrix
- Removal of degradation products
- Apoptosis or return to non-resorbing state
What is bone modelling?
Bone changes shape based on mechanical forces or physiological influences
What is bone remodelling?
Bone is renewed to maintain bone strength and mineral homeostasis
How does PTH act on the bone?
stimulates osteoclasts to resorb bone and liberate Ca2+
- reaction to low blood Ca2+
What effect does calcitonin have on the bone?
- STIMULATES osteoBLASTS
- inhibits osteoclasts
- inhibits Ca2+ reabsorption from intestine and in kidney
Does estrogen encourage or discourage bone formation?
Encourages it
- lengthens lifespan of osteoblasts
- mediates effects of ROS
- regulates RANKL-induced osteoclast differentiation
- modulates bone-resorbing cytokines
What are some bone-resorbing cytokines?
IL-1, IL-6, TNF-alpha, M-CSF
What are two types of bone formation/ossification?
- Intramembranous
2. Endochondral
What are the main differences between the two types of bone formation/ossification?
Intramembranous - formed w/out cartilage framework & develops into flat bones
Endochondral - formed onto a cartilagenous model & develops into bones of extremeties and axial skeleton which bear weight
What are the stages of intramembranous ossification?
- Aggregation (of mesenchymal cells)
- Migration (mesenchymal cells)
- Formation of 3-D spongy bone network (by osteoblasts)
- Formation of periosteum
What are the steps to Endochondral ossification?
- Develop Hyaline Cartilage Model
- mesenchymal cells –> chondrocytes –> secrete matrix
- interstitial and appositional growth - Develop Bony Collar - becomes periosteum
3 & 4. Death of Central Chondrocytes & Invasion of Blood Vessels
- chondrocytes hypertrophy & secrete ALP –> calcification
- blood vessels grow through thin bony collar - Formation of Primary Ossification Centre = first site where bone begins to form
- Growth of Endochondral Bone - get epiphyseal cartilage
7 & 8. Secondary Ossification Centres
- ossification centres form at proximal and distal end
- cartilage is resorbed and replaced by spongy bone
9 & 10. Skeletal Maturity - epiphyseal closure; only remaining cartilage is on articular surfaces
What is an osteon?
The structural unit of bone
- formed around a central Haversion Canal (vascular & nerve supply)
- contains concentric lamellae, osteocytes (lacunae), and canaliculi (which connect the osteocytes)
- grows from the outside in (osteoblasts are central)
Trivia question: What is the last bone to stop growing?
Clavicle (usually around age 21)
Trivia question: What is the densest bone in the animal kingdom?
An elephant’s tail
What is the process for increasing length of a bone?
Proliferation of cartilage at epiphyseal plate
Metaphysis needs to be reshaped to form new diaphysis (b/c head of bone will be further from centre.
How is remodelling cortical bone different than remodelling spongy bone?
Cortical bone is arranged in osteons.
Spongy bone is arranged in trabeculae.
What are the phases of bone remodelling?
- Activation (recruiting osteoclasts)
- Resorption (by osteoclasts)
- Reversal (switch from resorption to osteoblast activity)
- Formation (by osteoblasts)
What is Wolff’s Law?
Bone will adapt to the loads placed on it
Why is bone the “best” tissue in the body?
- it heals itself with its own tissue
- it renews itself with its own tissue
- it responds to the demands placed on it
Where are different types of marrow located within bones in an adult?
Yellow marrow (fat) is located in the medullary cavity in spongy bone in diaphysis of long bones. Red marrow (hematopoietic) is located in flat bones and epiphysis of long bones.
What is the physis?
What is another name for this?
Growth plate
aka. epiphyseal plate
What is a Salter-Harris Type I Fracture?
No bone injury but growth plate detaches
What is a Salter-Harris Type II Fracture?
Fracture proximal to growth plate with a portion of metaphysis breaking off with the head of bone.
What is a Salter-Harris Type III Fracture?
Fracture where chunk of epiphysis and physis comes off
What is a Salter-Harris Type IV Fracture?
Fracture goes straight through epiphysis, physis, and metaphysis.
What is a Salter-Harris Type V Fracture?
CRUSH INJURY! Physis loses functionality.
What are cement lines?
Uncalcified matrix with little collagen that forms around osteons/new haversion systems
- cement the new new haversion system to older osteons
