lec 24- limb development in vertebrates

Question 1

when do limbs start to form?

Answer 1

when the embryo has developed a body with neural tube, somites, notochord, and a head

Question 2

when do the regions of the embryo that will form the limbs become determined?

Answer 2

early in development

Question 3

if some regions of limb formation are removed, can the embryo still form normally?

Answer 3

yes, the remaining cells can compensate for the missing part and a normal structure can form

Question 4

in chick embryos, what forms first when it comes to limb formation?

Answer 4

-the limb bud forms along the sides of embryos that are about 3 days old
-limb buds form by proliferation of the lateral plate mesoderm

Question 5

what are limb buds made of?

Answer 5

they are composed of a mesenchymal with an outer covering of ectoderm

Question 6

what is mesenchyme?

Answer 6

loose connective tissue

Question 7

what do the mesenchymal cells give rise to?

Answer 7

the skeletal elements and connective tissues of the limb, the muscles develop from cells that migrate into the limb from the somites

Question 8

what forms at the tip of the limb bud?

Answer 8

a thickening of ectoderm known as the apical ectodermal ridge

Question 9

what occurs in a limb bud?

Answer 9

-presumptive muscle cells migrate into the limb from the adjacent somites
-behind the proliferating cells, mesenchymal cells begin to differentiate into cartilage
-beneath the ridge is a region of proliferating undiffernetiated cells called the progress zone

Question 10

how does limb formation occur?

Answer 10

-over time the limb bud grows and lengthens along the axis that extends from shoulder (proximal) to the finger or wing tip (distal)
-skeleton is laid down from proximal to distal end
-mesenchyme cells condense, turning into cartilage then bone
-limb is patterned along AP axis and DV axis

Question 11

are the mesoderm in the limb regions determined to form limbs before limb buds form?

Answer 11

yes

Question 12

how do the Hox genes help limbs form before limb initiation?

Answer 12

-Hox genes are expressed in lateral plate mesoderm
-one set of Hox genes is expressed in the region of wings/forelimbs
-a second in the intervening regions that act as repressors
-a third set in posterior regions
-the transcription factor Tbx 5 determines the forelimbs and is controlled by Hox genes
-the transcription factor Tbx 4 determines hind limbs and it gains help from Pitx1
-Tbx4 and 5 activate FGF10 expression and FGF 10 induces expression of FGF8 in the overlying ectoderm which will form the Apical Ectodermal Ridge
-FGF8 from the ridge maintains FGF10 expression in the mesoderm, creates a positive feedback loop

Question 13

what evidence is their that Tbx4 and 5 determines the identity of the limb and FGF is the ligand that initiates limb development?

Answer 13

1. Mutant embryos that fail to produce FGF ligands or lack an FGF receptor cant form limbs buds
2. When FGF is applied to the interlimb region between the prospective limb buds an extra limb forms, if its closer to Tbx5 a forelimb forms and if its closer to Tbx4 a hindlimb forms, a chimeric wing forms in the middle (has both)

Question 14

what are the three axes of the limbs?

Answer 14

proximal-distal = shoulder to hand
anterior-posterior = digit 1 to 5
dorsal-ventral = side away from the body to the side next to the body

Question 15

what are the two organizing regions in limb formation?

Answer 15

the apical ectodermal ridge (AER) and the zone of polarizing activity (ZPA)

Question 16

what does the AER do?

Answer 16

controls limb buds outgrowth and proximal-distal patterning

Question 17

what does ZPA do?

Answer 17

specifies position along the anterior-posterior axis

Question 18

what happens to cells in the progress zone?

Answer 18

proliferate and start to differentiate as they leave the progress zone

Question 19

what happens when AER is removed?

Answer 19

limb development ceases

Question 20

what happens when an extra AER is added?

Answer 20

leads to duplicated structures

Question 21

what happensif AER is removed but FGF8 or FGF4 signals are grafted on?

Answer 21

normal development

Question 22

what evidence shows that FGF is a key signal needed to form the ridge?

Answer 22

1. FGF8, FGF4, and two other FGFs are expressed in the AER
2. when FGF8 and two other FGF genes are removed from the ridge, truncated limbs form

Question 23

what are the two models that have been proposed to explain patterning along the proximal-distal axis?

Answer 23

1. progressive specification (or timing) model
2. the two signal model

Question 24

explain the progressive specification model:

Answer 24

-proposes the positional values are specified based on length of time cells spend in the progress zone
-cells that spend longer in the progress zone would experience more AER signals than cells that leaver quicker

Question 25

explain the two signal model:

Answer 25

-proposes that diffusible signals specify positional values and the structures then differentiate progressively in sequence as the limb grows out -cells in the early limb bud would acquire positional values from two opposing signals: retinoic acid from the proximal region causing proximal fates and FGF signals from the AER specifying for distal fates -cells that perceive low levels of both will acquire an intermediate fate