L18: Limb Development

Question 1

define limb disc

Answer 1

thickened area on the side of the embryo that will give rise to the limb

the actual primordia for the limb

Question 2

peripheral region around the limb disc

Answer 2

the morphogenetic field

can give rise to the limb if the disc is removed

Question 3

what happens if the limb field is removed?

Answer 3

no limb will form

Question 4

removing part of a limb disc

Answer 4

remaining portion will form normal limb

Question 5

splitting a limb disc into 2 halves and preventing them from fusing

Answer 5

each half will form a normal limb

Question 6

fusing 2 limb discs together

Answer 6

a single normal limb forms

Question 7

list the roles of the following:
tbx4
tbx5
gli 3
hand 2

Answer 7

4 — initiates future hind limb

5 — initiates future forelimb

3 — expressed anteriorly and fixes AP axis

2 — expressed posteriorly and fixes AP axis

Question 8

initiation of limb development

Answer 8

**Tbx5/4 are the initial initiators of limb development

1. they signal mesoderm to express FGF-10
2. which signals ectoderm to express FGF-8
3. which acts back on the mesoderm

paraxial mesoderm using retinoic acid as signal based upon Hox code

Question 9

list the axes that define limb development

Answer 9

dorsoventral
ant-post
proximodistal

Question 10

transplant a limb disc to the opposite side

Answer 10

AP is reversed
no other axes are changed

normal limb will form

Question 11

rotate a limb disc 180 degrees

Answer 11

both AP and DV axis are reversed

normal limb develops but still only AP axis is reversed

Question 12

Slack transplanted a strip of flank tissue from just posterior to the limb disc to just anterior of the limb disc

Answer 12

the resulting limb consists of 2 post halves arranged in mirror image fashion

conclude that the post tissue = post organizing center
—it uses retinoic acid to diffuse across tissue for signaling

Question 13

Riddle: suggested that the morphogen expressed by the posterior organizing region might be the RNA product of Shh gene

treated the anterior part of a limb bud w/ retinoic acid

Answer 13

caused the cells in this region to behave like post organizing region

conclude that retinoic acid in not the morphogen but capable of activating shh genes

Question 14

Hoxb8

Answer 14

determines position of ZPA via its highest conc

expression is induced by retinoic acid

Question 15

Shh

Answer 15

signal produced by ZPA

maintain structure and function of AER

induces expression of gremlin

Question 16

gremlin

Answer 16

inhibits BMP-2

inhibits gli-3 in post part of limb bud

Question 17

BMP-2

Answer 17

inhibits FGF-4 in AER

Question 18

gli-3

Answer 18

inhibits expression of shh in anterior part of limb bud

it itself is expressed in the anterior part of limb bud

Question 19

define AER

Answer 19

apical ectodermal ridge

2 layers:
outer flattened periderm
inner simple columnar layer

separated from basal lamina by limb bud mesoderm

located at border btwn dorsal ectoderm and ventral ectoderm

Question 20

AER and limb bud outgrowth

Answer 20

interacts w/ underlying limb bud mesoderm

promotes continued outgrowth of limb

outgrowth promoting signals = FGFs and Wnts

Question 21

limb bud mesoderm

Answer 21

involved in determination of actual limb morphology

Question 22

removal of AER results in ?

Answer 22

cessation of any further limb outgrowth

Question 23

cells in distal limb bud express ?

Answer 23

msx1

Question 24

cells in posterior part of limb bud are under the influence of ______ and develop into ?

Answer 24

retinoic acid

proximal limb components

Question 25

limb bud mesenchyme transplanted under epidermis

Answer 25

a limb will form there

Question 26

effects of limb mesoderm on overlying ectoderm is an example of ?

Answer 26

instructive induction

Question 27

grafting extra AER over limb bud mesoderm

Answer 27

additional limb forms

Question 28

mesoderm produces AEMF to do ?

Answer 28

apical ectodermal maintenance factor

the maintain AER in healthy state

Question 29

posterior necrotic zone

Answer 29

will become axilla

Question 30

mesenchymal cells destined to form limb bud musculature will migrate from ?

Answer 30

somite ventral dermomyotomes

Question 31

why is cartilage formation restricted to central core of limb bud ?

Answer 31

due to inhibitory restraints caused by the overlying limb bud ectoderm

which produces wnt-7a

Question 32

mesenchymal pre-cartilage condensations express ? and _____ is expressed later in cartilage. ______ are later restricted to perichondrial cells .

hypertrophic cells express ____ which may induce expression of _____.

Answer 32

BMP-2/4

BMP-3

BMPs

ihh - indian hedgehog
BMP-6

Question 33

differentiation of cartilaginous skeleton occurs in a ________ sequence.

digit formation occurs in the ____ to _____ direction.

Answer 33

proximodistal sequence

V to 1 direction

Question 34

list the sequence of stages in joint formation

Answer 34

1. limb skeleton forms as precartilaginous rods
2. cells traverse to future joint area
3. BMP is needed to maintain cartilage development
4. noggin inhibits bmp
5. cell necrosis
6. loose CT fills area
7. articular cartilages form on sides of joint cavity
8. mesenchymal cells form the other components

Question 35

what is the receptor for scatter factor

Answer 35

c-met

Question 36

sequential events in limb muscle formation

Answer 36

1. prox cells of limb bud produce scatter factor
2. premuscle cell express c-met
3. 30-100 premuscle cells migrate to bud from each somite
4. migrating cells express pax3 and n-cadherin
5. dif into muscle depend on wnt6 from bud ectoderm
6. condensation
7. myogenic cells condense further into 2 muscle masses (flexors/extensors)

Question 37

CT and limb muscle formation

Answer 37

1. CT associated w/ muscle masses expresses Tcf4

2. CT determines morphology of individual muscles

Question 38

motor axon growth into muscle masses

Answer 38

medial axons in spinal cord grow into ventral muscles

lateral axons in sc grow into dorsal muscles

ant muscles - ant axons
post muscles - post axons

Question 39

nerves – removal of a forelimb

Answer 39

nerves of brachial plexus remain smaller than they would have been

ganglia cells may be reduced by 50%

Question 40

nerves – adding an extra limb

Answer 40

nerves to the normal limb also supply the extra limb

nerves and ganglia increase in number/size

Question 41

nerves – forelimb in cut out prior to nerve outgrowth and transplanted somewhere else

Answer 41

brachial nerves will deviate from normal path to reach the transplanted limb and innervate as normal

if transplanted very close brachial plexus will develop new branches to supply it

Question 42

normal path of nerves is blocked by an obstacle

Answer 42

avoid it
go around it

and still reach their proper location

Question 43

if a transplanted limb is placed too far away or an obstacle is too great

Answer 43

local spinal nerves will be recruited but they cannot provide proper innervation

so limb does not have normal function

**thus only brachial/lumbar plexus nerves have ability to innervate a fully functioning limb

Question 44

initial limb vasculature

Answer 44

• -fine capillary network
• -specific channels form
• -resulting in central art
• -capillary beds connect art to marginal sinus
• -marginal sinus drains into venous channels

overlying ectoderm inhibits formation of blood vessels