Bone Development

Question 1

Types of bone growth

Answer 1

•Intramembranous ossification
• Endochondral ossification •
NB: Ossification = bone formation

Question 2

Intramembranous ossification

Answer 2

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.

Question 3

Mesenchymal cells

Answer 3

are multipotent stem cells that can be found in the bone marrow that are capable of forming skeletal tissues

Question 4

Endochondral Ossification

Answer 4

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

Question 5

growth (epiphyseal) plate.

Answer 5

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.

Question 6

Regions within the growth plate

Answer 6

Reserve Zone Proliferative Zone
Zone of maturation within hypertrophic zone Zone of degeneration within hypertrophic zone
Zone of calcification within hypertrophic zone

Question 7

Reserve Zone

Answer 7

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

Question 8

Proliferative Zone

Answer 8

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

Question 9

Zone of maturation and degeneration within hypertrophic zone

Answer 9

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

Question 10

of within zone

Answer 10

further preparation of matrix for calcification, further chondrocyte growth in size

Question 11

Zone of calcification within hypertrophic zone

Answer 11

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

Question 12

If the growth plate is involved in a fracture management is

Answer 12

more complex.

Question 13

If the proliferative zone of the growth plate is involved then the prognosis is

Answer 13

poor.

Question 14

Modelling

Answer 14

– defines skeletal development and re-shaping of bones throughout life. Osteoblast (bone forming cells) and osteoclast (bone resorbing cells) activity is not coupled.

Question 15

Remodelling

Answer 15

– 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

Question 16

Remodelling cycle

Answer 16

Quintessence 
Resorption 
Reversal
Formation
Mineralisation

Question 17

Remodelling cycle

Answer 17

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

Question 18

Bone healing

Answer 18

• Primary / Direct Healing

* Secondary / Indirect Healing – Most common – uses endochondral and intramembranous ossification

Question 19

Primary / Direct healing

Answer 19

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

Question 20

Secondary/ indirect healing- haematoma

Answer 20

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

Question 21

Secondary/ indirect healing- inflammation

Answer 21

Recruitment and activation of inflammatory and osteoptrogenitor cells- osteoblasts and condrocytes
Clearance of necrotic tissues

Question 22

Secondary/ indirect healing- callus formation

Answer 22

Soft and hard
Differentiation of MSCs according to the mechanical environment
Initial stabilisation of fracture, then replaced by calcified tissue

Question 23

Secondary/ indirect healing- granulation tissue

Answer 23

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

Question 24

Secondary/ indirect healing- remodelling

Answer 24

Long process 
Lamellae aligned  with the forces of the cell
Resorption of remaining cartilage 
Restoration of haversian system
No scar formed