Ossification of bone and disease Flashcards
What is intramembranous ossification?
-The replacement of mesenchyme with bone
What is endochondral ossification?
-The replacement of hyaline cartilage template with bone
Where does intramembraneous ossification occur?
- In the formation of flat bones
- In the widening of long bones
Describe intramembraneous ossification
- Mesenchyme template of flat bones is invaded by osteoblasts from the periosteum
- Osteoid becomes mineralised and forms bone tissue spicules
- These bone tissue spicules develop to form trabeculae which radiate out from a central point which used to be the primary ossification centre
In endochondral ossification of long bones, does all the cartilage get replaced to bone immediately?
-No the epiphyseal growth plates remain to allow development of long bones
Describe endochondral ossification
- Initial cartilage model in embryo
- Collar of periosteal bone appears at the shaft
- Central cartilage calcifies, nutrient artery penetrates supplying osteogenic cells, primary ossification centre formed
- (postnatal) Medulla becomes cancellous, cartilage forms epiphyseal growth plates and epiphyses develop secondary ossification centres
- (pre-puberty) Epiphyses ossify and growth plates continue to move apart, lengthening bone
- (mature) Epiphyseal growth plate replaced by bone, articular cartilage remains
Describe how growth occurs at the epiphyseal growth plates (think zones)
- Zone of reserve cartilage
- Zone of proliferation-> Chondrocytes proliferate, enlarge and secrete matrix
- Zone of hypertrophy-> Chondrocytes become very large
- Zone of calcified cartilage -> Chondrocytes degrade and the matrix calcifies
- Zone of resorption-> Columns of calcified cartilage becomes mineralised forming bony spicules which develop into bone
Can you tell the difference histologically between intramembraneous and endochondral ossification?
-No they are both arranged in the same way with haversian systems
What inheritance pattern is osteogenesis imperfecta?
-Autosomal dominant
What is the cause of osteogenesis imperfecta?
-Mutation in the TI collagen gene
What morphological and mechanical abnormalities can bones show in osteogenesis imperfecta?
- Bones are often thin and bowed/curved with growth retardation
- Bones are highly susceptible to fracture due to the weakened structure of collagen fibres
How is osteogenesis imperfecta treated?
-Orthopaedic devices such as rods being insterted into the medulla
Why does ostogenesis imperfecta have medicolegal importance?
-The recurrent fractures can look like deliberate harm to the child
What is achondroplasia?
-Short-limbed dwarfism
What is the cause of achondroplasia?
-An autosomal dominant point mutation in the fibroblast growth factor receptor gene which results in a gain of function
How does the gain of function of the FGFR gene in achondroplasia result in short-limbed dwarfism?
Stops the action of the growth plates by:
- Decreased endochondral ossification
- Inhibited proliferation of cartilage in growth plates
- Decreased cellular hypertrophy
- Decreased cartilage matrix production
