bone development

Question 1

limb bud formation

Answer 1

forelimb bud forms earlier, hindlimb bud forms later

Question 2

limb bud

Answer 2

contains precursors of skeletal elements and tendon sheaths, muscles and vessels migrate in later, so all limb and limb girdle bones lateral plate mesoderm

Question 3

limb elongation

Answer 3

proximo-distal differentiation is mediated by the apical ectodermal ridge

Question 4

normal elongation

Answer 4

stylopod, zeugopod, autopod

Question 5

late ablation

Answer 5

stylopod, zeugopod

Question 6

early ablation

Answer 6

stylopod

Question 7

limb orientation

Answer 7

crania-caudal orientation generated by the zone of polarising activity which lies caudally

Question 8

polydactyly

Answer 8

common cause is ectopic medial expression of ZPA-related genes

Question 9

apoptotic digit separation

Answer 9

paddle has to form digits, forms clefts early pn in the paddle - cells start to apoptose and form digits

Question 10

syndactyl

Answer 10

when digits don’t separate

Question 11

bone-endochondral ossification

Answer 11

mechanism of bone formation in most trunk and limbs - mesenchymal condensation in portion of limb

Question 12

1. blastemal condensation

Answer 12

large, tightly-packed mesenchymal cells

Question 13

2. chondrification

Answer 13

cells differentiate into chondrocyte - the cartilage model

Question 14

3. hypertrophy

Answer 14

chondrocytes at the centre stop dividing and swell - interstitial growth
connective tissue around model is compressed into perichondrium

Question 15

4. early mineralisation

Answer 15

central chondroblasts start to mineralise matrix, perichondrial cells become osteoblasts – cortical bone collar – compact bone

Question 16

5. vascular invasion

Answer 16

invades through cortical bone, accompanied by osteoblasts - these osteoblasts lay down primary spongiosa, eventually cancellous bone

Question 17

6. further lengthening of diaphysis

Answer 17

chondrocyte proliferation and hypertrophy, marrow cavity enlarges

Question 18

7. secondary centres in epiphyses

Answer 18

follows same processes: chondrocyte hypertrophy – vascular invasion – ossification

• no marrow cavity in epiphyses
• diaphysis thickens due to traction on the periosteum.
• leaves a think disc of cartilage between epiphyses and diaphysis

Question 19

8. growth plates

Answer 19

cause most pre and post natal growth , one at each end of most long bones and vertebral centra

Question 20

growth plate structure

Answer 20

1. reserve cartilage - hyaline
2. proliferation - columns form
3. maturation - stop dividing and swell
4. hypertrophy and calcification
5. cartilage degeneration, osteoblast invasion
6. osteogenesis - mineralisation on cartilage spicules the metaphysis
   - - shaft lengthens

Question 21

joint formation

Answer 21

proximally in the limb joints form in the pre-existing gaps between blastemal condensations
in the autopod, joints may act to divide condensations

Question 22

joint formation mechanism

Answer 22

1. a mesenchyme ‘joint plate’ infills the gap
2. joint plate capitates and differentiates into capsule and synovial membrane
3. synovial cells are worn off the articular surfaces by metal movements in utero

Question 23

limb muscle formation

Answer 23

limb muscle precursors are not made within the limb bud, but invade it in to form the myotomes
forms from ventral hypomere