Lecture 7
Which bone cell builds bone, and which one breaks it down?
Osteoblasts build bone; osteoclasts break it down (resorb it).
Which ossification pathway builds long bones by replacing a hyaline cartilage model?
Endochondral ossification
Modeling or remodeling: which one changes bone size or shape?
Modeling
What forms first when blood vessels rupture at a fracture?
A hematoma
What type of cartilage makes up both the epiphyseal (growth) plate and articular cartilage?
Hyaline cartilage
In the growth plate, which zone has chondrocytes that ENLARGE, and which zone comes right before it?
The hypertrophic zone; the proliferative zone comes right before it.
During appositional growth, which cell works on the OUTSIDE of the bone and which on the INSIDE, and what does each do?
Outside (periosteal surface): osteoblasts build new bone, so the diameter increases. Inside (endosteal surface): osteoclasts resorb bone, so the medullary cavity enlarges.
A week-2 X-ray shows the break bridged by fibrous tissue and cartilage. Which stage is this?
Soft callus formation (about weeks 1–3)
Spongy bone has no osteons or central canals. How do its osteocytes get nutrients?
By diffusion from blood vessels in the marrow spaces. The trabeculae are thin enough that no central canal is needed.
Achondroplasia disrupts growth plate cartilage. Why do patients have short limbs but a normal-sized skull vault?
Limb bones form endochondrally and need a cartilage growth plate to lengthen. The skull vault forms by intramembranous ossification, with no cartilage model to disrupt.
Osteoclasts use two tools to resorb bone. What does each one dissolve?
Acid dissolves the mineral (hydroxyapatite). Proteolytic enzymes digest the organic matrix, especially collagen.
In stage 4, woven bone is replaced by lamellar bone. Using what you know from L7, why is lamellar bone stronger?
Its collagen fibers run in alternating directions in adjacent lamellae, which helps the bone resist twisting and crack propagation. Woven bone's collagen is disorganized.