Bone development and ossification

Question 1

What is ossification (bone formation)?

Answer 1

removal of cartilage by osteoclasts to make space for bone marrow

Question 2

intramembranous ossification - how is bone formed

Answer 2

formed directly from mesenchymal cell condensation in the membrane

Question 3

intramembranous ossification - what bones are formed in this way?

Answer 3

skull, cortical bone shafts of long bones

Question 4

intramembranous ossification - stages of formation

Answer 4

• embryonic CT/ mesenchyme proliferates
• differentiates
• directly to osteoprogenitor cells
• form osteoblasts
• osteoblasts synthesise matrix (osteoid)
  then : matrix deposition, formation of periosteum, vascularisation, and continued growth and remodelling

Question 5

intramembranous ossification - matrix deposition and mineralisation

Answer 5

• collagen fibres randomly arranged in irregular bundles and there are large and numerous osteocytes = woven bone

Question 6

intramembranous ossification - vascularisation details

Answer 6

bone formation continues to form a network of trabecular bone
- haematopoietic bone marrow forms between trabeculae following incorporation of blood vessels

Question 7

intramembranous ossification -continued growth and remodelling details

Answer 7

later woven bone is remodelled by osteoclasts and replaced by lamellar bone

Question 8

endochondral ossification - how is bone formed

Answer 8

cartilage is deposited by chondroblasts in the shape of the required bone and is surrounded by the perichondrium

Question 9

what are chondrocytes?

Answer 9

chondroblasts embedded In their own matrix

Question 10

endochondral ossification - stages of formation

Answer 10

1. cells of the perichondrium differentiate to osteoprogenitor cells and osteoblasts - they lay down the bony collar around diaphysis (cartilage model)
2. impermeable bony collar shuts supply of nutrients to chondrocytes- degenerate and cartilage matrix becomes calcified - restricts diffusion further
3. capillaries invade the periosteum and bony collar and bring osteoclasts with them- they resorb calcified cartilage to excavate a central cavity (marrow space)
   at the same time, osteoprogenitor cells migrate in with blood vessels to form osteoblasts - lay down bone matrix around the cartilage core that remains

Question 11

endochondral ossification - what occurs at the epiphyses?

Answer 11

chondrocytes undergo rapid proliferation, forming longitudinal column cells - increases the length of developing bone

Question 12

endochondral ossification - what happens when chondrocytes age?

Answer 12

get further from extending epiphyseal surface

they hypertrophy and begin to calcify the cartilage matrix - restricts nutrients and cells die

Question 13

endochondral ossification - more on formation of bone

Answer 13

end walls between empty lacunae break down, opening up the longitudinal tunnels in the calcified cartilage, and blood vessels bring more osteoblasts to lay down bone on top of the remaining calcified cartilage
- early spongy bone is remodelled by osteoclasts and osteoblasts to give mature lamellar bone

Question 14

where are primary ossification centres found?

Answer 14

mid shaft

Question 15

where are secondary ossification centres found?

Answer 15

epiphyses

Question 16

where do growth plates form?

Answer 16

between epiphyses and diaphysis

Question 17

structure of bone

Answer 17

epiphysis, then metaphysis and then middle section is diaphysis

Question 18

when is cartilaginous growth plate formed?

Answer 18

formed during endochondral ossification - transition between epiphyses and metaphysis of long bones

Question 19

what is the function of cartilaginous growth plates?

Answer 19

responsible for longitudinal growth of bone

Question 20

what are the 4 distinct zones of the epiphyseal growth plate

Answer 20

resting zone
proliferative zone
hypertrophic zone
calcification/ossification zone or primary spongiosa

Question 21

what happens in growth plate

Answer 21

resting reserve zone
proliferative - proliferation, columns, clusters
hypertrophic - cells enlarging in their lacunae
calcification
degeneration/degradation
ossification front/ primary spongiosa

Question 22

what are the types of bone growth?

Answer 22

appositional

interstitial

Question 23

what is appositional growth?

Answer 23

matrix deposition upon pre-existing matrix - addition onto free surfaces
occurs in bone and cartilage

Question 24

what is interstitial growth?

Answer 24

expansion of matrix from within (internal a=swelling)

cartilage only

Question 25

what does length and width of a bone depend on?

Answer 25

results from a combination of both appositional and interstitial growth