Bone Formation and Metabolism Flashcards
What is meant by ‘lamellar’ bone?
Mature bone with a highly ordered collagen structure
What is meant by ‘woven’ bone?
Immature bone with a randomised collagen structure
What are the different types of lamellar bone?
Cortical(compact) bone and spongy (cancellous/trabecular) bone
Describe the composition of the extracellular matrix of bone
35% is organic element (osteoid) with type I collagen for tensile strength, proteoglycans for compressive strength and growth factors. 60% is inorganic and largely calcium hydroxyapatite. 5% is water
Name the cells present in bone.
Osteoclasts, osteoblasts, osteocytes and osteoprogenitor cells
Describe the role of osteoblasts
Form new bone
Describe the role of osteoclasts
Degrade bone (desorption)
Describe the role of osteocytes
Maintain bone integrity
Describe the role of osteoprogenitor cells
Are stem cells derived from mesenchymal cells which can differentiate into osteoblasts to form new bone
Define ‘ossification’
Laying down new bone material by osteoblasts
Name the two different types of bone ossification
Intramembranous ossification and endochondral ossification
Define ‘intramembranous ossification’
Formation of bone from within a condensed layer of membrane (primitive mesenchymal tissue) which occurs in the development of flat bones such as the skull
Define ‘ endochondral ossification’
Formation of bone from cartilage model as in long bone development
Describe the process of intramembranous ossification
1) In mesenchyme tissue, cells differentiate into osteoblasts -> many centres of ossification (COO) formed
2) Osteoblasts secrete osteoid at these centres which mineralise to form spicules of bone
3) COO size increases as more osteoblasts formed from differentiation, COO grow around foetal blood vessels until centres meet & fuse to form trabecular bone and mesenchyme condenses on outer surface to form vascularised periosteum
4) More fusion, until outer layer continuous with woven bone. Then remodelling so outer woven -> compact, and inner -> lamellar spongy bone and red bone marrow develops in space in spongy bone.
What is a ‘primary ossification centre’?
First part of bone to start ossifying (in diaphysis/centre of bone)
What is a ‘secondary ossification centre’?
Site where bone formation continues (epiphysis.bone ends)
Describe the formation of the primary ossification centre in endochondral ossification.
1) Perichondrium around shaft of cartilage model converted to periosteum (with osteogenic potential) and mesenchymal cells differentiate into osteoblasts here to form collar of bone.
2) Chondrocytes in centre of diaphysis hypertrophy leaving thin intervening matrix which calcifies and cartilage cells die
3) Nutrient artery perforates bony collar, enters diaphysis to carry osteoprogenitor cells and primitive mesenchymal cells which form osteoblasts and secrete osteoid to form bone on remnants of calcified cartilage - forming trabecular bone which is the primary ossification centre
Describe the formation of the secondary ossification centre in endochondral ossification - completion of bone formation.
At epiphyseal ends the chondrocytes hypertrophy and die, the intervening matrix calcifies, and the epiphyseal arteries carry osteoprogenitor cells into the area where they differentiate into osteoblasts to form trabecular bone (secondary ossification sites). The bone continues to replace the cartilage in the epiphyses until articulating cartilage and epiphyseal growth plate (between epiphysis and diaphysis) are only cartilage that remains.
Name the 5 zones present in the histology of endochondral ossification
zone of reserve cartilage, zone of cartilage proliferation, zone of maturation and hypertrophy of chondrocytes, zone of degeneration and calcification of chondrocytes, zone of osteogenesis
At what age do bones stop growing?
18-25
Which is the last bone to fully ossify?
Clavicle
What is ‘appositional growth’
The widening of the bones (growing out sideways instead of upwards)
Describe the process of appositional growth.
Periosteum cells differentiate into osteoblasts and secrete new bone matrix which forms ridges on either side of periosteum blood vessels until ridges fuse and enclose blood vessels to form tunnel of bone moving out sideways.
Name two reasons for bone remodelling.
It renews bone prior to deterioration, and redistributes the bone matrix along the lines of mechanical stress.
Describe the process of bone remodelling
Osteoclasts secrete HCl and protein-digesting enzymes beneath bone surface to break down collagen fibres and dissolve minerals. These proteins and minerals then enter the osteoclasts to exit into the interstitial fluid (and then blood) to increase serum calcium levels (bone desorption). Osteoblasts then fill the lacuna with osteoid which then mineralises to create new bone formation (takes 7-10 days)
Describe the role of calcium in bone metabolism.
Low serum calcium levels stimulates the secretion of parathyroid hormone from the parathyroid glands. Conversely, high serum calcium levels stimulate secretion of calcitonin from the thyroid glands (C-cells).
What is the normal serum level of calcium?
2.2-2.6mmol/L
Describe the role of parathyroid hormone in bone metabolism.
PTH promotes: calcium reabsorption in kidney (and phosphate excretion), bone resorption by increasing activity and number of osteoclasts (osteoblasts have PTH receptor and express RANKL, osteoclast precursors have RANK receptor leading to differentiation), also stimulate synthesis of 1,25-dihydroxyvitamin D which increase calcium reabsorption from the gut
Describe the role of oestrogen in bone metabolism
Decreases calcium absorption in the gut, and also inhibits osteoclasts (bone resorption)
Describe the role of glucocorticoids in bone metabolism
Decrease calcium absorption in the gut and increase bone degradation (bone desorption)
Describe the role of calcitonin in bone metabolism
Rising serum calcium levels stimulate the thyroid gland to release calcitonin which stimulate bone deposition and inhibits osteoclasts, and also reduce the calcium uptake in the kidneys
