limb development

Question 1

evolution of tetrapod limb?

Answer 1

fin fold/wolffian evolved into pectoral fin then chiridium - struts

Question 2

how does the limb bud form

Answer 2

precursors of skeletal elements and tendon sheaths, muscle and vessels migrate, limb and girdle bone.

Question 3

how does the limb bud elongate

Answer 3

proximo-distal differentiated is mediated by the apical ectodermal ridge as limb lengthens the AER induces the fate of the underlying tissue

Question 4

what is the limb composed of

Answer 4

a stylopod, zengopod and autopod

Question 5

what happens with progressively earlier ablasion of AER

Answer 5

causes loss of more proximal elements

Question 6

late ablasion?

Answer 6

no autopod

Question 7

early ablasion

Answer 7

no zengopod or autopod

Question 8

how is the craniocaudal orientation generated

Answer 8

by zone of polarising activity lying caudally. grafting a ZPA on the cranial edge of the limb bud results in a mirror image duplication

Question 9

what is polydactyly in cats caused by

Answer 9

ectopic medial expression of ZPA related genes

Question 10

how is limb patterning achieved

Answer 10

apoptic digit separation is programmed cell death, number of digits is dependent on species

Question 11

what is the condition where digits do not separate

Answer 11

syndactyly

Question 12

what is the name of the mechanism of bone formation in most of the trunk and limbs

Answer 12

bone endochondral ossification

Question 13

what are the 3 main stages of bone endochondral ossification

Answer 13

mesenchymal (fetal connective tissue) condensation > cartilage model > ordered mutlicentre process of ossification

Question 14

outline the whole process of bone endochondral ossification 7

Answer 14

1 blasternal condensation 2 chondrification 3 hypertrophy 4 early mineralisation 5 vascular invasion 6 further lengthening of diaphysis 7 secondary centres in epiphyses

Question 15

blasternal condensation

Answer 15

large tightly packed mesenchyme cells

Question 16

chondrification

Answer 16

cells differentiate into chondrocytes - THE CARTILAGE MODEL

Question 17

hypertrophy

Answer 17

chrondrocytes at centre will stop dividing and swell = interstitial growth. connective tissue around the model is compressed into perichondrium the inner surface of which is chondrogenic = appositional growth

Question 18

early mineralisation

Answer 18

central chondroblasts start to mineralise the matrix. perichondral cells become osteoblasts > cortical bone collar > compact bone

Question 19

vascular invasion

Answer 19

through cortical bone accompanied by osteoblasts. these lay down primary spongiosa which eventually becomes cancellous bone

Question 20

further lengthening of diaphysis

Answer 20

chrondrocyte proliferation and hypertrophy, marrow cavity enlarges

Question 21

secondary centres in epiphyses

Answer 21

same process: chondrocyte hypertrophy vascular invasion > ossification

Question 22

what is the difference of the bone endochondral ossification in epiphyses

Answer 22

no marrow cavity in epiphyses. diaphysis thickens due to traction on periosteum. leaves thin discs of cartilage between epiphysis and diaphysis

Question 23

what are growth plates

Answer 23

cause most prenatal and post natal growth and are found at each end of most long bones and vertebral centra (only in accessory carpal and tibular tarsal)

Question 24

when do growth plates close

Answer 24

after maturity - bridged by bone, and are perforated by vessels leaving a radiological mark/scar

Question 25

which species have 2 epiphysis in the embryo digit

Answer 25

MCIII, MTIII, P1 and P2 all have two
epiphyses in the embryo. Each only has one by birth – at the ‘fetlock end’.
These close between 6-18 months
Unclear if P3 ever has true epiphyses or growth plates (does in humans).

Question 26

what are the functional types of growth plates

Answer 26

in compression e.g. radius, tension e.g. ulnar, ‘flake epiphysis’ late forming and variable e.g. iliac crest. evolutionary relics?

Question 27

what are evolutionary relic growth plates

Answer 27

not true growth plates, but fusions between bones e.g. scapular

Question 28

why is the scapular different?

Answer 28

distal parts are limb bone like but the blade is dermomyotome

Question 29

what is the structure of growth plates

Answer 29

1. reserve cartilage - hyaline 2 proliferation - columns form 3 maturation - stop dividing and swell 4 hypertrophy and calcification 5 cartilage degeneration with osteoblast invasion 6 osteogenic zone - mineralisation on the cartilage specules the metaphysis.
   SHAFT LENGTHENS @ DIAPHYSIS

Question 30

where do joints form

Answer 30

between pre existing gaps between blastemal condensations

Question 31

outline joint formation

Answer 31

1 a mesenchyme joint plate infills the gap 2 joint plate cavitates and differentiates into capsule and synovial membrane 3 synovial cells are worn off the articular surfaces by fetal movements in utero

Question 32

where are muscles formed

Answer 32

limb muscle precursors invade into the limb bud from the myotomes at epimere and hypomere

Question 33

what is angular limb deformity

Answer 33

in fast growing animals and bones such as ulnar and tibia. growth is from the conical distal growth plates leading to a deformed limb of overgrowth and a slight bend in the bone

Question 34

how are bones repaired after a fracture

Answer 34

have to let the bone remodel and regrow. controlled by cytokines and hormones