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What are the inner and outer surfaces of bone called?
Endosteum and periosteum
True or false: bone is very vascular?
What two types of canals are present in compact bone?
Volkmann's canals and Haversian (central) canals
Describe the structure of a Haversian system or Osteone
In the middle of an osteone is a central canal. Osteoblasts form concentric lamellae around these central canals (between 4-20 depending on the stage of growth). Within each of these lamellae the orientation of collagen fibres is different - this gives compact bone its strength.
What cell type constantly remodels Haversian systems?
What are formed during remodelling of Haversian systems?
Describe the formation of a Haverisan system in the diaphysis of a long bone
- Longitudinal ridges form along the diaphysis, and periosteal cells differentiate into osteoblasts. A periosteal capillary is found in the groove. New bone begins to extend adjacent ridges towards each other.
- The ridges fuse and the groove becomes a bony tunnel enclosing the blood vessel.
- Additional bone lamellae are deposited around the tunnel which is then converted into the Haversian canal containing a blood vessel.
- The Haversian vessel continues to receive blood through the canals of Volkmann extending obliquely across the diaphysis. Note that the Haverisan vessel is surrounded by concentric lamellae. Multiple Haversian systems form, and such appositional growth widens the shaft of the bone. A similar widening of the medullary cavity occurs.
- When the bone reaches full size, outer and inner circumferential lamellae provide the boundaries of the compact bone consisting of Haversian systems. The interstitial lamellae represent remnants of preexisting Haversian systems replaced by new Haversian systems during remodelling. Remodelling occurs though out life and is part of normal bone maintenance. As one aversion system is formed by the activity of osteoblasts another system is dismantled by osteoclasts and then replaced or rebuilt.
Explain the difference between intra-membranous ossification and endochondral ossification
Intra-membranous ossification is when bone forms directly from osteogenic connective tissue. There is no pre-existing cartilage model. Whereas endochondral ossification his when there is replacement of a hyaline cartilage model with bone.
Give examples of where intramembranous and endochondral ossification take place in the body
- Intramembranous - mandible, maxilla, flat bones of the skull
- Endochondral - growth of long bones in embryo and juvenile
During intramembranous ossification where do cells change to become osteoblasts and start producing new bone?
The osteogenic region
Describe the stages involved in endochondral ossification - replacement of the cartilage model
- A longbone is made entirely out of cartilage. On its outer surface it will have perichondrium which over time will become vascularised. The cells in the middle of the cartilaginous model, as it is elongating, are no longer able to be nourished by diffusion as bone is being laid down by the periosteum. This prevents cells in the middle from getting nourishment and causes the chondrocytes to atrophy. This process also causes release of vascular endothelial cell growth factor so more blood vessels are attracted in to give the bone a blood supply. At the same time, cartilage cells die, attract minerals and become mineralised inside a mineralised matrix.
- At the next stage, vessels penetrate through the boney wall into the area where chondrocytes are dying off. As the blood vessels comes in it brings with it CT, including CT from the periosteum which contains osteo-progenitor cells. There is then formation of a primary centre of ossification. The chondrocytes and mineralised matrix is dissolved away and replaced by bone.
- Growth of long bones continues by endochondral ossification at the epiphyseal growth plate. The middle of the primary centre of ossification sends off blood vessels towards the two ends of the bone. The process of bone deposition replaces any cartilage in the middle of the bone. The bone is remodelled by osteoclasts so we end up with a marrow cavity.
- At the end of the long bone there is another site of hypertrophic chondrocytes as another blood vessel forms a second centre of ossification.
- The growth plate separates the primary and secondary centres of ossification. As long as the chondroblasts in the growth plate can keep laying down cartilage it will keep the centres of ossification separate and the bone will continue to grow in length.
What is a growth plate?
A block of hyaline cartilage that separates the primary and secondary centres of ossification by constantly trying to replace itself.
Describe what is happening at the epiphyseal growth plate during endochondral ossification
In the epiphyseal cartilage there are chondroblasts producing daughter cells which produce more cartilaginous matrix. As the process continues they get further away from the epiphyseal cartilage and cells move into the reserve zone. In this zone the chondrocytes align in concert with each other. As they move further away from where proliferation occurs the cells are changing - in this area they start to die off. In the hypertrophic zone the cells that were chondrocytes die, attract minerals, become encased in mineralised matrix and wait for the advancing front of the vascular zone to come in and dissolve them away. The cells are then laid down as osteoblasts and the bone proper is formed.
Describe the termination of growth and closure of the epiphyseal growth plate
- Blood vessels from the diaphysis and epiphysis intercommunicate
- All the epiphyseal cartilage is replaced by bone, except for the articular surface
- The epiphyseal growth plate is replaced by an epiphyseal line. This process occurs gradually from puberty to maturity and the long bone can no longer grow in length.
Describe the stages in compact bone remodelling
- Osteoclast precursors are recruited to the haversian canal and differentiate into osteoclasts. Osteoclasts are lining the bone lamella facing the canal and start the bone resorption process of the inner lamella and consecutive lamellae toward the outer lamella. Interstitial lamellae are residuals of the remodelling osteon.
- Additional osteoclast precursors are recruited as lamellar resorption progresses slightly beyond the boundary of the original osteon. When osteoclasts stop removing bone, osteoblasts appear.
- Osteoblasts reverse the resorption process by organising a layer inside the resorption cavity and starting to secrete osteoid. The cement line indicates the boundary of the newly organised lamellae. New bone lamellae continue to be deposited towards the centre of the osteon.
- Osteoblasts continue laying down bone and eventually become trapped within the mineralised bone matrix and become osteocytes. A new osteone or haversion system has formed.
What happens in osteoporosis?
More bone is reabsorbed than is subsequently produced
Describe trabecular bone remodelling
Trabecular bone remodelling occurs on the bone surface. In contrast to cortical bone remodelling which occurs within an osteone. The trabecular endosteal surface is remodelled by this mechanism similar to cortical bone remodelling: osteoclasts create a resorption space limited by a cement line. Then osteoblasts line the cement line surface and start to deposit osteoid until new bone closes the resorption space.