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
Can you be homozygous in achondroplasia?
-No it is fatal
What is the result of excessive growth hormone before puberty?
-Gigantism
How does excessive GH cause gigantism?
-Promotion of epiphyseal growth plate activity
What is the result of excessive GH in an adult?
-Acromegaly (widening of bone through increased periosteal growth)
What is the result of decreased GH before puberty?
-Pituitary dwarfism
What is the most common cause of altered GH levels?
-Benign pituitary tumour (adenoma)
How do sex hormones influence growth?
- Influence the development of ossification centres
- Control pubertal growth spurt
What is the result of excessive sex hormones in puberty?
-Retards bone growth due to early closure of the epiphyseal growth plates
What is the result of deficient sex hormones in puberty?
-Prolonged bone growth and tall stature as epiphyseal growth plates persist till later in life
What is the cause of cretinism?
-Untreated thyroxine deficiency during development
Can thyroxine deficiency damage be reversed in neonates?
-Yes with prompt administration of thyroxine within a time-window
What is rickets?
-A childhood bone disease characterised by soft bones due to lack of vitamin D
What are the characteristics of rickets?
- Insufficient calcium deposition for rigidity
- Soft and malformed bones
- Distortion of the skull bone (bossing)
- Enlargement of the costochondral junctions in ribs
What is the cause of osteomalacia?
-Lack of calcium or vitamin D in adults
What are the common causes of osteomalacia? (what causes lack of vit D)
- Poor diet
- Lack of sunlight
- Intestinal malabsorption
- Liver/kidney damage
What are the common symptoms of osteomalacia?
- Bone pain
- Back pain
- Muscle weakness
What are the characteristics of the trabeculae like in osteomalacia?
-Trabeculae have large amounts of non-mineralised bone (osteoid) making them weaker
What sites are commonly effected in osteomalacia?
- Neck of femur
- Ribs
- Spine
What is osteoporosis?
-A metabolic bone disease caused by a decrease in bone mass to the point where bone no longer provides adequate mechanical support
What is the cause of decreased bone mass in osteoporosis?
-Enhanced bone resorption relative to deposition
Why are bones susceptible to fracture in osteoporosis?
-Increased resorption causes thin traneculae, making them more prone to fracture
Why is osteoporosis associated with age?
-Outer surfaces of trabeculae are constantly remodelled, with ageing the resorption bays formed by osteoclasts are incompletely filled
What are the two types of postmenopausal causes of osteoporosis?
- T1-> Increased osteoclast activity due to oestrogen withdrawal
- T2-> generally occurs over 70 due to attenuated osteoblast function
How do genetic factors play a role in osteoporosis risk factors?
-Bone density is higher in black races
Why can lack of exercise be a risk factor for osteoporosis?
-If the bones are immobilised there is an acceleration of bone loss
