Week 25 Flashcards
(35 cards)
What are the two main types of bone in a typical long bone?
Compact bone
Trabecular bone
What are the three types of cells in bone and their roles?
Osteoblasts: Synthesise uncalcified extracellular matrix called osteoid, which calcifies to form bone.
Osteocytes: Mature osteoblasts that dwell in lacunae, monitor minerals and proteins to regulate bone mass.
Osteoclasts: Large multinucleate cells from monocytes, resorb bone by releasing H⁺ ions and lysosomal enzymes.
What makes up the extracellular matrix in bone?
30% organic matrix
70% mineral (hydroxyapatite)
25% water
Matrix is organized into layers called lamellae
Bone calcifies when calcium and phosphate ion concentrations rise
What is the function of Haversian and Volkmann’s canals?
Haversian canals:
Formed by concentric lamellae
~50 µm diameter
Contain blood capillaries and nerve fibres
Part of an osteon (Haversian system)
Volkmann’s canals:
Run perpendicular to Haversian canals
Interconnect Haversian canals and connect to the periosteum
Canaliculi: Channels that communicate between Haversian canals and osteocytes in lacunae
Compare woven bone and lamellar bone.
Woven bone (primary bone):
Found in embryonic development and fracture repair
Osteoid with randomly arranged collagen fibres
Temporary and replaced by lamellar bone
Lamellar bone (secondary bone):
Found in adult skeletons
Highly organised mineralised osteoid sheets
Stronger than woven bone
Includes compact and spongy types
Covered by periosteum (outer) and endosteum (inner)
What are the features of compact bone?
Forms the outer ‘shell’ of bone
Lamellae organized into concentric circles around Haversian canals
Haversian canals connected by Volkmann’s canals
Osteocytes located in lacunae between lamellae
Lacunae are connected by canaliculi
Describe the structure and function of spongy bone.
Found inside compact bone, makes up the interior of most bones
Honeycombed appearance with large spaces
Bony matrix: 3D network of fine columns (trabeculae)
Spaces between trabeculae filled with:
Yellow marrow (adipocytes)
Red marrow (haematopoietic stem cells)
No Volkmann’s or Haversian canals
Light and porous; withstands multidirectional force
What are the two mechanisms of bone formation?
Endochondral ossification:
Hyaline cartilage is replaced by osteoblasts secreting osteoid
Example: femur
Intramembranous ossification:
Mesenchymal (embryonic) tissue is condensed into bone
Example: temporal bone and scapula
Both produce primary bone first, then it is replaced by secondary bone
What is bone remodelling and how does it occur?
Mature bone is resorbed and new bone is formed
Carried out by osteoclasts forming a cutting cone
Nutrients reabsorbed, then osteoblasts lay down new osteoid
Occurs primarily at sites of stress and damage
What is osteoporosis and its types?
Osteoporosis:
Decrease in bone density, fragile bones, increased fracture risk
Caused by osteoclast activity outweighing osteoblast activity
Type 1 (Postmenopausal):
Due to decreased oestrogen after menopause
Oestrogen increases osteoblast and decreases osteoclast activity
Type 2 (Senile):
Occurs after age 70
Type 3 (Secondary):
Due to coexisting disease (e.g. chronic renal failure)
What are rickets and osteomalacia?
Rickets (children):
Vitamin D or calcium deficiency
Poor mineralisation of osteoid → pliable bones
Distorted epiphyseal plates → skeletal deformities
Osteomalacia (adults):
Vitamin D or calcium deficiency during remodelling
Poorly mineralised osteoid
Leads to weak bones and increased fracture risk
How do skeletal muscles change with different types of training?
Resistance exercise increases muscle mass (cross-sectional area) and strength.
Endurance exercise stimulates mitochondrial biogenesis, promoting transformation to slow-twitch fibre types.
Submaximal aerobic exercise increases insulin-dependent glucose uptake.
What are the roles of bone in plastic adaptation?
Modelling: Growth
Remodelling: Adaptation
Removal and repair of damaged bone
Mineral homeostasis
What do osteoblasts and osteoclasts do?
Osteoblasts build bone
Osteoclasts chip bone (resorb bone)
What are osteocytes and where are they located?
Osteocytes are osteoblasts cocooned in lacunae (cavities within bone matrix).
They detect signals through a network of canaliculi.
What triggers bone remodelling and how is it detected?
Triggers:
Increased strain
Damaged tissue
Calcium homeostasis: reduced serum calcium → parathyroid hormone secreted by parathyroid glands
Detection:
All detected by osteocytes via a network of dendrites
What happens during the resorption phase of bone remodelling?
Osteoclasts are recruited to the remodelling site
Osteoblasts secrete enzymes (metalloproteinases) that degrade unmineralized bone to expose mineralised bone
How do osteoclasts act on bone during the resorption phase?
Osteoclasts bind to mineralised bone
Pump hydrogen ions into bone, dissolving mineralised matrix
Rest of the organic matrix is degraded by collagenolytic enzymes
What happens in the reversal phase of bone remodelling?
Removal of matrix debris by reversal cells (possibly phagocytes or differentiated osteoblasts)
Preparation of bone surface for bone deposition
Down-regulation of osteoclast recruitment
Promotion of osteoblast recruitment
What do osteoblasts do during the formation phase?
Osteoblasts secrete molecules to form bone matrix:
Collagen (type I)
Other proteins and lipids
This forms new osteoid
Mineralisation of the bone matrix using calcium (hydroxylapatite)
When and how does the bone remodelling process end?
Process finishes once an equal amount of resorbed bone has been replaced
Osteoblasts:
Undergo apoptosis
Or differentiate into osteocytes and become embedded in the bone matrix
Where does bone remodelling occur?
Bone remodelling occurs in both trabeculae (spongy bone) and compact bone0
How does plasticity complicate evolutionary interpretations?
Many adaptations may arise from plasticity, not genetic mutation
Example: gluteus maximus may have enlarged in individuals due to upright walking and endurance running
These changes may then spread through the population
Therefore, not all fossil features are inherited—some reflect short-term activities
What happens to bone density with age or inactivity?
Astronauts lose 1–2% bone density per month
Elderly lose 0.5–1% per year
Bone density increases until 20s, then decreases and can only be maintained
Weight-bearing exercise is critical
