Devo Lect 18 - Vertebrate Limb

Question 2

Asymmetry

Answer 2

Ant, post, proximal, distal parts of limbs very different

Question 3

Comparing limbs across species

Answer 3

Same bones and organization, just different proportions based on function

Question 4

Disorders in limb development

Answer 4

Polydactyly: extra digits; Syndactyly: fused digits; Polymelia: extra limbs; phocomelia: shortened limbs

Question 5

Formation of limb bud

Answer 5

Mesenchyme cells migrate to limb field, proliferate, form AER on limb buds (essential); limbs formed by stage 23 (day 56)

Question 6

AER

Answer 6

Apical ectodermal ridge; forms on top of the the limb bud mesenchyme cells. Needed for development of distal limb structures. Removal results in truncated limb, length depends on when removed

Question 7

FGF-10 and FGF-8

Answer 7

Mesenchyme cells secrete FGF-10 which cause epithelium to form AER; AER secretes FGF-8 to maintain mesenchyme cells just underneath; Mesoderm-ectoderm interactoin

Question 8

Saunders’ experiment (Fig 16.8)

Answer 8

Formation of limb axis - proximal-distal; mesoderm proliferation under AER; removal and addition of AER and mesoderm; AER allows proliferation, induced mesoderm tells the limb what to be, FGF responsible for signaling

Question 9

Figure 16.8

Answer 9

Summarize this

Question 10

Anterior/Posterior formation of limb bud

Answer 10

Defined before limb bud present; smal block of mesoderm tissue = ZPA; transplanting it caused posterior development of digits on both sides of limb

Question 11

ZPA

Answer 11

Zone of polarizing activity; determines ant/post axis; releases Shh, forms a gradient (high = post., low = ant.)

Question 12

Shh

Answer 12

Development of vertebrae; Somite to bone; from Hensen’s node to determine R/L symmetry; controls polarity of limbs

Question 13

Role of Hox genes in limb

Answer 13

TFs, genes arranged in series on chromosome; expressed in order along limb axis; knockouts result in lack of only one structure (ie Hox 11 -/- lack forearm, but the rest is normal)

Question 14

HoxA/D KO experiments

Answer 14

Stops shortly after bud; if they then add HoxD, hand developed on there

Question 15

Hox genes and snakes

Answer 15

Expression is different; Hox genes expressed in certain regions in chick embryo, but the boundaries of those genes in snakes is different; some snakes form vestigial limbs, but lack Shh

Question 16

Hox genes in fins vs mammals

Answer 16

Early Hox expression similar, later it is much higher in AER in mammal; important step in evolution

Question 17

Cell death zones

Answer 17

apoptosis, esp. in the digits; triggered by BMPs, although these are expressed in ducks and chickens; BMPs are blocked by gremlin in the duck

Question 18

Gremlin experiment

Answer 18

Put Gremlin beads in the webbing of developing chick embryo, apoptosis did not occur, proves gremlin is an antagonist of BMP