Bone Development Flashcards
Types of bone growth
•Intramembranous ossification
• Endochondral ossification •
NB: Ossification = bone formation
Intramembranous ossification
Intramembranous ossification is the process of bone development from fibrous membranes. It is involved in the formation of the flat bones of the skull, the mandible, and the clavicles (flat Bones). Ossification begins as mesenchymal cells form a template of the future bone.
Mesenchymal cells
are multipotent stem cells that can be found in the bone marrow that are capable of forming skeletal tissues
Endochondral Ossification
Endochondral ossification is the process by which growing cartilage is systematically replaced by bone to form the growing skeleton.7 This process occurs at three main sites: the physis, the epiphysis, and the cuboidal bones of the carpus and tarsus.
New bones are formed from a cartilage template. • Most bones in the body form this way
growth (epiphyseal) plate.
By birth most of the cartilage skeleton has been replaced with bone. Some areas of the cartilage model remain post partum, to allow for continued bone growth through to adulthood when skeletal maturity is achieved.
The cartilage regions that remain in developing long bones in young animals is known as the growth (epiphyseal) plate.
Regions within the growth plate
Reserve Zone Proliferative Zone
Zone of maturation within hypertrophic zone Zone of degeneration within hypertrophic zone
Zone of calcification within hypertrophic zone
Reserve Zone
the region closest to the epiphyseal end of the plate, contains small chondrocytes within the matrix. These chondrocytes do not participate in bone growth; instead, they secure the epiphyseal plate to the osseous tissue of the epiphysis
Proliferative Zone
the next layer toward the diaphysis, contains stacks of slightly-larger chondrocytes. It continually makes new chondrocytes via mitosis.
The chondrocytes increase in number rapidly and line up in stacks
Zone of maturation and degeneration within hypertrophic zone
contains chondrocytes that are older and larger than those in the proliferative zone. The more mature cells are situated closer to the diaphyseal end of the plate. In this zone, lipids, glycogen, and the enzyme alkaline phosphatase ( excreted by chondrites) accumulate, causing the cartilaginous matrix to calcify. The longitudinal growth of bone is a result of cellular division in the proliferative zone along with the maturation of cells in the zone of maturation and hypertrophy.
Chondrocides degenerate and die, leaving the lacunae empty and allowing the osteoprogenitor osteoblasts ( bone producing cells) cells to enter
preparation of matrix for calcification, chondrocyte growth
of within zone
further preparation of matrix for calcification, further chondrocyte growth in size
Zone of calcification within hypertrophic zone
chondrocyte death allows calcium release, allowing calcification of matrix
Chondrocides degenerate and die, leaving the lacunae empty and allowing the osteoprogenitor cells, osteoblasts ( bone producing cells) cells to enter and start laying down bone matrix into which calcium is deposited
The point where new Bone Is formed to add to the length of the long bone
If the growth plate is involved in a fracture management is
more complex.
If the proliferative zone of the growth plate is involved then the prognosis is
poor.
Modelling
– defines skeletal development and re-shaping of bones throughout life. Osteoblast (bone forming cells) and osteoclast (bone resorbing cells) activity is not coupled.
Remodelling
– Renewal of the bony skeleton throughout life. Adapting bones to meet the load placed upon them. Cyclic process reliant on coupled activity between osteoblasts and osteoclasts
Remodelling cycle
Quintessence Resorption Reversal Formation Mineralisation
Remodelling cycle
Bone lining cells or resting osteoblasts line the bone
Osteocytes are engulfed in the bone matrix with extensions that connect to bone lining cells and other osteocytes. Sense strain on bone.
Osteocytes sense strain and osteoclasts ( biggest cell with multiple nuclei) are recruited to the area.
Osteoclasts reabsorbs the bone by secreting acid into reabsorption pit between osteoclast and bone surface
This breaks down bony matrix and releases calcium that can be used elsewhere
Macrophages move into the area to clean up and induce osteoblast differentiation.
Osteoblasts then line up along the bone surface
Osteoblasts lay down and mineralise new bone then return to resting state
Bone healing
- Primary / Direct Healing
* Secondary / Indirect Healing – Most common – uses endochondral and intramembranous ossification
Primary / Direct healing
Relies on immobilisation of fracture site to reduce strain between fragments
• Edges must touch exactly
• Usually requires internal fixation
At the site of the fracture groups of osteoclasts congregate. they create cavities in the digest of the fracture site. Osteoblasts move to the site and deposit bone mineral
The bony matrix is laid down in layers (lamellae)
If bone fragments are close together the lamellae are organised longitudinally along the length of the bone. Blood vessels then penetrate the lamellae creating a haversian system eg blood vessel runs through the middle of layers of bone
If bone is too far apart for this lamellae are arranged perpendicular. Makes bone weaker and requires remodelling, adds time
No callus formed
Secondary/ indirect healing- haematoma
Form when bones are first fractured
A clot of blood cells that form a callus
Activation of coagulation cascade
Changes of local environment
Inflammatory cells and molecules released increasing blood flow
Secondary/ indirect healing- inflammation
Recruitment and activation of inflammatory and osteoptrogenitor cells- osteoblasts and condrocytes
Clearance of necrotic tissues
Secondary/ indirect healing- callus formation
Soft and hard
Differentiation of MSCs according to the mechanical environment
Initial stabilisation of fracture, then replaced by calcified tissue
Secondary/ indirect healing- granulation tissue
Active proliferation of osteoprogenitor cells- osteoblasts proliferate and secrete bony matrix into woven bone. Woven bone is then replaced with the stronger lamellae
Angiogenesis- blood vessel formation and infiltration: essential as it brings nutrients takes away waste aids inflammation and supply’s osteoprogenitors
Extracellular matrix production
Secondary/ indirect healing- remodelling
Long process Lamellae aligned with the forces of the cell Resorption of remaining cartilage Restoration of haversian system No scar formed
